Underlying nature of specific language impairment

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@ 1992 Association for Child Psychology and Psychiatry
J Child Psychol Psychial Vol. 33, No. I, pp. 3-66, 1992
Prioted in Great Britain
D V. M. Bishop
Introduction
Specific language impairment is diagnosed where there is a failure of normal language
development that cannot be explained in terms of mental or physical handicap, hearing
loss, emotional disorder or environmental deprivation. In the past, "developmental
dysphasia" or "developmental aphasia" were widely used to refer to this condition,
but these terms have fallen into disfavour in the U.K. and U.S.A., largely because
they misleadingly imply that we are dealing with a single condition with a known
neurological basis. These me<;iical terms are, however, still popular in continental
Europe, and among some paediatric neurologists inEnglish-sp~aking countries (e.g.
Rapin, 1987). Most specialists in the U.K. and U.S.A. prefer the more neutral terms
"developmental language disorder", or "specific language impairment" (SLI), and
it is the latter that will be adopted here.
Much of the extensive literature on SLI is concerned with documenting the
difficulties experienced by these children with different aspects of language. A smaller
body of work has concentrated on formulating psychological models that explain SLI
in terms of impairment in a particular aspect of cognitive processing. This paper aims
to bring together these approaches and to assess how far specific linguistic deficits
can be explained in terms of particular cognitive impairments. Each of the following
hypotheses will be reviewed and evaluated in turn.
Hypothesis 1. Underlying linguistic competence is intact, but there is an impairment
in the processes that are involved in converting this underlying knowledge into a speech
signal, i.e. the problem is an output disorder.
Hypothesis 2. SLI results from impairment of auditory perception, which influences
the course of language acquisition.
Keywords: Language impairment, child cognition, linguistics
Acceptedm anuscript received5 July 1991
Requestsjroerp rintsto : Dr Dorothy Bishop,M RC Applied PsychologyU nit, 15 ChaucerR oad, Cambridge
CB2 2EF, U.K.
'J,

4 D. V. M. Bishop
Hypothesis3 . There is an isolated impairment of the specializedli nguistic mechanisms
that have evolved to handle language processing.
Hypothes,s4 . There is a generalized deficit in conceptuadl evelopmentht at affects, but
is not restricted to, language processing.
Hypothesis5 . Learning strategiesa re abnormal, with a failure to apply appropriate
hypothesis-testing procedures.
Hypothesis6 . The problem is not with handling particular types of mental operation,
but rather arises becauseo f limitations in the speeda nd capaciryo j thei nformation-processing
system.
The paper will conclude by considering some general principles for future research
that are suggested by this review.
1. Language Impairment as Output Disorder
Information-processing accounts for cognitive functions draw a distinction between
mental representationosf information, and the cognitive operationsth at are involved in
transforming information from one representation to another.
Associated with any language stimulus will be a range of different representations.
A spoken utterance impinging on the ear is initially encoded in the auditory system
in terms of the acoustic characteristics of the stimulus. This representation must then
be converted into a more abstract representation in terms of a seq~ence of speech
sounds. This will not contain information about superficial acoustic details, such as
those characteristics specific to a particular speaker: the sound /b/ will be treated the
same irrespective of incidental features such as pitch or presence of background noise.
Familiar sequences of sounds are then matched against existing lexical representations
which are maintained in a long-term store. If we hear an unfamiliar word then no
lexical representation will be activated. A representation of the abstract grammatical
structure of the utterance will be derived from an analysis of the pattern of content
words and grammatical morphemes, and this information will be combined with
knowledge of meaning of lexical items to derive a representation of the propositional
content of the utterance.
A similar hierachy of levels of representation is implicated in the process of speech
production which starts with a representation of the meaning of the intended message,
and ends with a specification of motor commands to the articulators. The process
of producing meaningful speech involves generating representations of the grammatical
structure needed to convey the relations expressed in the message, accessing appropriate
lexical representations that specify the sequence of speech sounds corresponding to
words with relevant meanings, and computing the articulatory correlates of a particular
sequence of sounds and the motor movements necessary to produce these.
If we adopt this kind of model, we are led naturally to the question of where in
the language processing chain the deficit of SLI children is located. For instance,
when a child produces a grammatically simplified utterance, is this because the
underlying representation of grammatical structure is inadequate, or is the problem
somewhere in the chain of processes that are involved in converting an abstract
grammatical representation into speech output? Note that an output disorder is distinct

Specific language impairment 5
from a peripheral impairment of motor control. The distinction may be clarified by
drawing an analogy with the situation where a person is asked to draw a bicycle and
ends up producing a woefully inadequate picture (as many adults do). Logically, one
can see that the poor drawing could indicate that the person had only a vague idea
of what a bicycle looked like (a disorder of a central representation), or it could be
that they had a perfect mental image of a bicycle, but lacked the ability to transform
this image into appropriate motor movements. This would correspond to an output
disorder. Contrast this with the person who is unable to execute a drawing because
of some peripheral problem such as paralysis or tremor.
Early accounts of SLI did not use this conceptual framework, but they implied
that many expressive language problems could be regarded as output disorders. Most
children were thought to have an "expressive" form of "developmental aphasia",
where the difficulties were not explicable in terms of peripheral motor problems (e.g.
dysarthria), nor in terms of defective understanding. This view ofSLI may be evaluated
with regard to the different components of language that are affected.
Output explanations of abnormal speech
Virtually all children with SLI have some abnormality of speech production (Haynes
& Naidoo, 1991) and any theory of the disorder should be able to account for this.
In some children, problems with speech sound production are the only obvious
signs of disorder. Traditionally, such cases were diagnosed as "functional articulation
disorders", and treated as quite distinct from more pervasive language disorders.
However, there is some evidence that this might be a false dichotomy, and that
disorders restricted to speech output are on a continuum with more severe conditions
where a wider range of language functions is affected. Evidence for continuity is of
two kinds. First, many children whose problems appear confined to speech sound
production do prove to have more widespread impairments when standardized
language tests are administered (Shriner, Holloway & Daniloff, 1969; Whitacre,
Luper & Pollio, 1970; Marquardt & Saxman, 1972; Saxman & Miller, 1973). Second,
if children are followed over time, the picture often changes from a child having general
difficulties with grammar and speech production, to one with isolated speech problems
(Bishop & Edmundson, 1987a; Scarborough & Dobrich, 1990).
The term "functional articulation disorder" implies that control of the articulatory
apparatus is defective either because of poor motor skills or inadequate oral sensory
feedback. However, in recent years, the emphasis has moved from articulatory
processes to consider other explanations of speech problems.
In terms of the framework introduced here, an output explanation of' 'functional
articulation disorders" maintains that the child has an adequate representation of
the phonological system of the language, and normal control of the articulatory
apparatus. What is lacking is the ability to convert an abstract phonological
representation into a set of motor commands to the articulators. Before we can evaluate
this hypothesis, we must consider alternative explanations.
One view, that will be considered in greater detail below, maintains that the
underlying problem affects input rather than output processes: abnormal speech is
seen as a secondary consequence of inadequate auditory perception. If children do

6 D. V. M. Bishop
not distinguish between two sounds perceptually, then it would be expected that they
could not learn to produce them distinctively.
Another possibility is that input processes are unimpaired, but the child fails to
develop a normal representation of the phonological system of the native language.
The problem is in abstracting the underlying structure from speech input. Those
without training in linguistics tend to assume that language contains a finite number
of speech sounds and the child's task is to learn the articulatory configuration
corresponding to each sound. This is not so. Languages vary in the ways in which
they partition the domain of possible speech sounds into sets that contrast differences
in meaning. The task confronting the child is not just leaming how to articulate sounds,
but also discovering which sounds signal a change in meaning and which do not.
For instance, French distinguishes the vowel sounds in the words "doux" and 'du",
whereas English does not (both sound like renderings of English "do"). Conversely,
English contrasts the initial consonant in "then" and "zen", whereas French does
not. English speakers of French frequently make errors on the doux/du contrast,
whereas French speakers of English confuse then/zen; not because of any inherent
perceptual or articulatory limitations, but because they do not have an adequate
knowledge of the phonological system of the language. It is possible that children
with SLI have a similar difficulty; they fail to learn which sounds are contrastive
in their language. This will be termed the learning hypothesis.
We can see, then, that output explanations for speech disorders in SLI are not
the only possible account: the problem could arise from auditory perceptual deficits,
or it might reflect difficulty in learning how to partition the universe of possible speech
sounds into a set of phonemes that can signal contrasts in meaning. Let us consider
what types of evidence may be used to choose between these competing hypotheses.
Circumstantial evidencefr om associatedm otor deficits. One line of evidence comes
from studies investigating other areas of motor function in children with SLI. Few
children with SLI have hard neurological signs, but several studies have found that
they are often immature on tests of motor skill (Stark & Tallal, 1981; Bishop &
Edmundson, 1987b; Robinson, 1987). While this evidence is only correlational, it
does give credence to the notion that speech difficulties may be but one indication
of general difficulties in programming coordinated sequences of motor movements.
Analysiso f speeche rrors.A seconda pproacht o distinguishing explanationso f speech
difficulties involves analysing the errors themselves. The traditional approach was
to classify" articulation errors" in terms of substitutions and omissions of particular
sounds. The involvement of linguists in the study of language impairments led to
radical changes in how abnormalities of speech production were assessed and
conceptualized. Grunwell (1982) noted that traditional error analysis failed to
distinguish between phonetic deviations and phonological disorders. A speech disorder
is appropriately described as a phonetic deviation when the child marks all the contrasts
that are used to signal differences in meaning in the adult system, but production
of particular sounds is distorted. A common example is where /r/ is produced as a

labiodental approximant lpJ (so that it sounds intermediate between a normal Irl and
Iw/). Although one perceives such speech as abnormal, the production of Irl and Iwl
is clearly contrasted, so that "ride" and "wide" are produced differently. In such
cases, the underlying phonological system is intact: the abnormality is in how
articulatory correlates of specific sounds are represented. One might wonder whether
the distorted production of particular sounds could be a consequence of distorted
perception: however, this seems implausible because any perceptual impairment should
apply when monitoring one's own speech output as well as when listening to the speech
of others, so should not result in a mismatch between perception and production.
(One is reminded of the claim that Modigliani's elongated figures may be attributed
to astigmatism. The fallacy is apparent when one considers how Modigliani's figures
would appear to the artist: if normal figures look elongated to him then his own
elongated drawings should appear even more abnormal).
Although there is a lack of systematic data on this point, most children with SLI
appear to have difficulties that go beyond phonetic distortions, in that they do not
make all the contrasts between phonemes that are required by the adult system and
so, for instance, many not distinguish Idl and Ig/. In such cases all three of the
hypotheses considered above are plausible: the difficulty could be in perceiving the
distinction between the speech sounds, in learning which sounds are distinctive in
the language, or in motor output process. Leonard (1982) found close similarities
between phon910gical errors of SLI children and those seen in normal development.
Analyses of the phonological production of normal young children indicates that their
errors are systematic in two respects. First, they involve entire classes of speech sounds.
To take a common example, if the child tends to produce "cat" as "tat", "goat"
as "doat" and "sing" as "sin" we may account for all these changes by specifying
that velar sounds are produced at the alveolar place of articulation. Second, many
changes operate in one direction only: e.g. "k" is produced as "t", but not vice
versa. Stampe (1969), who was interested in explaining patterns of phonological
development across different languages, termed these systematic patterns "phonological
processes" and argued that they operated to simplify speech output by merging
contrastive pairs of phonemes to the more easily articulated form. This, then, is an
output explanation of phonological errors which assumes that the child's lexical
repr~sentations of the phonological forms of words are accurate, but simplification
occurs at the articulatory stage. Stampe's account is attractive because it can explain
certain of the characteristics of common processes that are not easily accounted for
in terms of perceptual or learning hypotheses. One of these is the fact that substitutions
tend to be undirectional. If the child treated Ik/ and It I as different exemplars of the
same phoneme one might expect either form to be produced interchangeably, but
this is not observed; Ikl is often produced as It/, but one seldom finds It I produced
as Ik/. Also, many processes operate at the suprasegmentallevel; thus children tend
to delete final consonants (e.g. "bag" becomes "ba"), reduced consonant clusters
(e.g. "string" becomes "ting") and omit weak syllables in polysyllabic words (e.g.
"banana" becomes "nana"). Furthermore, there may be consonant harmony,
whereby the production of a sound is influenced by other sounds earlier or later in
the word (e.g. "daddy" is pronounced correctly but "doggie" becomes "goggie").

8 D. V. M. Bishop
Such patterns are not easily explicable in terms of difficulty in perceiving certain
consonant distinctions, or as the consequence of a phonological system that collapses
certain categories. However, they are readily explained in terms of simplification of
articulation.
Grunwell (1981) has argued against applying this type of output explanation to
phonological disorders on the grounds that children with such difficulties are usually
able to imitate speech sounds in isolation, but this is not incompatible with an output
explanation. If motor movements were slow, clumsy or poorly coordinated, one can
see that a child might succeed on the simple task of imitating one sound, but reveal
problems on more taxing tasks which involve integrating a sequence of sounds into
a fluent word form.
There is a third pattern of speech error that has been described in SLI children,
which is not readily categorized as either phonetic or phonological. This is where
the child's speech production is unpredictable, both in terms of whether a particular
word will be pronounced correctly, and in terms of which sound will be substituted
if an error is made. Accuracy of speech production is more a function of length of
the utterance than of its phonological constitution. A common way of identifying this
type of speech problem in children is to contrast ability to repeat isolated speech sounds
(p - p - p, t - t - t - , or k - k - k -), with ability to repeat a sequence of different
sounds (e.g. p - t - k). Many children who are able to repeat isolated sounds have
difficulty with the p - t - k sequence. By analogy with acquired neurological disorders,
this has been termed "developmental verbal dyspraxia" (Edwards, 1973), but the
diagnosis remains controversial, and defining criteria are inconsistently applied.
Evidencefr om perceptuatl asks. The evidence reviewed so far indicates that many of
the phonological problems observed in SLI children are compatible with an explanation
in terms of output processes and are hard to explain in terms of a simple perceptual
or learning theory that maintained that the child simply collapsed certain phonemic
contrasts into a single category. However, as we shall see in the next section, there
is one set of evidence that is difficult to reconcile with an output explanation, namely
the finding that most SLI children, including those regarded as having dyspraxic
disorders of motor programming, are impaired on a range of phonological processing
tasks ~hat do not involve any speech output.
One way of explaining such findings is to propose a new version of a perceptual
or learning theory, in! which the problem is not viewed as failure to distinguish between
contrastive phonemes, but as a more basic inability to segment the speech stream
into phonemes. This hypothesis will be considered more fully below. An alternative
approach is to retain an explanation in terms of defective speech output and to explain
perceptual problems as a consequencoef the output difficulties. Winitz (1969), for
example, suggested that repeated mispronunciation of words affected the child's
perceptual system so that contrasts which were misproduced would eventually also
be misperceived. One way of testing this idea is to consider speech perception in
children whose speech production is impaired for physical reasons, i.e. dysarthric
children, where one would expect a similar effect to be observed. Bishop, Byers Brown

and Robson (1990) carried out such a study, comparing two groups of people with
cerebral palsy, matched on age and non-verbal ability: a group with speech difficulties
(dysarthria or anarthria) and a control group with normal speech. Speech-impaired
individuals were impaired relative to controls on speech discrimination when assessed
using a task that involved judging whether minimal pairs of non-words were the same
or different, but they were not impaired when the same contrasts were tested using
Locke's (1980) procedure (see below), which does not require the child to retain
unfamiliar strings of sounds. This study indicated that speech difficulties can influence
performance on speech discrimination tests, depending on which assessment method
is used.
Grammatical impairment as a secondary consequenceo f phonological disorder
Phonological and grammatical errors do tend to co-occur in the expressive language
of SLI children and Leonard, Sabbadini, Leonard and Volterra (1987) considered
whether failure to produce certain grammatical morphemes might reflect the operation
of common phonological processes in the speech of children with SLI. It could be
argued that their tendency to omit grammatical morphemes that form phonologically
complex clusters (e.g. Itsl in "cats", Iptl in "helped") reflects a process of cluster
reduction, and that omission of unstressed sylables (e. g. "is" in "he is big") arises
through a process of weak syllable deletion. In support of this view, Panagos and
Prelock (1982) found that syntactic errors increased with syllabic complexity in a
sentence repetition task, and Paul and Shriberg (1982) noted that in some SLI children
omissions of grammatical morphemes could be totally accounted for in terms of
phonological processes such as final consonant deletion.
However, data collected by Leonard et al. (1987) showed very clearly that expressive
phonological limitations alone cannot explain the range of morphological deficits in
English-speaking children with SLI. The children were much more likely to produce
the regular noun plural (e. g. dog - dogs, cat - cats) in obligatory contexts than the third
person singular verb (e.g. go -goes, kick -kz'cks) or the contracted copula ('s) although
all these grammatical forms involve adding Isl or Izl to the stem.
Another re~son for rejecting an explanation of grammatical deficits in terms purely
of output processes is that this could not account for the comprehension problems
seen in these children. Bishop (1979) found that most children with expressive
grammatical disorders were impaired on a test of grammatical comprehension, even
though their deficits might not be obvious in casual conversation when they could
rely on context and redundancy to decode language.
Output explanations of vocabulary deficits
SLI children do poorly on tasks of naming vocabulary, raising the question of
whether the child's long-term store of words (the "mental lexicon") is impoverished
or whether the child has adequate lexical representations but cannot retrieve these.
Problems of lexical access are common in acquired aphasia, where the patient knows
the word, and may be able to generate it if provided with a cue such as the first sound,
but suffers from a severe form of the "tip of the tongue" phenomenon that may affect
even the commonest words (see Lesser, 1978, for a review). Inability to access an

10 D. V. M. Bishop
intact lexical representation corresponds to an output deficit in the conceptual
framework adopted here.
Dollaghan (1987) argued that vocabulary deficits in SLI children reflect problems
in retrieval rather than perception or storage of words. However, the evidence for
this conclusion was ambiguous. She studied a group of 4 - 5-year old SLI children,
none of whom had major phonological problems that might interfere with the ability
to pronounce novel words, but all of whom had significant limitations of expressive
syntax. Subjects were exposed to a novel word in a game. They were asked to hide
a pen and fork, before being asked to hide the "koob", which was the term used
to refer to an oddly shaped white plastic ring. The novel word was presented once
only. Comprehension was then assessed by placing these three objects on the table
together with two unfamiliar objects, to discover whether the child could select the
correct item when asked for fork, pen and koob. They were then asked to name all
three items. SLI and control children did not differ on the comprehension task, but
there was a striking difference on the production task. Only one of the 11 SLI children
produced the word "koob" compared with seven out of 11 control children. Also,
the control children were more likely to produce some of the phonemes of the novel
word correctly, even if they did not produce the whole word. The mismatch between
the successful performance by SLI children on the comprehension task and their poor
performance on production led Dollaghan to conclude that the problem was one of
retrieval rather than storage. However, children could have succeeded on the
comprehension task without retaining any phonological information about the novel
word, simply by selecting the funny object that they had seen before when presented
with a novel word. A proper test between explanations in terms of perception, storage
and retrieval would require that children be taught more than one novel word and/or
that they be given "catch trials" on a comprehension test in which they were asked
to select an object to correspond to a novel word that they had not previously
encountered.
If the main difficulty for SLI children is in retrieval of adequately stored lexical
representations, then it should be possible to improve performance dramatically by
providing retrieval cues. Kail, Hale, Leonard and Nippold (1984) studied the
effectiveness of retrieval cues on the free recall of word names by SLI children. They
found that SLI children were poor at free recall overall, but they did not improve
any more than control children when retrieval cues were provided. They concluded
that the problem for SLI children is in the initial storage of lexical representations,
rather than in lexical access.
Output explanations of memory deficits
One striking deficit seen in most SLI children is limitation of immediate memory
span (see, e.g. Haynes & Naidoo, 1991). Contemporary models of working memory
(Baddeley, 1986) stress the importance of an "articulatory loop" that is used to maintain
items in memory by a process of rehearsal and to translate material from visual to
verbal form. According to this model, people who speak slowly rehearse fewer words,
so speech rate will be a limiting factor in memory span. It is therefore predicted that
individuals with expressive phonological problems would have reduced spans. Kamhi,

Catts, Mauer, Apel and Gentry (1988) found that SLI children were poor at short-tenn
memory tasks and favoured this form of explanation for their findings, suggesting
that the problem lay in planning an articulatory program for a complex phonological
sequence. Gathercole and Baddeley (1990), however, in a similar study found that
SLI children's performance was affected by the number of syllables in the non-words,
but not by articulatory complexity. They included non-words that incorporated
consonant clusters as well as others that used only single consonants, and they found
that this had no effect on performance by SLI children.
If slow speech rate causes memory limitations, then we should find memory deficits
in children whose speech is impaired for purely physical reasons, i.e. cerebral palsied
children with dysarthria or anarthria. There is some disagreement on whether this
is the case. Bishop and Robson (1989) found no differences in memory function
between cerebral palsied individuals with nonnal speech and those who were dysarthric
or anarthric, and they concluded that rehearsal does not depend on covert articulation.
However, in a study by Raine, Hulme, Chadderton and Bailey (1991) speech-disordered
children were found to be impaired on tests of short-term memory,
regardless of whether there was a physical basis for the speech difficulties.
Evaluation: output explanationso f SLI
A major problem for output explanations of language difficulties is that most SLI
children have deficits in phonology, vocabulary and grammar even when these abilities
are tested receptively without requiring any speech production. The only way such
results could be accounted for in terms of theory of output disorder would be by
maintaining that limited ability to speak leads to restricted receptive language
development. Studies of children who are unable to speak because of physical handicaps
allow us to assess the validity of such an explanation. There is some evidence that
suggests that expressive speech difficulties can restrict short-term memory for verbal
materials, and this could lead to receptive deficits on tasks where there is a heavy
memory load. However, many children with total anarthria develop normal receptive
language skills well in excess of those seen in children with SLI (Bishop et al., 1990).
Although output difficulties may lead to secondary impairments in how children handle
language processing tasks, it does not seem feasible to treat all receptive difficulties
as secondary consequences of expressive problems. We need to consider alternative
explanations.
2. Language Impairment as an Auditory Disorder
Eisenson (1972) was one of the first to popularize the notion that auditory
impairments were the cause ofSLI. He maintained: "The use of the term developmental
aphasia, or one of its synonyms, implies that the child's perceptual abilities for auditory
(speech) events underlies his impairment for the acquisition of auditory symbols. His
expressive disturbances are a manifestation of his intake or decoding impairment"
(p. 69). He regarded SLI as a unitary disorder, ranging in severity from profound
receptive aphasia at one end, to mild syntactic and phonological deficits at the other.
He proposed that all disorders on this spectrum had the same underlying cause:

12 D.V. M. Bishop
auditory perceptual impairment. The evidence for auditory deficits in SLI will first
be reviewed before going on to consider how well this theory can account for the
range of different language impairments seen in this condition.
Evidencefo r impairment in processingr apid transients timuli
By far the most comprehensive investigations of auditory processing in SLI have
been carried out by T~l~ and her colleagues in a series of studies conducted over
the past two decades. On the basis of initi~ studies of a group of 12 SLI children
recruited from a British residenti~ school, T~l~ and Piercy (1973a,b) concluded
that language-impaired children had a selective deficit in processing rapid or brief
sign~s in the auditory mod~ity. Later work, however, using larger samples of
American children suggested that auditory perceptu~ problems are but one indication
of a more gener~ized impairment in perception and production of rapid sequences.
In ~l these studies care was taken to devise experiment~ tasks that could be
performed with no verb~ instructions and which involved contrasting only two stimuli.
The child is presented with a box which has two response panels. First he or she is
trained to press panel A in response to stimulus 1. Then stimulus 2 is presented on
sever~ occasions, and the child trained to press panel B. Once this has been mastered,
random sequences of stimuli are given, and the child presses the appropriate panel
for each one. The aim of the test procedure up to this point is to establish that the
child can discriminate the stimuli adequately. Various manipulations may then be
introduced, either by reducing the duration of each stimulus, curtailing the interv~
between stimuli, or increasing the length of the sequence of stimuli. Using this
procedure, Tall~ and Piercy (1973a,b) demonstrated that performance ofSLI children
with non-verb~ auditory stimuli was cruci~ly dependent on timing. When stimuli
were of brief duration or when the interv~ between them was very short, performance
declined dramatic~ly. In contrast, no deficit was found when visu~ stimuli were
presented in an~ogous tasks. This study provided experiment~ support for Eisensons's
(1972) claim that "the aphasic child's basic perceptu~ impairment (is) one for auditory
perception for speech at the rate at which speed is normally presented" (p. 66, my it~ics).
Tall~ (1976) argued that the observed deficits were not simply a consequenocfe im mature
language skills, because the pattern of performance seen in SLI children did not
resemble that ofnorm~ children at any age. When the interv~ between stimuli was
long, SLI children performed better than younger control children, but they did
substanti~ly worse than this group when the interv~ was less than 305 msec (see
Fig. 1).
T~l~ and Stark (1981). conducted a larger investigation of 35 SLI children aged
from 5 to 9 years, selected by stringent and objective criteria. In this study
discrimination of auditory and visu~ stimuli was explicitly compared. The auditory
stimuli included complex tones, as used by T~l~ and Piercy (1973a,b), as well
as synthesized speech sounds (ba and da). The visu~ stimuli were letter-like
forms.
The results agreed in many respects with those from the British sample, but there
were ~so points of difference. As in the earlier studies, there were striking differences
between SLI and control children in their ability to discriminate tone pairs when
a variable inter-stimulus interv~ was used (T~l~, Stark, K~lman & Mellits, 1981).

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Fig. 1. Relationship between inter-stimulus interval and perception of brief tone pairs by SLI children
compared to younger normal controls (based on Tallal, 1976).
The SLI group was also significantly impaired in ability to process sequences of more
than two stimuli, even when the interval between stimuli was relatively long.
There were, however, some interesting differences from previous work. Some SLI
children performed poorly with auditory stimuli that were neither transient nor rapid.
Two children failed to reach criterion on a task that sim~y required them to learn
to associate a different response with each tone. A further seven children performed
at chance level with auditory sequences of two tones even when there was a long interval
between stimuli.
Another striking difference from earlier work was that the SLI group was deficient
relative to controls with visual as well as auditory stimuli on tests of sequencing, rate
processing and serial memory. Post hoc analysis indicated that this finding was a function
of the age of the subjects: the younger SLI children were impaired in visual and
auditory modalities, whereas the older ones were impaired with the auditory stimuli
only.
~his age effect could reflect sampling factors, but it could also indicate that the
profile of impairment changes as the child grows older. To investigate this possibility,
Bernstein and Stark (1985) traced 29 children from the original Stark and Tallal (1981)
study and retested then on perception of synthetic ba - da contrasts 4 years after the
initial testing, at which time 23 children still met criteria for SLI. Overall, there was
a substantial improvement in children's performance. When first tested, 19 of the
SLI children had failed to discriminate reliably between these synthetic speech sounds,
whereas on retest 24 of them reached criterion, and there was no overall difference
between the SLI group and controls. When presented with sequences of stimuli at
a fixed inter-stimulus interval, performance of SLI children was variable. Many of

D. V. M. Bishop
them did very well, but a few made many errors. All those who did pass this subtest
were able to reach criterion on a rate processing subtest that included trials with brief
inter-stimulus intervals. Furthermore, most of the SLI and control children could
repeat sequences of four or five items on a serial memory subtest.
Bernstein and Stark (1985) noted: "Considering time 2 results alone, we could
not conclude that specific language impairment in older children is caused by perceptual
deficits in rapid rate processing of phonemes" (p. 28). However, there are two points
to note when interpreting these results. First, performance was near ceiling levels
on retest, so it could be the case that SLI children still had an auditory perceptual
deficit, but the tests were too insensitive to detect this. The second point is that it
would be erroneous to conclude that the language disorder was not caused by perceptual
impairments just because no perceptual impairment was evident at time 2. As Bernstein
and Stark concluded: "Language disabilities may result when inadequate processing
of sensory information occurs during early childhood although the original processing
deficit may no longer exist" (p. 28).
Auditory deficit as the explanationfo r phonologicapl roblemsi n SLI
A crucial question is whether the perceptual deficit described by Tallal and colleagues
can account for the types of phonological problems observed in SLI children. One
approach to studying the relationship between auditory perception and phonological
impairment is to consider whether phonological errors are made on the sounds the
child has difficulty discriminating. On the basis offmdings with non-verbal tone stimuli,
Tallal and colleagues made predictions about the types of speech sounds that SLI
children should find difficult to discriminate and produce. These were broadly
confirmed (Tallal & Piercy, 1974; Tallal, Stark & Curtiss, 1976; Tallal & Stark, 1981),
with poor discrimination found for stop consonants, such as ba and ga and relatively
good performance with vowels, which are characterized by steady-state rather than
transient acoustic features. Tallal and Piercy (1975) went on to show that the advantage
for vowels over consonants could be reversed by altering temporal characteristics of
synthetic speech stimuli, i.e. stretching formant transitions of consonants or truncating
vowels. However, it was found that the perceptual deficit of SLI children was not
limited to discrimination of temporal cues. They also did poorly when asked to
discriminate sa and sha, which differ on spectral cues.
Another way of investigating the relationship between auditory perception and
phonological production is to consider whether individual differences in phonological
status are related to auditory perceptual processing. Frumkin and Rapin (1980)
subdivided a group of SLI children according to whether they had any phonological
impairment at the time of testing. Children with phonological disorders showed the
characteristic deficit described by Tallal and Piercy. They had difficulty in
discriminating between synthetic ba and da, but their performance improved when
the duration of the formant transition was increased from 40 to 80 msec. In contrast,
SLI children with normal phonology were unimpaired on this task, but they were
poor at discriminating between brief vowel sounds, and had difficulty in reporting
the order of pairs of consonants presented in quick succession. This intriguing study
suggested that auditory perceptual problems may underlie several distinct varieties~

of SLI, but impairment in processing rapid transitional information is the deficit that
is most clearly linked to phonological competence.
Auditory deficit as the explanationfo r phonologicali mpairmentsi n milder disorders
Much of the research relating phonological impairment to auditory perception has
been conducted on children diagnosed as having" phonological disorders", who do
not have the broader range of linguistic impairments that characterize SLI. As noted
above, however,. a case can be made for treating such disorders as on a continuum
with SLI, representing a mild form of the disorder. If this characterization is correct,
we should expect to find similar underlying causes, and it is interesting therefore to
consider how far an auditory deficit theory can account for phonological difficulties
in children with no other obvious language problems.
A substantial body of work has been concerned with the question of whether
expressive phonological problems can be explained in terms of deficits in auditory
discrimination of speech sounds, but results from such studies have been contradictory,
with some researchers reporting auditory discrimination deficits and others failing
to confirm these. To some extent these discrepancies may reflect population differences.
Some studies concentrated on children whose problems appear to have been restricted
to phonetic distortions, whereas others included children with limited or abnormal
phonological systems. Some investigations excluded children with more widespread
language difficulties whilst others did not. Stark and Tallal (1988) did include a
subgroup of children with isolated articulation disorders in their study, but they
reported that these children were unimpaired on tests of rapid auditory processing,
unlike the SI..I subgroup.
Locke (1980) and Seymour, Baran and Peaper (1981) pointed out that another
reason for discrepant results lay in the unsatisfactory procedures used to test auditory
discrimination. Most tests used in research settings adopted one of two paradigms.
The first involves presenting the child with words that constitute" minimal pairs",
i.e. they differ by one phoneme, e.g. goat - coat. The child is shown a set of pictures
and has to select the one whose name is spoken. A major disadvantage of this approach
is that it is difficult to find a vocabulary of minimal pairs of words that are pictureable
and familiar to young children. This limits the phoneme contrasts that can be assessed,
and many tests adopting this format include many contrasts that are seldom confused
in children's speech.
An alternative approach is to use non-words, in a same - different paradigm. The
child hears a minimal pair, such as gub - guv and must say if they are the same or
different. However, this type of task is often impracticable with young children, who
may quickly tire attending to pairs of meaningless verbal stimuli. Given that there
is a 50% chance of giving a correct answer by guessing, it is necessary to use a long
sequence of items to get a sensitive index of performance.
Locke (1980) proposed a novel approach in which the child is shown a picture with
a familiar name (e.g. dog) and has merely to judge whether or not the tester says
the name correctly (e.g. "is this a gog?"). This procedure overcomes the major
problems of other test methods, and also has the advantage that one can design a
test individually for a child so as to assess whether contrasts that are not distinguished
in the child's speech are also misperceived. Bird and Bishop (1991) included a

D. V. M. Bishop
test based on Locke's procedure in a study of 14 children with phonological disorders
but normal receptive vocabulary. They found that this group performed significantly
less well than control children, individually matched on age and non-verbal ability.
However, there was wide individual variation, and several children had no difficulty
in discriminating phonological contrasts that they did not produce distinctively in
their speech. There are two possible ways of viewing this result. One is to conclude
that children with phonological impairments are heterogenous, with some having
perceptual deficits and others not. If this is so, one would expect to find other
characteristics correlated with presence of perceptual deficit, e.g. the type of
phonological problem or its prognosis. An alternative interpretation is suggested by
the longitudinal study by Bernstein and Stark (1985) described above. They found
that many SLI children who demonstrated perceptual impairments when young no
longer did so when seen a few years later. It may be that a perceptual impairment
early in life is sufficient to cause a phonological disorder, which persists even after
the perceptual impairment has resolved. Thus to demonstrate a link between expressive
phonological problems and perceptual impairment it may be necessary to study children
at an early age.
Bird and Bishop (1991) found that, whereas some of their subjects did have
difficulty discriminating between phonemes, many more of them had a different type
of problem, namely a difficulty in perceiving phoneme constancy across different word
contexts. Consider the sound Isl when saying "soup" or "see". In "soup" the lips
are rounded, whereas in "see" they are spread, in each case anticipating the following
vowel. There are both acoustic and articulatory differences between these sounds,
yet in English they are classified as exemplars of the same phoneme. To test ability
to perceive such constancy a task was devised in which children were shown a puppet
who liked things that began with the first sound of his name, "Sam". They were
then required to judge, e.g., whether the puppet liked the "sock" or the "ball".
The contrasting sounds (in this case, Ibl and Is/) were selected to be perceptually
distinctive, avoiding sounds that children had difficulty in producing contrastively.
In another task, children were given training in rhyme generation and then asked
to produce rhyming words (e.g. "tell me something that rhymes with cat"). These
tasks revealed substantial deficits in children with phonological problems. They could
not judge that the initial sound of Sam was the same as the initial sound in sock, and
hence match Sam with sock rather than ball, despite the substantial differences between
the sounds Isl and Ib/. Also, they were very poor at rhyme generation. Such results
are hard to explain in terms of poor discrimination. If the child had difficulty in
distinguishing two phonemes, then certain classes of sounds would be collapsed
together. For instance, if the child treated It I and Ik/ as instances of the same sound,
we might then expect that when asked for a rhyme for cat, the response "back" would
be given. However, this was not the type of error that was observed. When asked
to generate rhymes, the commonest type of response seen in phonologically impaired
children was for the child to give a semantic associate (e.g. replying "dog" or
"hamster" to "cat"). These children seemed to have no idea of what was required
of them, despite repeated demonstration. Bird and Bishop (1991) explained these
findings by arguing that children with phonological problems failed to segment words
into phonemes. jusczyk (1986) proposed that young children in the early stages of

language acquisition do not operate at the level of the phoneme. Rather they make
distinctions between larger, un-analysed chunks of sound. Only gradually do they
learn to recognize common elements in the words that they have mastered, and this
helps them to organize their growing vocabulary and to imitate new words that they
hear on the basis of production rules for particular phonemes. Bird and Bishop (1991)
proposed that children with phonological problems continue to analyse speech
in this immature manner, learning each new word as an entire unsegmented pattern.
Consequently, they do not appreciate that words are composed of a small number
of building blocks, and their language learning is inefficient and protracted.
It remains open to question whether this failure to learn phonological principles
is a consequence of auditory perceptual limitations. This could arise, perhaps, if slow
and inefficient processing of auditory signals led to masking of later phonemes by
earlier ones. What is clear is that the problems go beyond failing to discriminate
between similar sounds: rather there is a failure to identify the basic units necessary
for efficient perception and storage of the sounds of words.

18 D. V. M. Bishop
and Piercy (1974, 1975), with adequate discrimination of steady-state vowels but very
poor performance with consonants. When the formant transitions of consonants were
extended, performance of these two children improved. The study was of interest
in demonstrating that children with severe comprehension problems do not appear
to be qualitatively different from those with milder forms of SLI: rather they have
the same types of auditory perceptual impairment, but in more pronounced form.
If, as Rapin et at.'s (1977) diagnostic label implies, the impairment in these children
is restricted to the processing of speech sounds, one should find normal perception
of non-verbal auditory stimuli. The tone stimuli used by Tallal and colleagues have
not been systematically applied to children diagnosed as having verbal auditory agnosia.
In most cases the only evidence against a more general auditory deficit is that perception
of environmental sounds is unimpaired. This, however, is not very satisfactory. Often
the child is offered choices between environmental sounds that differ substantially
in acoustic characteristics and performance is near ceiling level. It may prove more
accurate to speak of a general" auditory agnosia" in these children, rather than a
specifically verbal impairment.
What if one bypasses the postulated auditory perceptual deficit and presents children
with visual forms of language? If an auditory processing problem were responsible
for comprehension failure, then it seems reasonable to predict that one should observe
much better understanding for written or signed language, provided the child had
had adequate opportunity to learn these. Denes, Balliello, Volterra and Pellegrini
(1986) presented a detailed case study of a child who had little speech and virtually
no understanding of spoken language, but could communicate adequately through
reading and writing. However, he did have a tendency to make grammatical errors
in written language. Grammatical deficits in comprehending visual language were
also found by Bishop (1982), who compared children with severe receptive language
disorders withYQunger normal children. Several of the children studied by Bishop
had been taught using an artificial sign system that mapped directly on to English,
so that one could sign any English sentence, complete with inflectional endings, and
so directly compare comprehension across modalities for similar materials. The results
were clear-cut: modality of sentence presentation had no effect on performance of
a group of children with receptive language disorders, whereas grammatical complexity
did. Thus, most children misinterpreted reversible passive sentences, regardless of
whether these were spoken, written or signed. At first glance, this looks like strong
evidence against an explanation in terms of auditory processing deficit. However, in
a study which will be described in greater detail below, Bishop (1982) demonstrated
that congenitally deaf children performed just like the SLI children, demonstrating
major problems in comprehending complex sentences, regardless of whether these
were spoken (and hence perceived by lip-reading), written or signed. This study
demonstrated that it is quite wrong to assume that an auditory deficit will affect
comprehension only in the auditory modality.
Most children with SLI do not have the severe comprehension problems described
in children with' 'verbal auditory agnosia", but nevertheless their understanding is
typically below age level. Several studies have investigated the nature of their difficulties
and found that SLI children do have distinctive problems in comprehending
grammatically complex sentences. Bishop (1979) found that SLI children had major

difficulties with reversible passive sentences, although they performed better with
reversible active sentences, whereas Van der Lely and Harris (1990), studying slightly
younger children selected on the basis of poor understanding, found poor performance
by SLI subjects on all items where word order was used to express thematic relations.
Leonard (1989) suggested that such impairments are consistent with an interpretation
in terms of perceptual deficit. To perceive the contrasts between' 'the man chases
the dog" and' 'the man is chased by the dog", the child must detect the -ed ending
on the verb and the preposition by, neither of which is strongly stressed. If the child
perceives both sentences as "man chase dog", then there will be problems in learning
how grammatical variants in word order alter meaning. * Note that one does not need
to maintain that the child never perceives the weakly stressed morphemes; even if the
child sometimes processes them, learning will be impeded because the relationship
between syntax and meaning becomes opaque.
Auditory deficit as the explanationfo r expressivger ammaticalp roblems
It is relatively straightforward to predict that an auditory perceptual impairment
will result in problems in comprehension, but it is less clear how the structure of
expressive language might be affected. Leonard et al. (1987) noted that the grammatical
errors made by SLI children tended to be concentrated on verb inflections, auxiliaries
and the copula (i.e. that part of the verb to be that connects subjects to predicate,
as in "he is rich"). Several explanations may be offered for this finding.
(i) The grammatical problems might be a consequence of expressive phonological
limitations. Evidence against this explanation has been reviewed above when
considering the "output disorder" hypothesis.
(ii) SLI children might be regarded as normal language learners confronted with
a systematically altered input, i.e. an auditory perceptual deficit.
(iii) There might be impairment of an innate module specialized for learning
grammar.
Leonard et al. (1987) and Leonard, Sabbadini, Volterra and Leonard (1988)
attempted to dissociate predictions made from these different hypotheses by comparing
expressive difficulties in two different languages, English and Italian. Eight English-speaking
children were contrasted with eight Italian-speaking children, all diagnosed
as cases of SLI. The English children all used word final Isl, Izl, It I and Id/ in singular
nouns such as bus and bed, so any failure to produce plural or past tense morphemes
could not be attributed to difficulty in producing these phonemes. The two groups
of children were matched in terms of mean length of utterance in words. Samples
of their language were collected, using pictures to elicit examples of grammatical forms
of interest. As can be seen from Table 1, Italian children did not show striking
differences between regular noun plurals and third person singular verb inflections,
both were produced in obligatory contexts at a much higher rate than was observed
in English children. While there was no overall difference between Italian and English
children in the production of articles, the Italian children used the feminine forms
-However, recent unpublished data by Vander Lely (1990) suggest that SLI children can discriminate
different grammatical forms. Her subjects performed consistently correctly on truncated passives, such
as "the man is chased".

20 D. V.M. Bishop
Table 1. Use of grammatical morphemes in obligatory contexts by English and Italian SLI children
book/books
ball/balls
dog/dogs
key/keys
libro/libri
palla/palle
cane/cani
chiave/chiavi
mean = 94.4%
S.D. = 9.8
range 71 - 100
mean = 79.3%
S.D = 27.1
range 18 - 100
Third person
singular
to buy/buys
to sell/sells
to open/opens
comprare/compra
vendere/vende
aprire/apre
mean = 7.9%
S.D. = 17.6
range 0 - 50
mean = 92.0%
S.D. = 11.3
range 68 - 100
Uncontracted
copula
the book is red il libro e' rosso mean = 51.9%
S.D. =27.0
range 0 - 78
mean = 76.5%
S.D. = 16.5
range 51 - 100
Contracted
copula
the book's red mean = 26.0%
S.D. = 20
range 0 - 53
mean = 6.8%
S.D. = 10.3
Range 0 - 26
mean 64.5%
S.D. = 36.8
range 16 - 100
provare/provato
vestire/vestito
to try/tried
to dress/dressed
Regular past
tense, not
involving
number/gender
agreement
he returned
she returned
mean = 61.8%
S.D. = 30.5
range 20 - 100
Regular past
tense involving
number/gender
agreement
lui eo tornato
lei eo tornata
Third person
plural
to see/they see vedere/vedono mean = 31.8%
S.D. = 34.3
range 0 - 75
Articles ending
in vowels
the, a la, una, i, Ie median = 55%
range 12 - 64
median = 74%
range 15 - 100
Articles ending
in consonants
median =7%
range 0 - 50
iI, un
of the definite and indefinite articles, la and una, significantly more often than the
corresponding masculine forms, il and un. Given that the number of obligatory contexts
for il and un was as high for la and una, it is difficult to explain this difference except
in terms of phonological structure: i.e. whether or not the article ends in a vowel.
Most errors with articles involved omission of the article rather than substitution of
an alternative. Finally, Leonard et al. (1987) also noted that Italian SLI children
correctly marked gender agreement of possessive pronouns and adjectives in nearly
all instances where this was required.

Specific language impairment
Leonard (1989) argued that many of the errors made by SLI children could be
accounted for by assuming that perceptual limitations impair the learning of specific
features. These include those represented by non-syllabic consonant segments (e.g.
in English, plural, possessive or third person singular -s, past -ed and contracted forms
of to be) and those represented by unstressed syllables (e.g. a, the, infinitive particle
to, complementizer that). On this interpretation, Italian SLI children find it easier
to learn verb inflections because most of these consist of stressed syllabic affiXes. Their
ability to handle gender agreement is attributed to the fact that the relevant affixes
have a clear relationship to one another; in most cases the final vowels of adjectives
match those of the nouns they modify.
One might ask why a perceptual problem should have such a selective effect on
morphological development. If children have problems learning that -ed marks the
past tense of a verb (e. g. played), why don't they have similar difficulty in perceiving
the final consonant in other contexts, e.g. in single words such as raid? Leonard argued
that, first, there is some evidence that SLI children do indeed have difficulty in
perceiving unstressed final consonants in single words, and second that when learning
the significance of the ed morpheme children must not only perceive /d/, they must
also relate played to play, hypothesize that /d/ is a morpheme, and work out its
grammatical function and semantic correlates.
Although a perceptual deficit explanation can account for many of the results, some
puzzles remain. English children were much more likely to produce the regular noun
plural -s than they were to produce third person singular -so This indicates that factors
other than perceptual salience are important. One possibility is that the semantic
correlate of the morpheme is easier to deduce for the plural than the third person.
However, if this were the explanation, one would expect a similar difference between
these two types of morpheme in Italian children, and this was not found. Parameter
setting theory, considered in more detail below, provides some clues as to how such
differences may be explained.
Perceptual impairment or memory impairment?
It was noted above that most children with SLI do poorly on tests of auditory-verbal
short-term memory, such as the digit span subtest of the Wechsler Intelligence Scale
for Children (Wechsler, 1974). Tallal and Piercy (1937b) included tests of memory
using non-verbal tone stimuli in their assessment of auditory processing by SLI
children. They found that, even where auditory stimuli were adequately discriminated,
SLI children frequently had difficulty in retaining sequences of more than three or
four items. One way of accounting for this result in terms of their theory is to suppose
that while perceptual processing is adequate to discriminate between pairs of speech
sounds, it is slow and inefficient. If the child were still processing stimulus 1 when
stimulus 2 arrived then stimulus 2 would be inadequately processed, and hence poorly
remembered. The critical question that remains to be answered is whether SLI children
have memory deficits over and above those that would be expected on the basis of
their perceptual impairment.
One way in which a perceptual deficit might affect memory processing would be
by influencing the way in which memorized material was encoded. Kirchner and
Klatzky (1985) carried out a study of free recall by SLI children that supported the

22 D. V. M. Bishop
idea that they encode material in terms of meaning rather than phonological
characteristics. The children were presented with pictorial stimuli to remember and
were explicitly instructed to rehearse the picture names aloud. Their rehearsal and
overall recall was less efficient than that of age-matched control children. Most
strikingly, and unlike control children, the SLI children made many intrusion errors
that involved giving the name of a semantic associate of an item from the list. This
would be consistent with the view that the pictures were remembered in terms of
meaning rather than sound. More recently, however, Gathercole and Baddeley (1990)
conducted a study that indicated that SLI children can and do encode words in
phonological form. Their subjects showed the normal sensitivity to phonological
characteristics of memorized materials, with poorer recall of similar word sets such
as bat, cap, cat than of distinctive sets such as bus, clock, hand. They were also poorer
at remembering pictures with long names (e.g. banana, elephant, ladybird) than those
with short names (e.g. boat,c at, egg),ju st like control children. This observation provides
evidence that children did rehearse the names of pictures rather than just forming
a semantic or pictorial representation of materials. However, the authors noted that
effects of phonological similarity were not found for lists of more than four items.
They suggested SLI children might switch to an alternative encoding strategy when
lists exceed their storage capacity. This would be consistent with the findings of
Kirchner and Klatzky (1985), who used a free recall procedure with lists of 12
items.
Gathercole and Baddeley (1990) challenged the view that memory limitations in
SLI children are a secondary consequence of auditory perceptual deficits. They found
that SLI children were extremely poor at repeating non-words. If this reflected a
perceptual problem, one might expect poor performance even on monosyllables, but
Gathercole and Baddeley showed that performance depended on the number of
syllables in the non-word, with much greater impairment on polysyllabic items. The
repetition scores of SLI children fell below even those of younger children matched
on receptive vocabulary. Similar results were reported by Kamhi and Catts (1986)
and Kamhi et al. (1988), who noted that SLI children were impaired at repeating
polysyllabic non-words when compared to reading retarded children as well as control
children. (On several other tasks that they used, reading retarded and SLI children
showed similar levels of impairment.) Another piece of evidence against an explanation
in terms of perceptual deficit was that the SLI children studied by Gathercole and
Baddeley (1990) were unimpaired on a phoneme discrimination task that involved
making same - different judgements about pairs of words and non-words. However,
this evidence is not conclusive, because performance of all groups was near ceiling
and the sound contrasts they tested were not chosen to reflect the types of phonological
errors made in the memory task. Gathercole and Baddeley suggested that the memory
difficulties they observed might arise because of limitations in storage capacity, leading
to fewer items being stored, or because phonological representations in memory are
less richly specified, leading to a less adequate memory trace. Although they did not
think that problems in distinguishing individual phonemes were responsible, they
did suggest that impairment of phonemic segmentation might be a factor that led
to degraded phonological representations in memory. This interpretation would mesh
well with the findings of Bird and Bishop (1991), discussed above (p. 14).

Regardless of whether or not the non ..word repetition deficit is explicable in terms
of a more basic perceptual deficit, it does have implications for vocabulary acquisition
in SLI children. The processes involved in repeating a non-word resemble those
implicated in forming a lexical representation of a new vocabulary item, and Gathercole
and Baddeley (1989) confirmed that in normal children, ability to repeat non-words
is strongly predictive of vocabulary growth. Limited vocabulary is common in SLI
(Haynes & Naidoo, 1991), and Gathercole and Baddeley's analysis provides a simple
explanation for this in terms of difficulty in retaining unfamiliar phonological strings.
A stringent test of this hypothesis would involve teaching SLI children novel words
varying in length. The prediction is that they should be disproportionately poor at
learning polysyllabic words.
Evaluation: the auditory deficit theory of SLI
The view of SLI as the consequence of auditory perceptual impairments has stood
the test of time remarkably well. The work ofTallal and colleagues has demonstrated
that these children do have problems in discriminating non-verbal auditory stimuli
when these are brief or rapid. It has sometimes been argued that the auditory deficit
hypothesis cannot account for the specific types of grammatical impairment seen in
many SLI children. For instance, Leonard (1979) concluded a review of the area
by stating that' 'The very nature of the restricted speech used by language impaired
children seems to suggest that auditory processing deficits may be a corollary to, rather
than a cause of language difficulties" (p. 227). However, more recent work by Leonard
and colleagues has revised that opinion and studies of receptive language function
by Bishop (1982) illustrate the dangers of assuming that one can specify precisely
how auditory impairment will affect grammatical development. The problems for
the future are not so much to test whether auditory limitations can hinder language
acquisition - they undoubtedly can and do - but rather to consider how wide a
range of the linguistic and non-verbal deficits seen in SLI children can be attributed
to this cause. One crucial question is whether the observed memory deficits are
secondary consequences of perceptual abnormalities or whether there is a primary
impairment in the memory system itself.
3. Linguistic Interpretations of SLI
One way of studying normal language acquisition is to attempt to simulate the
process by computer, but to date all attempts to do so have foundered. No-one has
succeeded in formulating a learning algorithm that will derive the grammar of any
language when provided with input from that language. This has been termed the
"learnability" problem and has led to the conclusion that humans must come to the
language learning task with some innate system specialized for grammatical processing.
The problem is then to discover what type of innate knowledge would constrain
learning to make it possible to master a grammar, while at the same time being flexible
enough to allow one to learn anyone of the diverse languages that might be
encountered. Given that grammatical difficulties are a hallmark of SLI, it may be
asked whether these children lack the postulated language-learning module and are

24 D. V. M. Bishop
therefore confronted with the same problem as the computers that are programmed
to extract grammatical regularities from language input.
In a review of the literature, Cromer (1978) noted that most theories of SLI
attributed the children's language problems to some underlying deficit in non-linguistic
processing, such as defective auditory perception, poor short-term memory, or
confusion in sequencing. Cromer argued that we had ignored the most obvious
explanation of SLI, which was that these children were unable to master those
grammatical relations that Chomsky (1965) had proposed were processed by an innate
"language acquisition device". Clearly, there is a danger of circular reasoning in
arguing that the child fails to learn language because of lack of a language acquisition
device, unless one can be more specific about what this device does, and hence predict
the nature of language errors that should occur. Recently, several accounts have been
proposed that make specific predictions about the types of language difficulty that
should be observed.
Failure to appreciate underlying hierarchical structure
Cromer (1978) noted that language learning involves extracting underlying
hierarchical structure from a temporal sequence. Consider the sentence: "The cheese
in the refrigerator is green". If language were interpreted by relating each word to
those nearest to it in the sentence, we would conclude that the refrigerator rather
than the cheese was green. Because we can utter only one word at a time, when
expressing complex relationships we need a grammar that allows us to keep track
of underlying relationships between sentence elements that are separated in the surface
structure. This involves appreciation of hierarchical structure, recognizing that some
components of the sentence are subordinate modifiers of other elements. If one lacked
the ability to extract hierarchical structure, this would have profound implications
for language learning. Some progress might be made by learning sentence frames
and sequential dependencies between word classes, but there would be severe difficulties
in understanding constructions where processes such as subordination were involved.
Cromer carried out two studies with a small group of SLI children with severe
comprehension problems ("receptive aphasics"). In both these studies he compared
children with receptive language disorders and deaf children. This is a potentially
powerful method, as the deaf children can indicate how far language limitations can
reasonably be attributed to auditory perceptual problems. However, these studies
were flawed by lack of comparability of deaf and SLI children on crucial variables.
In the first study, Cromer (1978) gathered samples of written language from children
with receptive language disorders and contrasted these with samples produced by
deaf children. He concluded that the written language of SLI children was characterized
by use of simple sentence patterns that could be interpreted in terms of sequential
dependencies. The deaf children attempted many more complex constructions,
although they frequently made errors with these. This exploratory study, was, however,
ambiguous. The differences between SLI and deaf children could have reflected
educational method rather than genuine differences in underlying disorder. The SLI
children came from a school where language was taught using a structured approach
through written sentences and they were explicitly trained to produce certain simple
sentence forms.

Setting aside for the moment such objections, it may be asked whether, if a
hierarchical planning deficit is implicated in SLI, this is a language-specific impairment
or a more general inability. In a subsequent study, Cromer (1983) investigated this
question using a task that involved extraction of hierarchical structure, but which
did not involve any language. Children were required to copy either two-dimensional
or three-dimensional patterns of the kind shown in Fig. 2. Previous work showed
Fig. 2. Two-dimensional stimulus for copying used in Cromer's (1983) study of hierarchical processing.
that normal young children use a chain strategy, starting their construction at one
side and working sequentially up the figure, across the middle and down the other
side (Greenfield & Schneider, 1977). Older children, however, usually start their
construction with the superordinate connecting level, and work down through the
hierarchy of levels. This involves interrupting the chain. Cromer scored children's
reproductions in terms of how far they followed the underlying hierarchical structure
of the pattern, and demonstrated that SLI children obtained low scores on a measure
of hierarchical organization compared to a control group of deaf children.
Unfortunately, in this study too the matching of SLI and deaf children was inadequate.
The deaf children came from a selective grammar school, entry to which was
determined by written examination, and their mean non-verbal IQ was 114. The
average non-verbal IQ of the SLI children was 99, and it seems probable that their
verbal skills were also lower than those of deaf children. The possibility cannot be
excluded that the differences in the model-copying task were simply a function of
language level and non-verbal ability.
Bishop (1982) noted that Cromer's theory predicted specific types of comprehension
problem in SLI children. Given a sentence such as "the book on the table is brown"
children should not make random sequencing errors, such as selecting a table on a
book rather than a book on a table, because they can appreciate sequential order.
They should, however, wrongly attribute the colour adjective to the nearest noun,
i.e. the table. Bishop devised a set of items designed to test this hypothesis, an example
of which is shown in Fig. 3.

26 D. V. M. Bishop
1 2
~

3
Fig. 3. Test item used by Bishop (1982) to assess comprehension of hierarchical relationships in complex
sentences. The child must select the item corresponding to "the circle on the star is black."
All had the structure "The X inion/under/behind the Y is Z", where X and Y
were nouns and Z a colour term. Understanding of all nouns and colours was first
established in a pretest. Test items were presented in three modalities: spoken, written
and signed using the Paget Gorman Sign System (a version of signed English). Results
with nine children with severe receptive language problems gave support to Cromer's
hypothesis that these children interpreted sentences sequentially without extracting
deeper hierarchical structure. Performance was very poor but errors were not random.
Instead, irrespective of modality of presentation, there was a tendency to attach the
adjective, Z, to the nearest noun, Y. Thus for the item in Fig. 3, they wo;lld select
picture 4 rather than the correct picture 2. This tendency was most marked when
the test was given using written presentation, where the child did not have to remember
the sentences.
However, when these same materials were given to profoundly deaf children, the
same pattern of performance was observed. Thus these results supported Cromer's
hypothesis as a descriptive account of the nature of the grammatical problems
experienced by children with receptive language disorders, but they challenged the
view that these problems arose because of a primary impairment affecting the language
acquisition device. In ;ddition, Bishop noted that studies of written language of
hearing-impaired children carried out in the U.S.A. yielded findings that were

strikingly similar to those obtained by Cromer (1983) with SLI children. Quigley,
Wilbur, Power, Montanelli and Steinkamp (1976) noted that deaf children tended
to impose a subject - verb - object pattern on sentences, and would connect the nearest
noun phrase and verb phrase, apparently treating English as if it had linear rather
than hierarchical structure. No-one would maintain that deaf children are born without
a language acquisition device: their language problems arise because the main input
to the language processor is visual rather than auditory. Bishop concluded that the
visual system is not well suited to processing temporal information, so that merely
presenting an auditory language in visual form will not overcome the language-learning
problems of those who cannot perceive aural input adequately.
In sum, Cromer's account of defective hierarchical processing in SLI makes accurate
predictions about the types of expressive and receptive problems these children
experience with grammatical structure. However, it is unlikely that inability to analyse
hierarchical structure is the primary problem. Rather, it seems that this difficulty
arises as a secondary consequence of auditory processing deficits that force children
to rely on the visual modality for language learning.
Semantic relations and the acquisition of argument structure
Recent theories of language acquisition have focused on the ways in which meaning
relationships are encoded in grammatical structure. When we produce a sentence,
we are describing relationships between properties, things, places and actions. The
verb is vital for expressing such relationships. Verb definitions may be regarded as
composed of a small set of basic semantic categories (thing, event, state, place, path,
property and manner) that form a scaffolding of grammatically relevant meaning
to which are added specific pieces of conceptual information (Pinker, 1989).
Note that a verb does not simply specify a particular type of action. It also entails the
existence of other sentence elements. Thus, the verb to fall entails that there is a noUn
acting in the role of theme, which is the grammatical subject of the verb. Other verbs
have more complex entailments. Consider the following examples.
Acceptable
John fell.
John smashed the egg.
John put the egg on the table.
Unacceptable
John smashed.
John put.
John fell the egg.
John put the egg.
John put on the table
These examples illustrate the different argument structures of the verbs. Fall (an
intransitive verb) has only a theme expressed as a subject, smash (a transitive verb)
requires an agent and a theme, expressed grammatically as subject and object. Put
entails an agent, theme and location, expressed grammatically as subject, object and
oblique object.
Knowledge of verb argument structure is crucial for grammatical language
production. Suppose the child relied solely on abstract rules, such as one stating that
subject-verb-object is a grammatical string. Such a rule would generate sentences

28 D. V. M. Bishop
such asJohnfell the egg orJohnput the egg. To be grammatical, a sentence must contain
the obligatory arguments specified by the verb's argument structure.
It follows that if the child has not learned adequate verb definitions, then
ungrammatical utterances will result. According to Pinker (1989), many
overgeneralization errors produced by normal young children arise for precisely this
reason. Utterances such as he get died, don't say me that are not the consequence of failure
to learn grammatical rules, but errors in learning verb semantics. Of course, knowledge
of verb semantics is not sufficient for grammatical language, it is necessary also to
map argument structure on to grammatical functions. For instance, there is a rule
mapping agents on to subjects and patients on to objects. According to Pinker (1989),
normal children do not have difficulty learning these linking rules, which he suggests
are innate.
Pinker's theory has barely been applied to the study of normal language acquisition,
let alone to children with disordered development. Nevertheless, it generates some
interesting predictions. Suppose that SLI children did indeed lack the innate component
of grammar: they might then have no knowledge of linking rules, but in other respects
be cognitively normal. On this view the child with SLI might have perfectly adequate
semantic representations of verbs, but be unable to work out how to express the
underlying argument structure grammatically.
This hypothesis is of interest because it predicts that the grammatical errors observed
in SLI children should differ from those seen in normal development. According to
Pinker, underspecified representations of verb meaning are the main cause of
overgeneralization errors in normal young children, and the progression to adult
sentence constructions develops as verb meanings become more finely tuned.
One way of evaluating this hypothesis is to compare different ways that children
learn word meanings. Pinker proposed that the child first deduces information about
argument structure from perceptual and conceptual analysis of the context in which
the word is used. Suppose the adult produces a bear who is punching the air and
moving forward, and tells the child "this iskaboozling". A probable deduction would
be that kaboozlew as an intransitive verb describing the action the bear performed.
Contrast this with the situation where the bear performs an identical action, but this
time in doing so hits a giraffe and makes it move forward. Here the natural deduction
would be that kaboozlew as a transitive verb describing the action of an agent (bear)
on a patient (giraffe). Pinker argues that by using contextual information to deduce
the meaning of words, the child can take the first steps to acquiring grammar. This
is termed semantic bootstrapping. * If SLI children are unimpaired in their ability
to learn verb meanings and argument structure from context, then we would predict
that they should perform normally on tasks designed to test semantic bootstrapping.
There is, however, another route to language learning, Landau and Glietman (1985)
noted that if the visual context was an important cue to meaning and structure, then
congenitally blind children should have major difficulties in language acquisition.
They reported a case study demonstrating this need not be so. To explain how a
.The tenD "bootstrapping" is borrowed from computer science, which likened the process of starting
a system from scratch to the operation of pulling oneself up by one's bootstraps.

blind child can learn to use verbs like "look" and "see" they proposed that, once
some grammatical knowledge is available, a child may perform a syntactic analysis
on an input sentence containing an unfamiliar word, and deduce the meaning of the
word from its syntactic characteristics. For instance, if an adult describing a cartoon
to a blind child says: "Tom really walloped Jerry", the blind child can deduce that
the word wallop refers to an action in which Tom was agent and Jerry was patient.
In effect, the child uses linking rules in reverse, to work out argument structure on
the basis of syntactic structure, i.e. syntactic bootstrapping. If SLI children have
difficulty in using linking rules, then they should find it hard to learn language this way.
An early study relevant to this issue was carried out by Leonard et al. (1982) who
contrasted vocabulary learning in 14 SLI children and 14 normal control children.
These children were exposed to novel words depicting objects or actions in a play
session. For instance, the adult would say "Here's the gourd" (object word) or "Watch
the baby kneel" (action word). Each word was produced five times in each of 10
sessions, and the child's comprehension was tested at the end of each session with
the command "give me the. . . " or "make the baby. . . ". Comprehension and
production of the words was then tested in the final session. Leonard et al. (1982)
were surprised to find that, overall, there were close similarities between SLI children
and control children in comprehension and production of the experimental words.
We know that SLI children do have weak vocabularies, and it had been anticipated
that this study would reveal that they were slow at learning new words. Leonard (1989),
however, pointed out that an analysis in terms of bootstrapping processes could clarify
these findings. He argued that the apparent ease of learning new words could be
a consequence of the fact that the new words were presented in an inflectionally bare
context, e.g. "Here's the gourd"; "Watch the baby kneel", and thus the main factor
determining learning was the child's ability to perform a conceptual analysis of the
object or action from which a semantic representation could be formed, i.e. semantic
bootstrapping. However, he suggested that in other situations where novel words
are presented in a range of grammatical contexts, syntactic bootstrapping assumes
importance and slower lexical acquisition would be anticipated.
Van der Lely (1990) carried out a study designed to compare semantic and syntactic
bootstrapping processes. Six children with SLI were compared with 17 younger
children who were matched on "language age". In the semantic bootstrapping task,
the child was shown toys performing novel actions, accompanied by novel words,
e.g. toy A jumps up and down on the back of toy B and the experimenter says "this
is voozing". The child's ability to infer grammatical relations was then tested by
asking him or her to (i) describe the behaviour of new toys carrying out the same
actions; and (ii) act out sentences such as "the horse voozes the lion" or "the lion
is voozed by the horse". In contrast, in the syntactic bootstrapping task no semantic
information or contextual cues were provided. The child was simply asked to make
up a meaning for a new word, and to show this to the experimenter by acting out
what he or she thought was meant by sentences such as "the lorry yols the car".
Responses were scored in terms of the semantic relationships between the toys. In
the example given, a child who appreciated the way in which syntactic structure usually
encodes thematic roles should make the lorry perform some action on the car, i.e.
lorry is agent and car is patient. Van der Lely (1990) found that SLI children did

30 D. V. M. Bishop
not differ from language-matched controls on the semantic bootstrapping task, but
they were significantly impaired on the syntactic bootstrapping task.
These results indicate that SLI children could form a semantic representation of
verb argument structure on the basis of contextual information. However, they seemed
unable to deduce thematic roles from syntactic functions. It is possible that they lacked
linking rules altogether, and succeeded on the semantic bootstrapping task simply
by using response strategies based on their knowledge of thematic roles. Van der
Lely (1990) favoured an alternative explanation, which was that SLI children could
use linking rules, but only in one direction: linking from a cognitive semantic
representation to a grammatical structure, but not vice versa. On either view, the
fundamental deficit in SLI is grammatical rather than semantic. However, this primary
grammatical deficit will lead to semantic deficits, because the child is unable to use
syntactic bootstrapping to deduce word meanings.
As with the work conducted by Cromer (1978), it cannot be assumed that, because
SLI children behave as if they are impaired in using linking rules, there is an innate
deficiency in a central language module. It is possible that inadequate auditory
perception could lead them to misperceive crucial grammatical features. An explanation
in terms of primary auditory deficit would be strengthened if similar deficiencies on
bootstrapping tasks were found in children with hearing loss.
"Feature blindness"
Current linguistic theories have proposed that marking of grammatical features
on lexical items is an independent component of grammar. Syntactico-semantic
features, which mark information such as number, gender and animacy, have two
main effects on surface structure: they influence the morphological form of the feature-marked
word, and they constrain the form of other items in the sentence. Thus in
English a plural noun is marked by a plural morpheme (/s/, Izl or hz/, depending
of the phonological form of the word). Plural marking on a noun constrains the form
of the verb of which it is subject, and the form of preceding determiners (some, or
two would be permissible preceding dogs but not dog, whereas a, one or this would be
allowable before dog but not dogs).
Clahsen (1989) studied grammatical errors produced by German children with SLI
and concluded that the grammatical problems of his subjects were confined to
morphology. Gender and number agreement in the noun phrase were often in error,
and subject - verb agreement caused great difficulty. Verbs were restricted lar,gely
to uninflected stem forms, infinitive forms and those suffiXed with -to However, some
kinds of verb morphology (e.g. rules for participles) were unimpaired. The children
were able to use word order to express thematic relations, but they were impaired
at using morphological case markers for accusative, genitive and dative. Clahsen
concluded that the German children he studied had some ability to use grammatical
morphology, but were selectively impaired in marking grammatical agreement.
Gopnik (1990a, b) and Gopnik & Crago (1991) also argued that the grammatical
impairment in SLI is confined to grammatical morphology, but they proposed a
more pervasive problem, which they termed "feature blindness", indicating a total
failure to master syntactico-semantic features. Gopnik and Crago (1991) reported striking
deficits shown by English-speaking individuals with SLI in using features such as

number, gender, animacy, mass/count distinction, proper names, tense and aspect.
Extracts from notebooks kept by some of the subjects illustrate their problems with
features such as number ("All the children got present"), aspect ("Carol is cry in
the church' '), and proper names ("A Patrick is naughty' '). It was noted that while
many utterances produced by SLI individuals may use such features in apparently
grammatical fashion, they do not apply the features systematically to mark contrasts
in meaning, and so may say "a boys", "a boy", "two boy" or "two boys". Rather
than concluding that they have a shaky knowledge of the adult grammatical system,
it was argued that it is more reasonable to say that their grammar lacks features. A
form such as final /s/ is regarded as a phonological variant with no associated meaning.
Thus grammatically correct utterances are generated by the same defective grammatical
processor as the incor;rect ones.
To investigate mastery of the plural feature, Gopnik and Crago (1991)
administered an experimental task in which subjects were shown a nonsense creature
and told, for example, "this is a zoop". They were then shown several such creatures
and asked: "These are. . . ?" It was found that subjects with SLI were poor at this
task and did not appear to have an internalized, unconscious set of rules for forming
plurals.
Another test assessed ability to use tense features. A sentence was presented in
one tense, and the subject then prompted to produce an analogous sentence in a
different tense. For example, the tester would say, "Every day the man walks eight
miles. Yesterday he. . . (walked eight miles)". The SLI individuals gave semantically
relevant answers but seldom made appropriate changes to verb tense. They also had
difficulty with a related test in which sentences were given to elicit similar forms with
different morphological endings, e.g. "There is a lot of sun. It is very. . . (sunny)".
In another test, subjects were asked to judge whether a sentence was grammatical,
and, if not, to correct it. The SLI subjects could judge which sentences were correct.
Gopnik and Crago pointed out that this is to be expected as their grammar does
generate such sentences. However, they were poor at detecting ungrammaticality
in sentences that had errors in feature-marking, and if they did detect the error they
had difficulty in correcting it.
Gopnik and Crago (1991) did not find any deficit in SLI subjects on comprehension
tests requiring understanding of such distinctions as reflexive/non-reflexive (" she
washes herself" vs "she washes her"), pronoun gender ("he holds him" vs "he holds
her"), passives and negatives ("the truck does not pull the car" vs "the truck is not
pulled by the car") and reversible possessives ("the mother's baby" vs "the baby's
mother"). They found that individuals whom they studied were able to produce some
complex sentences such as "I know how to play basketball", could detect the
ungrammaticality of sentences that did not represent obligatory verb argument
structure (e.g. "he puts"), and did not make such errors in their spontaneous speech.
It was therefore concluded that their grammatical impairment was specific to syntactico-semantic
features and did not affect mastery of thematic relations.
The feature-blindness hypothesis provides an interesting perspective on grammatical
problems in SLI, but the current evidence produced in its support is sparse, and may
be open to alternative interpretation. Gopnik and Crago (1991) themselves found

32 D. V. M. Bishop
several aspects of feature-marking to which SLI subjects did appear to be sensitive.
For instance, they performed correctly on a comprehension task that involved
responding to commands such as "Point to the book" vs "Point to the books". If
they regarded the plural inflection as a meaningless phonological variant, performance
should be at chance. This discrepant result was explained by suggesting that noun
plurals such as "books" were learned as complete lexical items referring to groups
of objects, so would have a separate lexical representation, rather than being generated
by applying a rule of adding a plural marker to "book". A similar explanation was
proposed for the fmding that some past tense forms were produced correctly, by arguing
that each one is learned as a new item, but the child does not generalize to new forms.
While this is an interesting possibility, it should be subjected to further testing. This
could be done by presenting SLI individuals with plural markers attached to novel
forms, e.g. by showing a nonsense animal and saying "this a zoop", and then
presenting a picture of a girl chasing a zoop and another of a girl chasing several
zoops, to see whether the subject could respond differentially to the instruction' 'the
girl chases the zoop" vs "the girl chases the zoops". Since the item "zoops" is novel
it cannot have a pre-existing lexical representation~ and so if correct performance
were achieved, this would provide evidence that SLI individuals do have some
awareness of the feature of number, even if they are poor at using it productively.
Gopnik and Crago (1991) make a bold claim that the deficit they have identified
indicates that a separate, innate component of the grammar is absent in these
individuals. If so, then the error pattern observed in SLI subjects should not be seen
in the course of normal development. This prediction needs to be tested by comparing
SLI subjects with younger normal children.
Another claim that requires more stringent testing is the assertion that the feature
problems shown by these subjects could not be explained in terms of an auditory
deficit of the kind proposed by Tallal and colleagues. Two lines of evidence have
been put forward to support this statement. First, Gopnik (1990a) noted that the deficits
are apparent in spontaneous speech, grammaticality judgements, writing and
repetition, and she concluded' 'because the deficits are apparent in all aspects of
language their roots probably lie in the underlying grammar rather than in a peripheral
processing system" (p. 715). The unsoundness of this argument is demonstrated by
Bishop's (1982) study, which found that peripheral hearing loss resulted in grammatical
deficits in comprehension of written and signed language. Second Gopnik and Crago
(1991) noted that SLI individuals do produce instances of phonological forms
involving rapid acoustic transitions (e.g. past tense Id/), but sometimes omit other
forms that are acoustically salient, ~.g. subject pronouns. However, as is apparent
from the studies of Leonard et at. (1987), a perceptual deficit theory can be adopted
without making the extreme prediction that children will omit all low phonetic
substance morphemes; instead it can be argued that contrasts signalled by such
morphemes will be especially difficult to learn, and this may have repercussions on
development of the entire grammatical system. It would be most interesting to see
how children with mild to moderate hearing loss would perform on Gopnik's tasks.
There are major discrepancies between the feature-blindness theory and other
explanations of grammatical impairments, not just at the theoretical level, but also
in terms of the empirical data cited in support. Gopnik (1990b) described a bilingual

Underlying nature of specific language impairment

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