normal EEG waveforms and characteristics

1. Normal EEG waveforms
Dr. Balaji B S

2. Scheme of presentation
• Introduction
• Normal waves
• EEG in sleep
• Quiz

3. Introduction
• Brain activity consists of rhythms named according to the frequency
• Rhythms designated with Greek letters
• Routine scalp EEG -frequency bandwidths between 1and 30Hz
• Faster frequencies-not seen in SEEG

4. Normal adult awake EEG
Clinically relevant frequency range
• Delta : Below 4/sec
• Theta : 4-7/sec
• Alpha :8-13/sec
• Beta : 14-30/sec
Background activity
• One dominant frequency (one that is most prominent or obvious in the record)
• Depends on the age of the subject and the state

5. Alpha rhythm
• 1st EEG pattern to be described by Berger in 1929
• Most prominent rhythm in normal awake record
• Frequency - 8-13 Hz
• Maximum over occipital region, shifts anteriorly during drowsiness
• Best seen in wakefulness with eye closed, physical relaxation &
relative mental inactivity
• Blocked/attenuated by attention, especially visual and mental effort

6. 41/F

8. Alpha rhythm
• Frequency is coupled to cerebral blood flow and falls when it is compromised
• Posterior dominant rhythm first appears around 3-4 months of age
• Alpha of 8Hz by 3 years in majority and 10Hz by 10 years
• Gradually falls in elderly, though not less than 8Hz
• 10% of adults have little or no alpha rhythm*
• Mostly due to genetic factors
• Blindness acquired early in life
Davis, Arch NeurPsych.1936

9. Alpha rhythm
Amplitude
• 15- 45 micro Volts in 75% of normal adults
• <15 micro Volts 6-7%
• Depends on inter electrode distance and the position of the electrodes in relation to the
potential field
• Faster alpha tends to be lower voltage
Reactivity
• Attenuate to opening of eyes
• Mental task like arithmetic problems
• More effectively by visual than other stimuli

10. 44/F

11. Alpha reactivity to eye opening

12. Alpha reactivity to mental arithmetic

13. Alpha rhythm
Symmetry
• Asymmetry in amplitude (<25%) is noted in around 60 % of the normal adults
• Higher amplitudes on right
• Dominant side has more mental activity that suppresses it
• Asymmetry should be assessed in both bipolar and referential montages
• Voltage asymmetry between sides
• >50% abnormal (if R>L)
• >35% abnormal (if L>R)
• Frequency asynchrony of 1Hz between sides abnormal

14. Alpha variants
• Rhythmic activity like alpha but with lower or higher frequencies
• Slow alpha variant – 3.5 to 6 Hz
• Fast alpha variant – 14 –20 Hz
• React to stimulation in the same way as ordinary alpha
• Alpha squeak: Just after eye closure, there is a brief period during which the posterior
dominant alpha rhythm may be higher in frequency and lower in amplitude

15. Alpha squeak

16. Alpha variants
• Temporal alpha
• Older people, independent alpha activity in temporal region
• Independent temporal alpha in breach rhythm-”Third Rhythm”
• Frontal alpha can occur with anaesthesia or as arousal response in children
• Generalised alpha with anterior dominance, without reactivity-Alpha coma

17. Alpha variants
Bancaud’s phenomenon
• Unilateral failure of alpha to attenuate with visual fixation/ mental alerting
The side lacking the blocking response is abnormal
Paradoxical alpha
• Drowsy individuals who awaken and open their eyes without immediate visual
fixation may have a paradoxical AR
(AR is absent with eyes closed and briefly present with eyes opened)
• This is most common in the context of sedation

18. Paradoxical alpha

19. Paradoxical alpha

20. Alpha and controversies
• Berger presumed cortical genesis of alpha with thalamic governance
• Various proponents
• Thalamic vs cortical origin
• Rhythm is most definitely cortical
• Although no evidence for synchronizing mechanism at cortical level found
• Enhanced rhythm in Yogis and Zen Buddhists
• May simply reflect the level of vigilance
• Alpha and intelligence*
• Gasser et al-individuals who are faster in retrieving information from memory
tend to have higher alpha frequency

21. Mu rhythm
• 1st described by Gastaut
• ‘Mu’: stands for ‘motor’
• Other names: Somatosensory alpha rhythm, Comb rhythm
• Ontogenetically occurs earlier than alpha in 1st year of life
• Somatosensory stimulation occurs before visual stimulation
• Mu rhythm is recorded over the sensorimotor cortex at rest
• More common in adolescents, females>males
• Morphology- Arciform, sharply contoured with negative polarity (comb like
appearance)

22. 11/F

23. Mu rhythm
• Frequency- 8-10 Hz
• Location- confined to the precentral-postcentral region
• Typically asymmetric, asynchronous with interhemispheric shifting
• When strictly unilateral, an ipsilateral breach rhythm or rolandic cortical
disturbance to be considered
• Abnormal- if persistent, unreactive, associated with focal slowing

24. Mu rhythm
• Blocked by contralateral extremity movement or even thought of movement
• Not blocked by visual fixation
• Present only in wakefulness
• Disappears with drowsiness & sleep
Features Alpha rhythm Mu rhythm
Distribution Occipital Central
Blocked by Eye opening Contralateral hand movement
Somatosensory stimulations
Effect of mental arithmetic Blocked Blocked

28. Beta rhythm
•Described by Berger
•14-30 Hz
•Low amplitude fast activity, present mostly in drowsiness
•May occur in awake state in alert, attentive individual engaged in mental arithmetic
•Develops from 6 months to 2 years age, over central and PHR
•As child grows, migrates anteriorly and becomes frontal predominant by adulthood

29. Types of beta activity
Frontal type:
• Frontal and central regions, present in drowsiness and occurs in bursts
• Related to sensorimotor cortex and reactivity to movement or tactile stimulation
Generalised beta activity:
• Diffuse distribution with no reactivity
• Induced/enhanced by drugs, may become prominent with amplitudes upto 25 microvolts

30. 14/F

33. Beta rhythm
Amplitude
• Usually < 20 μV , in 70% it is 10 μV or less
• > 25 μV  usually regarded as abnormal
• In presence of defects of dura/scalp/bone  Beta activity may be enhanced due to low
impedance (Breach activity)
Symmetry
• Asymmetry, upto 35% is considered normal
• May result from differences in skull thickness
• Persistent asymmetry > 35% abnormal

35. Clinical significance
First to show diminished voltage in the presence of:
• Cortical depression - contusion, ischemia, atrophy or cystic defect
• Presence of an intervening fluid collection somewhere between the cortex and the
electrode sites- subdural or epidural fluid collection
• Vertex beta should prompt a search for cortical dysfunction affecting the frontal region

36. Theta rhythm
• 1st described by Walter
• Frequency range of 4 to 7/sec or 4 to 7.5/sec
• Theta frequencies and theta rhythms predominantly seen in
• infancy and childhood,
• states of drowsiness and sleep
• Frontal / frontocentral region
• Normal adult waking record contains only a small amount of intermittent theta
frequencies and no organized theta rhythm

37. Theta rhythm
Frontal theta can be facilitated by
• Heightened emotions
• Concentration and Mental activities like problem solving
• Hyperventilation
• Amplitude <15 μV
• In awake adults, it is abnormal only if present in persistent focal bursts or runs
• In elderly, intermittent bitemporal 4-5Hz activity especially left temporal theta,
may occur in 1/3rd of asymptomatic individuals

38. 3y/M

41. 8/M

42. Lambda waves
• Sail waves
• Biphasic / triphasic sharp transients of positive polarity in occipital regions
• Triangular or saw-toothed shaped
• Most frequent in adolescents-seen in 2/3rd
• Awake patient, while scanning a complex visual environment
• Usually bilaterally synchronous
• Duration – 100 to 250 ms, Amplitude - < 50 microvolts

43. Lambda waves
• VEP produced by rapid shifting of images across the retina during the course of
saccadic eye movements
• Resembles VEP to low frequency photic stimulation
• Activities to eliminate lambda waves
• Eye closure
• Diminution of illumination
• Having the patient stare at a blank white card
• Often confused with occipital spikes
• That are more prominent in drowsiness and sleep
• Lambda waves disappear on eye closure

44. 9/F

46. Posterior slow waves of youth
• Arrhythmic slow waves superimposed on alpha rhythm
• Moderate to large amplitude slow waves with duration of 200-400ms
• Age range: 6-12yrs, more often in girls
• Usually in occipital region, occasionally extending to parietal and posterior
temporal region
• Reactive like alpha rhythm, alpha usually superimposed on the ascending limb of
slow waves
• Mimics spike-wave activity
• Distribution, frequency, reactivity, morphology are clues to distinguish

47. 20/F

49. Phi rhythm
• Posterior slow rhythm
• 3-4Hz
• Has no harmonius relationship to posterior alpha rhythm
• Distinct from the background and occurs within 2 seconds of eye
closure
• Commonly occurs when alert and after concentrated visual attention
-reading or picture or pattern scanning
• Frequent occurrence after hyperventilation

50. 26/F

52. Normal EEG in sleep
NREM
• Drowsiness & stage I - alpha dropout, appearance of theta and frontocentral beta, vertex
waves, POSTS
• Stage II- Sleep spindles, K complexes
• Stage III & IV- Delta predominates
REM

53. Vertex waves
• Stage I and II of NREM sleep
• Diphasic or triphasic sharp waves seen over the vertex or central regions
• Small positive deflection followed by large negative wave, 300-400 ms
• Usually bilaterally symmetrical and synchronous
• Less conspicuous in elderly, in children seen in frontal regions and called F waves

56. Asymmetric V
wave
Symmetric V
waves

57. F waves

58. Positive occipital sharp transients of sleep (POST)
• Surface positive sharp waves
• In occipital regions during Stage I- II sleep
• Similar to lambda waves, but a higher amplitude and duration
• Single or in runs of 5-6Hz
• Often bilateral synchronous, but can be asymmetrical
• 50-80% healthy individuals, more in young, decreases in elderly
• Not seen in blind people
• Often confused with occipital IEDs
• Monophasic appearance
• Positivity in referential montage helps to distinguish

59. 20/M

61. Occipital spikes-Bipolar

62. Occipital spikes-Referential

63. Sleep spindles
• Sigma waves
• 10-14 Hz sinusoidal spindle shaped activity lasting 1-2s, at intervals of 5-15s
• Maximally seen over the central regions
• Symmetrical and synchronous
• Prolonged runs are seen in patients on benzodiazepines
• Spindles are characteristic of stages II and III sleep
• With deeper sleep, frequency decreases and better expressed over frontal
regions

66. Sleep spindles
• Appear by 2months of age, synchronous by 18 months
• Generated by reticular nucleus of thalamus-maintains rhythmicity
• Persistent asymmetry of vertex waves and sleep spindles is abnormal
• Absence of sleep spindles is a common finding in epileptic encephalopathies

68. K complexes
• 1st described by Loomis
• K denotes knock since its initial description was in patients who were woken up
from sleep by a sudden knock
• Appear by around 5 months of age, predominantly midline
• Believed to be evoked response to arousal stimuli
• Broad biphasic or polyphasic waveforms seen in stage II and III NREM sleep
• Duration 500 -1000 ms
• They have sharp and a slow component, which is followed by sleep spindle(14Hz)
• Sharp component is usually diphasic and resembles vertex waves with maximum
amplitude over vertex regions

70. Delta rhythm
• Described by Walter
• <4 Hz frequency
• Stage 3: (Moderately deep sleep) 20 - 50 % of the rhythms are symmetrical delta
waves
• Stage 4: (deepest sleep) Over 50% are delta rhythms with amplitude > 300 μV
• Normal in children in less than 10 years of age
• Focal anterior temporal delta found in 1/3rd of people >60 years of age
• An anterior dominant rhythmic delta may occur in sleep onset in elderly
• Resembles FIRDA, not present during wakefulness

73. Midline theta activity of drowsiness or Ciganek rhythm
• 4-7 Hz rhythmic theta activity, maximum midline vertex
• Sinusoidal or arciform-resembles Mu rhythm
• Not reactive as Mu, slow frequency, occurring in drowsiness and alert state
• Fronto-central region, during drowsiness is a normal variant

76. REM
• Usually does not occur before 60-90 minutes of sleep onset
• Low voltage polyrhythmic activity
• Periods of slow alpha-sawtooth waves over frontal/vertex regions
• Occur in bursts with ocular movements
• Chin EMG is silent
• Rapid eye movements
• Irregular respiration

78. EEG 1

79. EEG 2

80. EEG 3

81. S-7uV/mm, LFF-1Hz , HFF-70
EEG 4

82. EEG 5

83. • Alpha and Beta rhythm - Berger (1929)
• Theta and Delta rhythm - Walter (1936)
• Gamma rhythm -………………. (1938)
• REM sleep - ……………………..(1950)
Question 6

84. References
• EEG in clinical practice: John R. Hughes
• EEG : Basic Principles ,Clinical Applications And Related Fields ;
Niedermeyer and Da Silva
• Current Practice Of Clinical Electroencephalography : Ebersole 4th
Edn.

85. Thank You

86. Activation methods
• Hyperventilation
• Photic stimulation
• Mental arithmetic

87. Hyperventilation
• Breathe deeply 20-30/minute for 3 minutes, eyes closed
• Best response in 8-12 years age. Response to HV decreases with age, not
performed >50 years
• Hypocapnia causing cerebral vasoconstriction and decreased blood flow
• Blood glucose level, less than 80mg%, causes more pronounced slowing
• Contraindicated in, stroke, SAH, MI, COPD

88. • Effects of HV on EEG include diffuse slowing in theta range initially, followed by
high amplitude diffuse delta activity called hyperventilation hypersynchrony or
hyperventilation induced high amplitude rhythmic slowing (HIHARS)
• Slowing is usually posterior dominant in children and anterior dominant in adults
• Slowing can persist upto 1 minute after cessation of HV

91. Photic response
• Diffuse light stimulation produces three main categories of electrographic response:
(i) photic driving
(ii) the photomyogenic (formerly referred to as photomyoclonic) response
(iii) the photoepileptiform response (PER) (also referred to as the photoparoxysmal
response [PPR])

92. Photic driving
• Consists of rhythmic, occipital-dominant waveforms that either show a one-to-one
relationship with each flash or appear as a harmonic (an integer multiple) or
subharmonic (an integer dividend) of the flash frequency
• Seen at stimulation rates between 5 and 30 Hz
• Large positive occipital sharp transients of sleep (POSTS) and lambda waves - are
predictive of a prominent driving response

94. Characteristics Photic driving Photomyoclonic Photoconvulsive
Distribution of
response
Posterior Anterior May be diffuse
Time locking + + May out last the IPS
Amplitude No change IPS rate No change
Elicitation of
response on eye
Closed Closed Closed and opened
Age Children and
elderly
Adults Any age
Significance Physiological Nonspecific Pathological if out
lasts IPS