The retina contains photoreceptors that detect light and send signals through bipolar and amacrine cells to retinal ganglion cells. Ganglion cell axons form the optic nerve and cross at the optic chiasm, with nasal fibers crossing to the opposite side. The optic tract carries signals to the lateral geniculate nucleus and superior colliculus. The LGN projects to primary visual cortex, which separates visual properties and sends them to different visual association areas for further analysis of color, shape, motion, and location.

1. RETINA-VISUAL PATHWAYS

5. Retina
 takes the information from its 100 million
photoreceptors about 1 million optic nerve
axons.
 Interposed between the photoreceptor layer
and the layer of ganglion cells (whose axons
form the optic nerve) is a layer containing
three kinds of interneurons

6.  Bipolar cells convey signals straight across
this layer, receiving inputs from
photoreceptors and synapsing on ganglion
cells.
 Horizontal cells spread laterally in the outer
synaptic layer, affecting transmission from
photoreceptors to bipolar cells.
 Amacrine cells have a similar role in the inner
synaptic layer, affecting transmission from
bipolar to ganglion cells

7.  ganglion cell axons travel along its vitreal
surface, so they need to cross the retina,
choroid, and sclera in order to leave the eye in
the optic nerve.
 at the optic disc, all the axons converge and
collect into groups that leave the eye through
small holes in the sclera.

8.  The sclera continues over the optic nerve as a
sheath continuous with the dura mater, much
as the spinal dura continues as the
epineurium of spinal nerves.
 The normal layers of the retina are absent at
the optic disc, which results in a blind spot in
the visual field of each eye

10. Central visual pathways

11.  Axons of retinal ganglion cells project to a
variety of places:
1. the superior colliculus, to help direct visual
attention;
2. other midbrain sites, for things like the
pupillary light reflex;
3. the hypothalamus, to help regulate
circadian rhythms;

12.  but mostly to the thalamus, for conscious
awareness of visual stimuli.
 Optic nerve fibers convey all the information
we will ever get about the shape, color,
location, and movement of objects in the
outside world.


13.  Different classes of ganglion cells emphasize
different properties of a visual stimulus
 these different properties begin to be sorted
out in the six-layered lateral geniculate
nucleus of the thalamus

14.  Different layers of the lateral geniculate then
project differentially to primary visual cortex
above and below the calcarine sulcus (also
known as striate cortex because of a stripe of
myelinated fibers that run through one of its
middle layers).

16.  Primary visual cortex then picks apart these
attributes a little more and parcels them out
semiselectively to distinct areas of visual
association cortex in the occipital and
temporal lobes.

18. CN II
 Extension of white matter of the brain-
enclosed in meninges
 No effective regeneration when divided
 Attached to anterior part of floor of 3rd
ventricle

19. Optic chiasma
 Nasal fibres of each CN II decussate and pass
to opposite optic tract
 Temporal fibres pass directly to optic tract of
their own side
 Right tract has fibres from right ½ of each
retina [nasal field of right eye and temporal
field of left eye]

20. Optic tract
 Passes around cerebral peduncle, high up
against temporal lobe , reaches side of
thalamus
Branches
1. Larger – lateral geniculate body [visual fibres
2. Smaller – bypasses LGB → superior
colliculus→ pretectal nuclei [re light reflexes]
3. Some fibres end in hypothalamus [ circadian
rhytm]

22. 1. Some fibres end in hypothalamus [ circadian
rhythm] circa= about; diem=a day
 superior colliculus also receives fibres from
1. Spinotectal/spinomesencephalic tracts
2. Auditory inputs via Inferior colliculus

23. Efferents from superior colliculus
1. Reticular formation
2. Inferior colliculus
3. Cervical spinal cord [tectospinal tract]
4. LGB→ pulvinar → visual association cortex

24. Optic Nerve, Chiasma, and Tract
 visual information from one side of the world
ends up in the contralateral occipital lobe.
 each eye looks at most of the right and left
half of the total visual field
 As a result, half of the output of each retina
needs to cross in the optic chiasm, and half
needs to stay uncrossed

26.  the crossed and uncrossed fibers
representing one half of the visual field
emerge from the chiasm as an optic tract.
 damage in front of the optic chiasm can cause
complete blindness of the ipsilateral eye,
 damage to one optic tract (or any part of the
visual system behind the chiasm) can cause
loss of the contralateral half of the visual field
of both eyes

27.  damage to one optic tract (or any part of the
visual system behind the chiasm) can cause
loss of the contralateral half of the visual field
of both eyes
 This deficit has the tongue-twisting name of
homonymous hemianopia ("blindness in the
same half of both visual fields")

31. Beyond primary visual cortex
 analysis of form and color is largely carried
out in ventral parts of the occipital and
temporal lobes
 analysis of location and motion takes place
more dorsally, around the junction of the
occipital, parietal, and temporal lobes.

32. dorsal stream reaching the area near the junction of the parietal,
occipital, and temporal lobes is particularly important for
analyzing the location and movement of visual stimuli;ventral
stream reaching the occipitotemporal gyrus is particularly
important for analyzing colors and shapes.

33.  damage to the occipitotemporal gyrus can
cause deficits in recognizing things visually
despite visual fields being intact.

34.  deficits can be fairly selective depending on
which part of the occipitotemporal gyrus is
damaged . For example
1. cortical color blindness [achromatopsia]
2. difficulty recognizing faces
[prosopagnosia]).

35.  damage near the junction of the parietal,
occipital, and temporal lobes can cause
difficulties in perceiving the motion of objects
or in "stitching together" multiple objects in
different locations into a unified scene.