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Chapter 10

PSL440Y1 Chapter Notes - Chapter 10: Achromatopsia, Pia Mater, Motion Perception

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The pathway serving conscious visual perception includes the lateral geniculate nucleus (LGN)
of the thalamus and the primary visual cortex. Information is funnelled through this
geniculocortical pathway and is processed in parallel by neurons specialized for the analysis of
different stimulus attribute. The striate cortex then feeds this information to different extrastriate
cortical areas in the temporal and parietal lobes.
The neural pathway leaving the eyes starting with the optic nerve.
The Optic Nerve, Optic Chiasm, and Optic Tract
The ganglion cell axons leaving the retina pass through three structures before forming
synapses in the brain stem (see Fig. 10.2):
o Optic nerve
o Optic chiasm
o Optic tract
The optic nerves exit the left and right eyes at the optic disks and combine to form the
optic chiasm. Here the axons originating in the nasal retinas cross from one side to the
other and this is called decussation (i.e. the crossing of a fiber bundle from one side of
the brain to the other). In this case the decussation is partial and following this crossing,
the axons of the retinofugal projections form the optic tracts.
Right and Left Visual Hemifields
Left visual hemifield: objects appearing to the left of the midline
Right visual hemifield: objects appearing to the right of the midline (see Fig. 10.3).
The central portion of both visual hemifields is viewed by both retinas and this region of space is
the binocular visual field. Objects in the binocular region of the left visual hemifield will be
imaged on the nasal retina of the left eye and on the temporal region of the right eye.
Rule of Thumb: optic nerve fibers cross in the optic chiasm so that the left visual
hemifield is” viewed” by the right hemisphere and the right visual hemifield is “viewed” by
the left hemisphere.
Targets of the Optic Tract
A small number of optic tract axons peel off to form synaptic connections with cells in the
hypothalamus and another 10% innervate the midbrain. Most innervate the lateral geniculate
nucleus (LGN) of the dorsal thalamus and these neurons give rise to axons projecting to the
primary visual cortex. This projection is called the optic radiation. Lesions in these areas will
cause blindness.
See Fig. 10.5:
A transaction of the left optic nerve would lead to blindness at the left eye only.
A transaction of the left optic tract would lead to blindness at the right visual field.
A midline transaction of the optic chiasm would affect only the fibers that cross the
midline; blindness would result in the peripheral visual fields on both sides.
Nonthalamic Targets of the Optic Tract
Direct projections to part of the hypothalamus play an important role in synchronizing biological
rhythms with the daily dark-light cycle.
Direct projections to part of the midbrain the pretectum control the size of the pupil and
certain eye movements.
10% of the ganglion cells project to the superior colliculus and these projections are
called the retinotectal projection.
o In the superior colliculus, a patch of neurons activated by a point of light commands eye
and head movements to bring the image of this point onto the fovea.
o Involve in orienting the eyes in response to new stimuli in the visual periphery
The right and left lateral geniculate nuclei are the major targets of the two optic tracts and each
LGN appears to be arranged in six distinct layers of cells numbered 1 through 6 (see Fig. 10.7).
The most ventral layer is layer 1. The layers of the LGN are stacked one on top of another and
are bent around the optic tract. The LGN is the gateway to the visual cortex and to conscious
visual perception.
The Segregation of Input by Eye and Ganglion Cell Type
The segregation of LGN neurons into layers suggests that different types of retinal information
are being kept separate.
At the LGN, input from the two eyes is kept separate.
o In the right LGN, the right eye (ipsilateral) axons synapse on LGN cells in layers 2, 3, and
5. The left eye (contralateral) axons synapse on cells in layers 1, 4, and 6 (see Fig. 10.8).
Two ventral layers, 1 and 2, contain larger neurons than the other layers, 3 through 6. The
ventral layers are thus called magnocellular LGN layers and the dorsal layers are called
parvocellular LGN layers.
P-type ganglion cells project exclusively to the parvocellular LGN
M-type ganglion cells project entirely to the magnocellular LGN.
Numerous tiny neurons also lie just ventral to each layer called the koniocellular layers. These
layers receive input from nonM-nonP types of retinal ganglion cells and also project to visual
cortex (see Fig. 10.9).
Receptive Fields
The visual receptive fields of LGN neurons are almost identical to those ganglion cells that feed
Magnocellular LGN neurons have large center-surround receptive fields which respond
to stimulation of their field centers with bursts of action potential. They are insensitive to
differences in wavelength and are most similar to M-type ganglion cells.
Parvocellular LGN cells are most like P-type cells. They have small center-surround
receptive fields and their responses are sustained increases in the frequency of action
potentials. Many of these cells exhibit colour opponency.
Receptive fields of cells in the koniocellular layers are center-surround and have either
light/dark or color opponency.
Within all layers of the LGN, the neurons are activated by only one eye (i.e. monocular) and ON-
center and OFF-center cells are intermixed.
Nonretinal Inputs to the LGN
The retina is not the main source of synaptic input to the LGN. The major input, constituting
about 80% of the excitatory synapses comes from primary visual cortex.
The LGN receives synaptic inputs from neurons in the brain stem whose activity is related to
alertness and attentiveness (e.g. see a flash of light when you are startled in a dark room). This
input does not directly evoke action potentials in LGN neurons but can powerfully modulate the
magnitude of LGN responses to visual stimuli.
LGN has a single major synaptic target: primary visual cortex. The primary visual cortex is
Brodmann’s area 17 and is located in the occipital lobe of the primate brain (see Fig. 10.10).
This area is also called V1 and striate cortex.
It is an organization whereby neighbouring cells in the retina feed information to neighbouring
places in their target structures (i.e. LGN and striate cortex) (see Fig. 10.11). There are three
important points to remember about retinotopy:
The mapping of the visual field onto a retinotopically organized structure is often
distorted because visual space is not sampled uniformly by the cells in the retina
o For example there are more ganglion cells with receptive fields in or near the fovea than
in the periphery. Thus the central few degrees of the visual field are overrepresented (i.e.
magnified) in the retinotopic map.
A discrete point of light can activate many cells in the retina and often many more cells
in the target structure, due to the overlap of receptive fields. Therefore the activity in
striate cortex is a broad distribution with a peak at the corresponding retinotopic location.
Don’t be misled by the word “map.” It is NOT a map.
Lamination of the Striate Cortex
Striate cortex has neuronal cells bodies arranged into about a half-dozen layers named by
Roman numerals VI, V, IV, III and II. Layer I, just under the pia mater, is devoid of neurons and
consists almost entirely of axons and dendrites of cells in other layers (see Fig. 10.12). The
striate cortex is about 2mm thick.