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Chapter 10

NROC64H3 Chapter Notes - Chapter 10: Prosopagnosia, Parietal Lobe, Motion Perception


Department
Neuroscience
Course Code
NROC64H3
Professor
Matthias Neimier
Chapter
10

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NROC64 Chapter #10:
The Central Visual System:
Retina extracts information about different facets of the visual image
More than 100 million photoreceptors in the retina but only 1 million axons leaving the eye
carrying info to the rest of the brain
Pathway leaving conscious visual perception includes the LGN of the thalamus and the primary
visual cortex (Area 17), V1 or striate cortex
The Retinofugal Projection:
Retinofugal projection - neural pathway that leaves the eye beginning with the optic nerve
o Centrifugal projection = away from the center
o Corticogugal projection goes away from the cortex
Retinofugal = away from the retina
The Optic Nerve, Optic Chiasm and Optic Tract:
Ganglion axons pass through three structures before reaching the brain stem
Components are: optic nerve optic chiasm optic tract
Optic nerves exit the right and left eyes at the optic disks and pass through holes in the floor
of the skull
Optic chiasm axons originating in the nasal retinas cross from one side to the other
o Decussation - crossing of a fiber bundle from one side of the brain to the other
o b/c only axons from the nasal retinas cross = partial decussation at the optic chiasm
Optic tracts axons form these tracts which run just under the pia along the lateral surfaces of
the diencephalon
Right and Left Visual Hemifields:
Left visual hemifield objects appearing to the left of the midline (vice versa)
o Objects in the binocular region of the left visual hemfield are imaged on the nasal retina
of the left eye and on the temporal retina of the right eye
o This is because the temporal regions are encoded by the opposite side of the brain
o Nasal are encoded by the same side of the brain (ipsilateral)
o Optic nerve fibers cross in the optic chiasm
Binocular visual field central portion viewed by both retinas
Targets of the Optic Tract:
Small # of optic tract axons form synaptic connections with cells in the hypothalamus
10% continue past thalamus to innervate the midbrain
Most innervate the LGN of the dorsal thalamus
o Neurons in LGN give rise to axons that project to the primary visual cortex
o Projection from LGN cortex = optic radiation
Lesions in the retinofugal projection cause blindness
Cut in the left optic nerve = blind in left eye
Cut in the left optic tract = blindness in right visual field
Midline cut in the optic chiasm = only affects fibers that cross the midline
Nonthalamic Targets of the Optic Tract:
Direct projections to the hypothalamus play an important role in synchronizing a variety of
biological rhythms including sleep, wakefulness with the daily dark-light cycle
Direct projection to the midbrain (pretectum) control the size of the pupil and certain types of
eye movement

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10% (150 000 neurons) of the ganglion cells in the retina project to a part of the midbrain
tectum called the superior colliculus (little hill)
Tectum of the midbrain major target of retinofugal projection in all nonmammalian vertebrates
o In these the superior colliculus = optic tectum
o Projection from retina superior colliculus is called the retinotectal projection in
mammals
Superior colliculus a patch of neurons activated by a point of light commands head, eye
movements to bring the image to the fovea
Box 10.1 David and Goliath:
Body size is regulated by secretion of growth hormone form the anterior lobe of the pituitary
gland
If anterior lobe swollen (hypertrophied) it produces excess amounts of hormone resulting in
body growth of unusually large proportions
This is called pituitary hypertrophy also disrupts normal vision
Loss of peripheral vision b/c enlarged pituitary, results in a loss of peripheral vision = tunnel
vision/bitemporal vision
The Lateral Geniculate Nucleus:
Right and left lateral geniculate nuclei in the dorsal thalamus = major targets of the 2 optic tracts
Each LGN appears to be viewed in 6 layers of cells (most ventral layer = 1)
Layers arranged like a pancake bent around the optic tract
LGN = gateway to visual cortex (visual perception)
The Segregation of Input by Eye and by Ganglion Cell Type:
LGN neurons receive synaptic input from the retinal ganglion cells
Segregation of LGN neurons in layers suggest that diff types of retinal info are being kept
separate
o Axons from M-type, P-type, nonM-nonP ganglion cells in the 2 retinas synapse of diff
LGN layers
o LGN: right eye (ipsilateral) axons synapse on LGN layers 2, 3, 5
o Left eye (contralateral axons synapse on layer 1, 4, 6
o Layers 1 and 2 contain larger neurons and the 4 more dorsal layer 3-6 contain smaller
cells
o Ventral layers = magnocellular LGN layers and dorsal layers = parvocellular LGN layers
o P-type ganglion cells in retina project to parvocellular LGN
o M-type ganglion cells magnoceullular LGN
o Koniocellular layers- tiny neurons b/w layers that receive inpit from the nonM-nonP
types of retinal ganglion cells and project to visual cortex
Info derived from 3 categories of retinal ganglion cells and from 2 eyes remains segregated
Retina give rise to streams of info that are processed in parallel
Receptive Fields:
Inserting a microelectrode into LGN can study potential discharges of geniculate neurons in
response to visual stimuli
Visual receptive fields of LGN neurons are almost identical to those of the ganglion cells that
feed them
Magnocellular LGN neurons: relatively large centre-surround receptor fields, respond to
stimulation with a transient burst of APsand are insensitive to diff in wavelength (like M-type
ganglion cells)
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Parvocellular LGN cells are like P-type retinal ganglion cells small centre-surround receptive
fields, respond to stimulation of receptive field centers with a sustained increase in the freq of
Aps, exhibit colour opponency
Receptive field of cells in koniocellular layers center-surround and have either light/dark
opponency
Within all layers of LGN the neurons are activated by only one eye
Non-Retinal Inputs of the LGN:
Retina is not the main source of synaptic input to the LGN
Major input (about 80% excitatory synapses) comes from the primary visual cortex
LGN also receives synaptic inputs from neurons in the brain stem (activity: related to alertness
and attentiveness)
LGN is the 1st site in the ascending visual pathway where what we see is influenced by how we
feel
Anatomy of the Striate Cortex:
LGN has a single major synaptic target: primary visual cortex (aka area 17, V1, striate cortex)
located in the occipital lobe, surround the calcarine fissure
Close correlation b/w structure and function
Retinotopy:
Projection from retina LGN and V1
Retinotropy organization whereby neighbouring cells in the retina feed info to neighboring
places in their target structures LGN and striate cortex
o 2-D structure of retina is mapped onto 2-D surface of the subsequent structures
o 3 important points:
Mapping of the visual field onto a tetinotopically organized structure is often
distorted (central few degrees of the visual field are overrepresented or
magnified in the retinotopic map)
Discrete point of light can activate many cells in the retina due to the overlap of
receptive fields
Don’t be misled by the word “map” perception is based on the brain’s
interpretation if distributed patterns of activity
Lamination of the Striate Cortex:
Neocortex and striate cortex have neuronal cell bodes arranged into about half a dozen layers
Just under the pia mater is largely devoid of neurons and consists entirely of axons and
dendrites
Full thickness of the striate cortex from white matter to pia = 2 mm
At least 9 distinct layers in reality of striate cortex, however 3 layers are combined
The Cells of Different Layers:
Spiny stellate cells small neurons with spine-covered dendrites that radiate out from the cell
body, primary in 2 tiers of layer IVC
Pyramidal cells outside layer of IVC, covered with spines, a single thick apical dendrite that
branches as it ascends toward the pia mater and by multiple basal dendrites that extend
horizontally
Only pyramidal cells send axons out of the striate cortex to form connection with other parts of
the brain
Axons of stellate cells make local connections only within the cortex
Inhibitory neurons which lack spines are in the cortical layers as well, these neurons only form
local connections
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