Visual System

1
Visual System
receptive field area of visual space which
activates an axonal fiber may be narrow or
broad depending upon function and/or location
also may be linked to particular characteristic
(ie. color or direction) lateral inhibition
situation where stimulation near the receptive
field blocks activation within the receptive
field serves to enhance contrast between
light/dark areas several different types of
contrast are perceived by the visual system
spatial contrast- changes in luminance between
receptive fields temporal contrast- difference
in luminance over time combined contrast- both
spatial and temporal change- ie. motion
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Visual System
the lens of the eye uses smooth muscles to focus
an inverted image on the retina-- the cornea
is fixed and does not change shape iris
controls amount of light entering fovea area of
sharpest viewing most dense photoreceptors pig
ment epithelia, or choroid surrounds retina,
blocks light sclera connective tissue around
eye and optic nerve retina has 5 layers, 3 of
cells light passes through all layers
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Retinal Structure and Function
outer nuclear layer photoreceptor cells closest
to the pigment epithelia essentially only rods
and cones inner nuclear layer retinal
interneurons (may be excitatory or inhibitory)
bipolar cells link photoreceptors to retinal
ganglion cells horizontal cells synapse on
synapses at photoreceptors to bipolar
cells amacrine cells synapse on synapses at
bipolar to ganglion cells ganglion cell layer
output neurons of retina (optic nerve) few
amacrine
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Retinal Structure and Function
rods and cones have outer segments that carry out
phototransduction photoreceptors are bound to
11-cis retinal which absorbs the light opsins
photoreceptor proteins 3 colored opsins, 1 rod
opsin- rhodopsin opsins are G-proteins,
activating transducin (Ga) which activates
a cGMP phosphodiesterase, reducing cGMP
concentrations cones (color vision) are not
sensitive enough for low light concentrated in
the fovea rods can detect single photons, but
adapt quickly at higher light levels shut down
in brightest light-- only cones are
active cGMP activates a cation channel light
hyperpolarizes photoreceptors photoreceptors
have graded potentials- no action potentials but
increased (darkness) or decreased (brighter)
from a mean level
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Retinal Structure and Function
2 types of bipolar cells- on or off types both
respond to glutamate, but use different
receptors (glu inhibits on-type
bipolars) on-bipolars work by removal of
inhibition off-bioplars function using kainate
receptors
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Retinal Structure and Function
most ganglion cells are also on or off types
connecting to the same type bipolar cell on
and off bipolars form synapses on different
laminae of the inner plexiform layer other
ganglion cells respond to switching ie.
signal change in luminosity, not intensity
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Retinal Structure and Function
horizontal neurons are GABAergic inhibitory
interneurons believed to control lateral
inhibition (ie. antagonistic surround) amacrine
cells synapse onto inner plexiform layers
contains diverse cell types and many
transmitters all ganglion axons leave through
the optic fiber layer to optic nerve there is a
blind spot where the optic nerve exits the
retina (optic disc) ganglion cells project to
essentially 2 regions of the brain- superior
colliculus and lateral geniculate of the
thalamus both regions are topographic connections
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Retinal Projections
retinal ganglion cells have different functional
properties beyond on-off generated by
amacrine cell interactions parallel sensory
pathways have each type of RGC projecting to a
different region, separating different types
of signals for independent processing 3
different regions of the lateral geniculate
receive inputs from RGCs P-ganglion cells
parvocellular dorsal-most region, small
cells M-cells magnocellular 2 ventral
layers, large cells I-cells intercalated
layers aka koniocellular (hardest to study)
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Retinal Projections
P and M cells differ in 5 main ways 1) P
receptive fields are smaller than M 2) P gives
sustained response, M transient 3) P transmit
slower, M faster 4) P is more color sensitive
than M 5) M is more sensitive to contrast than P
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Motion Detection
motion sensitivity develops in retinal ganglion
cells but extends beyond simple motion ganglion
cells basically detect onoff stimuli
relatively large receptive fields, so it does
not localize motion well in visual cortex,
position information is more exact AND
direction of motion is also important ie.
vertical vs horizontal gives different
results cortical region MT also detects motion,
but looks over very broad receptive fields
to detect 2 objects moving in oppositive
directions
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Lateral Geniculate Nucleus
in lower vertebrates such as chick, the optic
nerves cross completely in the optic
chiasm in carnivores, half of the axons cross
the midline so that visual fields overlap
nasal axons cross (contralateral) temporal
axons do not (ipsilateral) inputs from the 2
eyes remain segregated in the LGN, in this
case by layer
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Visual Cortex
LGN neurons from both sides project to V1 of the
occipital cortex first location where neuron
responses are binocular- either eye V1 has
orientation specificity-- elongated shapes in one
direction but not another stimulates many of
these neurons best 2 main cell types in a
heirarchy for V1 simple cells have on-and-off
subregions complex cells have orientation
specificity, but process input from simple
cells simple cells are more common in neurons
connecting directly to LGN axons complex cells
are more distantly connected processes
generalizations about the visual field
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Visual Cortex
V1 has several processing streams making up
parallel processing the 3 regions of the LGN
project to different subdivisions of visual
cortex layer 4Ca receives inputs from
magnocellular neurons layer 4C/3 receives
inputs from parvocellular neurons
koniocellular neurons project to layers 23 in
'blobs' topographic mapping is retained in V1-
functions are segregated by layer ocular
dominance columns (inputs driven by
primarily 1 eye) lie near each other over
the map columns next to each other overlap by
50, maintaining topography
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Visual Cortex
'blobs' of konicellular projections are labeled
by high cytochrome C cells that are color
selective, poorly oriented, and monocular in V2,
neurons are arranged in parallel stripes of
high and low activity with blobs projecting to
thin high activity stripes thick stripe
neurons project to the 'dorsal stream' thin
and pale stripe neurons project to 'ventral
stream'
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Visual Cortex
about 50 of the cortex is involved in visual
processing-- vast amounts 30 subdivisions
each have different processing functions many
regions connect in a hierarchical fashion, but
stronger connections ie. more of the 'drive'
means there is generally a feedback circuit as
well processing regions of the brain are clearly
function specific temporal lesions selectively
damaged recognition rather than acuity
parietal lesions damaged acuity or perception
as opposed to recognition 'ventral stream' goes
to temporal areas 'dorsal stream' goes to
parietal areas
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Visual Cortex
area MT (aka V5) is strongly dominated by motion
95 of neurons are highly selective for
direction of motion MT takes inputs from V1 and
further processes them so it can measure
motion over a large area neurons here are less
sensitive to shape, although extended objects
drive neurons here better than isolated
features features are very general, it is the
direction that matters