Download October 29

The Central Visual System
Transduction
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Photoreceptors release the neurotransmitter
glutamate (glu) when depolarized.
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Only ganglion cells have action potentials.
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Depolarized in the dark.
Hyperpolarized by light.
Photoreceptors produce graded response that
provides input aggregated by bipolar cells.
Magno ganglion cells receive input from rods,
parvo ganglion cells from cones
Bipolar Cell Receptive Fields
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The receptive field is the area of the retina
capable of changing the bipolar cell’s
membrane potential
Two kinds of receptive fields:
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OFF cell – excitatory
ON cell – inhibitory
OFF and ON refers to light, not the cell
Center and surround are opposites
Edge Detection
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The center-surround organization of the
receptive fields of ganglion cells exaggerates
the contrast at borders.
Visual processes “fill in” what occurs between
borders (edges).
Contrast effects occur because we notice
variations, not absolute magnitudes of light.
Color Contrast
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Cones respond to specific wavelengths of
light that determine hue.
Color cells have complementary surrounds
that heighten contrast and strengthen their
signal.
Opponents are: red/green, blue/yellow.
Color Opponency
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Certain colors are never seen in combination:
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Reddish green, bluish yellow.
Red and green mix to form yellow; yellow and blue mix
to form white.
Hering’s opponent process theory – perceptual
cancellation occurs because colors are processed as
opponent pairs.
Color cells have complementary surrounds that
heighten contrast and strengthen their signal.
Color Processing
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The brain compares responses of three types
of cone cells.
Inputs from the three types of cones are
combined in different ways.
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The brain computes responses of specific cones
but also all cones in the retina (background) to
compensate for ambient light (constancy).
Area V4 responsible for color constancy –
damage results in loss of color experience.
Visual Fields
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Each eye has a visual field that overlaps the
visual field of the other eye.
Each eye’s visual field is divided in half –
called a hemifield.
The right hemifield of each eye is viewed by
the left hemisphere of the brain.
The left hemifield of each eye is viewed by
the right hemisphere of the brain.
Some Terminology
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The suffix “fugal” means to flee
Retinofugal refers to where the axons of the
optic nerve go after they leave (flee) the
retina.
Decussation – crossing of a bundle of fibers
(axons) from one side of the brain to the other.
Tract – a bundle of fibers going the same way
Retinotopic Mapping
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The relationship between an image in the
world, its impact on the retina, and the retina’s
projection to the cortex is maintained.
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This is called topographic mapping.
Stimulation of neighboring retinal locations
results in stimulation of corresponding areas
of the LGN, superior colliculus, and occipital
cortex (primary visual cortex).
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Relationships between areas are maintained.
Types of Ganglion Cells
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Magnocellular (M cells) – large cells that
receive input from rods.
Parvocellular (P cells) – small cells that
receive input from cones.
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Blob pathway – concerned with color perception.
Interblob pathway – concerned with shape/form.
Koniocellular (nonM-nonP) – small cells
involved in color vision (not well understood).
Mapping Within the LGN
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Optic nerve carries information from ganglia
to LGN. Crosses at optic chiasm.
Separate layers are maintained for each eye
and for each type of cell (M and P).
Interneurons project from areas of the LGN to
striate cortex (also called primary visual
cortex or V1).
Mapping in the Striate Cortex
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Separate layers from LGN to striate cortex are
maintained in ocular dominance columns.
M, P, & non-M/P cells enter the cortex at
different levels of layer 4 of the visual cortex.
Information is combined by pyramidal cells
that synapse at higher levels in the striate
cortex.
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Input from both eyes is combined at layer 3.
Stages in Edge Detection
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Retinal bipolar cells have center-surround
receptive fields.
LGN ganglion cells respond to contrast and
change in visual input.
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Center-surround (on-off) receptive field.
Neurons in the visual cortex have rectilinear
receptive fields with excitatory and inhibitory
zones.
Edge Detectors
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Hubel & Weisel found simple cells
responding to edges at different orientations.
Complex cells in the visual cortex collect onoff data from multiple cells to form edges.
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Complex cells provide positional invariance.
M-channel cells are orientation and direction
selective, for motion detection.
P-IB channel cells analyze object shape.
Extrastriate Pathways
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Parallel processing of visual information from
the striate cortex.
Three pathways:
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Color processing – P blob cells, goes from V1 to
V2, then V4, then inferior temporal cortex.
Shape processing, depth perception – P interblob
cells, go from V1 to interior temporal cortex.
Motion & spatial relations – M cells, V1 to V2,
then MT (V5), to parietal cortex.
Equiluminance
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Holding brightness constant permits the study
of the contribution of color to perception.
Results:
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Brightness, not color, is important to motion
detection, perspective, relative sizes, depth
perception, figure-ground relations, visual
illusions.
Motion is a cue for distinguishing among objects.
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Things that move together belong together.
Complex Forms, Motion
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Processing of form occurs outside the visual
cortex – inferior temporal cortex.
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Not organized retinotopically.
10% selective for specific images (hands, faces).
Processing of motion occurs in middle
temporal area (MT or V5), then parietal lobe.
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Used for seeing moving objects, pursuit eye
movements, guidance of bodily movement
Binding Mechanisms
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How is information from the separate, parallel
pathways brought together and associated?
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Cells may identify patterns of synchronous
activity.
Treisman & Julesz – combination requires
attention.
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A pre-attentive process detects the major outline
of an object.
An attentive process notices, selects & highlights
combinations of features.
Visual Agnosias
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Existence of distinct agnosias for aspects of
perception suggests that these abilities are
localized to areas selectively damaged.
Achromatopsia – good perception of form
despite inability to distinguish hues.
Prosopagnosia – inability to recognize faces
as particular people (identity). Can recognize
that it is a face, and tell the parts.
Development of the Visual System
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Pathways are developed before birth.
Fovea develops in the first four months after
birth – ability to see detail.
Connections between layers in visual cortex
develop with experience, after birth.
Visual acuity becomes adult-like by 12
months.