The Central Visual System

Transduction Photoreceptors release the neurotransmitter glutamate (glu) when depolarized. Depolarized in the dark. Hyperpolarized by light. Only ganglion cells have action potentials. 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 The receptive field is the area of the retina capable of changing the bipolar cell’s membrane potential Two kinds of receptive fields: OFF cell – excitatory ON cell – inhibitory OFF and ON refers to light, not the cell Center and surround are opposites

Edge Detection 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 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 Certain colors are never seen in combination: 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 The brain compares responses of three types of cone cells. Inputs from the three types of cones are combined in different ways. 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 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 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 The relationship between an image in the world, its impact on the retina, and the retina’s projection to the cortex is maintained. 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). Relationships between areas are maintained.

Types of Ganglion Cells Magnocellular (M cells) – large cells that receive input from rods. Parvocellular (P cells) – small cells that receive input from cones. 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 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 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. Input from both eyes is combined at layer 3.

Stages in Edge Detection Retinal bipolar cells have center-surround receptive fields. LGN ganglion cells respond to contrast and change in visual input. Center-surround (on-off) receptive field. Neurons in the visual cortex have rectilinear receptive fields with excitatory and inhibitory zones.

Edge Detectors Hubel & Weisel found simple cells responding to edges at different orientations. Complex cells in the visual cortex collect on-off data from multiple cells to form edges. 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 Parallel processing of visual information from the striate cortex. Three pathways: 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 Holding brightness constant permits the study of the contribution of color to perception. Results: 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. Things that move together belong together.

Complex Forms, Motion Processing of form occurs outside the visual cortex – inferior temporal cortex. Not organized retinotopically. 10% selective for specific images (hands, faces). Processing of motion occurs in middle temporal area (MT or V5), then parietal lobe. Used for seeing moving objects, pursuit eye movements, guidance of bodily movement

Binding Mechanisms How is information from the separate, parallel pathways brought together and associated? Cells may identify patterns of synchronous activity. Treisman & Julesz – combination requires attention. A pre-attentive process detects the major outline of an object. An attentive process notices, selects & highlights combinations of features.

Visual Agnosias 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 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.