The Central Visual System

1. The Central Visual System

2. 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

3. 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

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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

10. 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.

11. 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).

12. 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).

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. 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

19. 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.

20. 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.

21. 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.