PHYSIOLOGY OF VISION

PHYSIOLOGY OF VISION

1. PHYSIOLOGY OF VISION By- Dr. Priyanka Raj

2. Visual impulse in photoreceptors Processing and transmission of visual impulse in retina Processing and transmission of visual impulse in visual pathway Analysis of visual impulse in visual cortex Three part system hypothesis of visual perception

3. Visual impulse in photoreceptors

4. PHOTOTRANSDUCTION STANDING POTENTIAL OR DARK CURRENT HYPER- POLARISING RECEPTOR POTENTIAL

5. Cone versus rod receptor potential Cone receptor potentisl has sharp onset and offset Rod receptor potential has sharp onset and slow offset Rod responses are proportionate to stimulus intensity at illumination levels below threshold for cones  rods detect absolute illumination Cone responses are proportionate to stimulus intensity at high levels of illumination when rod responses are maximal  generate response to change in light intensity above background

6. Processing and transmission of visual impulse in retina

7. Receptor potential from photoreceptors  electronic conduction  other cells of retina NEUROTRANSMITTERS IN RETINA- Glutamine  excitatory. Rods & cones GABA Glycine Dopamine amacrine cells Acetylcholine Indolamine

8. Physiological activities in retinal cells HORIZONTAL CELLS Phenomenon of lateral inhibition- Horizontal transmission of signals in outer plexiform layer (PR->bipolar cells) Minute spot of light  central most area excited  area around (surround) inhibited Enhance visual contrast by lateral inhibition Concept of receptive field- The influence area of a sensory neuron Receptive field of horizontal cells is very large as compared to the photoreceptor cell

9. BIPOLAR CELLS 1 st order neurons of visual pathway. Some bipolar cells depolarize while some hyperpolarize  provide opposing excitatory and inhibitory signals Depolarizing cells respond to glutamate (excitatory) Some are directly stimulated by photoreceptors  excitatory others are indirectly inhibited by horizontal cells Receptive field of bipolar cells has got a centre-surround antagonism

10. AMACRINE CELLS Receive information at the synapse of bipolar cell axon with ganglion cell dendrites  temporal processing at the other end of bipolar cells Electrically produce depolarizing potentials & spikes  act as generator potentials in ganglion cells Help in temporal summation and initial analysis of visual signals

11. GANGLION CELLS Electrical response of Bipolar cells  modified by amacrine cells  ganglion cells  action potential  signals to brain On & off centre ganglion cells these produce propagated spikes On centre  increase their discharge Off centre decrease their discharge Depending upon their function  W, X, Y ganglion cells

12. W X Y Small (diameter <10 μ ) Medium (dia= 10-15) Large (dia upto 35) 40% of all ganglion cells Most numerous (55%) Fewest (5%) Dendrites spread widely in IPL Small fields (dendrites do not spread) Very broad dendritic field Pick up signals from rods Pick signals from at least one cone Pick up signals from widespread retinal area Responsible for Rod vision in dark Detect directional movements Responsible for colour vision Respond to rapid change in visual image Sustained cells Transient cells

13. Processing and transmission of visual impulse in visual pathway

14. OPTIC NERVE, CHAISMA & OPTIC TRACT Axons of RGC  optic nevre Single optic nerve fibre can be excited only by a specific stimulus falling on a restricted area of retina  receptive field

15. LAERAL GENICULATE BODY 2 principal functions RELAY STATION- Relay visual information from optic tract to visual cortex (geniculocalcarine tract) The signals from two eyes are kept apart in LGB TO “GATE” THE TRANSMISSION OF SIGNALS- Control the passage of visual signals to visual cortex Receive gating (inhibitory) controls from- Primary visual cortex  corticofugal fibres Reticular area of mesencephalon

16. Retinotopic projection Ganglion cell axons project a detailed spatial representation of retin on LGB with precise point-to-point localization LGB  6 layers 1,4,6  input from contralateral eye 2,3,5  input from ipsilateral eye Each layer  point-to-point representation of retina present Along a line perpendicular to layers  receptive fields of cells are identical

17. P cells project to the parvocellular layers of the lateral geniculate nucleus. known as midget retinal ganglion cells  small sizes of their dendritic trees and cell bodies. 80% of all retinal ganglion cells part of the parvocellular pathway. receive inputs from relatively few rods and cones. have slow conduction velocity respond to changes in color but respond only weakly to changes in contrast unless the change is great M cells project to the magnocellular layers of the lateral geniculate nucleus. known as parasol retinal ganglion cells  large sizes of their dendritic trees and cell bodies. 10% of all retinal ganglion cells part of the magnocellular pathway. receive inputs from relatively many rods and cones. have fast conduction velocity can respond to low-contrast stimuli, but are not very sensitive to changes in color

18. Electrophysiological properties Receptive fields of P & I cells of LGB are similar to RGCs & optic nerve axons All geniculate receptive fields process on-center/off-center configuration Fields as sustained (X) & transient (Y) is maintained High degree of peripheral suppression in geniculate receptive field. Larger “off” periphery cancels effects of “on” centre. They are sensitive in responding to spatial differences in retinal illumination. Geniculate relay cells have binocular receptive fields.

19. OPTIC RADIATIONS Composed of axons of LG relay cells which project to visual cortex on same side Central portion  macular fibres Dorsal fibres  upper retinal quadrants Ventral fibres  lower retinal quadrants

20. Analysis of visual impulse in visual cortex

21. 1. Retinopic organization Striate Area 17  visuosensory retina Peristriate area 18 & 19  visuopsychic area Modified nomenclature V1  area 17 V2  most of 18 V3  narrow strip over anterior part of area 18 V4  within area 19 V5  posterior end of the superior temporal gyrus point-to-point representation Ganglion cell axons  LGB  visual cortex

22. Layers of Primary Visual Cortex Six distinct layers – Layers I, II and III – are thin and contain pyramidal cells Layer IV – thickest layer. Further subdivided into a, b, ca and cb. Layer V and VI – relatively thin.

23. Connections of Primary Visual Cortex Geniculate afferents the axons from the lateral geniculate nucleus terminate generally in layer IV. The rapidly conducted signals from the Y retinal ganglion cells terminate in layer IV ca. Visual signals from X ganglion cells in the retina terminate in layers IVa and IVc. This pathway transmits accurate point to point and color vision. Subcortical connections Reciprocal connections returning from striate to LGB arise from pyramidal cells of layer VI. Axons from pulvinar to striate cortex terminate among dendrites of layers I and V.

24. Corticocortical connections Fibres to extrastriate visual regions arise from pyramidal cells of layers II and III os the striate cortex. Fibres to contralateral striate cortex also arise in layer III. Reciprocal connections from these regions are made predominantly by fibres that terminate in layer II and III of striate cortex.

25. PHYSIOLOGY OF VISUAL CORTEX Retinal ganglion cells & lateral geniculate neurons  respond to both diffuse retinal and spot stimulus Cortical neurons  stimulus in form of straight line, bar or edge presented in a proper spatial orientation  orientation & configuration receptive field differ in visual cortex Aspects of physiology: Concept of receptive field of striate cortex Columnar organization of striate cortex Serial v parallel analysis of visual image Role of extra-striate cortex in visual functions Psychophysiological aspects of visual functions

26. Concept of receptive field of striate cortex Hubel & Wilson  named cortical cells as 3 receptive field types

27. SIMPLE CELLS Found mainly in layer IV of the primary visual cortex (area 17) Form the 1 st replay station within the visual cortex Respond to bars of light, lines or edges in a particular orientation only The orientation of a stimulus most effective in evoking a response is called “ receptive field axis orientation” Receptive fields  arranged in parallel bands of “on-areas” & “off-areas” FUNCTION: Role in detection of lines and borders in different areas of retina Detect orientation of each line/border  horizontal/vertical/inclined

28. COMPLEX CELLS Found in cortical layers above and below layer IV of areas 17, 18, 19 Require preferred orientation of linear stimulus but are less dependent upon the location of a stimulus in the visual field Respond maximally when stimulus is moved laterally without change in orientation On and off areas cannot be mapped in their receptive fields Receive input from both eyes  called binocular 4 types of receptive field a/c preferred stimulus Activated by a slit-nonuniform field Activated by a slit-uniform field Activated by an edge Activated by a dark bar

29. FUNCTION- Detection of lines, bars and edges specially when they are moving Perception of features, orientation and movement of objects Simple + complex cells = feature detectors

30. HYPERCOMPLEX CELLS Found in cortical layers II & III of areas 17, 18, 19 All properties of complex cells + require the line stimulus to be of specific length Hubel & wiesel = 6 types (4 lower + 2 higher) hypercomplex cells Dreher = class I & class II

31. Columnar organization of striate cortex ORIENTATION COLUMNS “vertical grouping of cells with identical orientation specificity” Unit of organization in the cortex Several million vertical columns in visual cortex On moving column-to-column  sequential changes in orientation preference of 5-10 degress

32. Depth perception  2 separate column systems Constant depth column  contains binocular units with exactly same retinal disparity for properly oriented stimuli Constant direction columns  points perpendicular to the center of contralateral eye Together  localize points in a 3D space

33. OCULAR DOMINANCE COLUMNS Independent system of columns which exist in visual cortex with respect to binocular input to cortical cells Simple cells  uniocular input; complex+hypercomplex cells  binocular input Neurons with receptive fields dominatd by one eye are grouped alternately into left and right eye columns A group of binocular complex and hypercomplex cells in layers II, III, V & VI that receive a stronger input from one of the two eyes, along with their cells in layer IV receiving uniocular input from the same eye are known as ocular dominance column

34. THE COLOUR BLOBS Primary areas for deciphering colours Interspersed among the primary visual columns Receive lateral signals from adjacent visual column and respond specifically to colour signals SERIAL Vs PARALLEL ANALYSIS Hierarchical model for cell interconnection Columnar organization of cortex Simple (monocular)  complex (binocular)  hypercomplex

35. EXTRASTRIATE CORTEX Neurons of straite cortex (area 17 or VI)  extrastraite cortex [area 18 (V2), area 19 (V2), V3 V4 MT]  strait cortex Pontifical cells  receive information from the feature detectors (simple & complex cells) Specialized extrastriate areas  Colour processing area  V4 (rhesus monkeytrial) Movement processing area  MT. cells show strong preference for stimuli moving in a particular direction Stereoscopic depth perception area  V2 & V3

36. PSYCHOPHYSIOLOGICAL ASPECT OF VISUAL FUNCTIONS Vision is related to verbal language and reading Visual cortex connects with tactile sensory motor auditoy, olfactory and speech areas Angular gyrus (area 40)of parietal lobe acts as visual memory centre for words by forming associations between visual and auditory centres Corpus callosum connets the two hemispheres and help perceieve the several qualities simultaneously and synthesize a unified picture Brain’s response to stimuli is in the form of an over all picture

37. “THREE-PART-SYSTEM” HYPOTHESIS OF VISUAL PERCEPTION

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