Lecture Overview

1. Lecture Overview Sensory coding issues Anatomy of the retina Receptive Fields Organization of striate cortex Feature analysis Spatial frequency analysis Visual association cortex

2. Definitions: Afferent vs. efferent Stimulus: any energy capable of exciting a receptor Mechanical Chemical Thermal Photic Sensory energies are measurable (unlike ESP)

3. Sensory Receptors Receptors: specialized nerve cells that transduce energy Act as dendrites that eventually induce A.P.�s Receptors are �mode� specific Only detect a small range of energy levels Eye: 400-700 nM Ear: 20-20,000 Hz Taste buds: specific chemicals �Law of Specific Nerve Energies�

10. Receptive Fields Receptive Field (RF): Those attributes of a stimulus that will alter the firing rate of sensory cell Can measure RF at each level of sensory system There are as many RF�s as there are cells in a sensory system

11. Sensory System Issues How many synapses (order of system) Degree of decussation (crossover) ? Projects to what area of thalamus? Projects to what area of cortex? Does cortex show �topical� organization? Does cortex show �columnar� organization? Modification of sensory coding? Experience, hormones?

12. Visual Systems Function of visual systems is to detect EMR emitted by objects Nature of visible light (400-700 nM) Functions of vision Locate figure vs. ground Detect movement (predator/prey?) Detect color (adaptive value of color?)

13. Eye Details An eye consists of: Aperture (pin hole, pit, or pupil) Lens or not? Photoreceptive elements (retina)

16. Cross-section through Retina Light passes through several layers of retina to reach the photoreceptors

17. Rods and Cones Rods: 120 million Light sensitive (not color) Found in periphery of retina Consist of stacked protein disks Low activation threshold Cones: 6 million Are color sensitive Found mostly in fovea One continuous membrane

19. Retinal Circuitry Photorec.: 1st order Bipolar: 2nd order Ganglion cell: 3rd order Rods: more diffusely connected to bipolar cells

21. Visual Transduction Photopigments Consist of opsin and retinal In the dark, NA+ channels are open -> glutamate is released Light breaks opsin and retinal apart-> Activates transducin (G protein)-> Activates phosphodiesterase-> Reduces cGMP -> closes NA+ channels Net effect: light hyperpolarizes the retinal receptor

24. Receptive Fields: Ganglion Cell Ganglion cells exhibit low baseline firing rates: Receptive fields: circular in shape with ring-shaped surround: �ON-Cell�: Light placed on center increases firing Light placed on surround decreases firing Get examples of stains from internetGet examples of stains from internet

27. Visual Pathways Within retina: Photoreceptor -> bipolar cell - > ganglion cell Beyond retina: Ganglion cell -> through optic chiasm -> lateral geniculate -> primary visual cortex (striate)

29. Retina-LGN Details Magnocellular system Cells from retina terminate in LGN layers 1,2 Carry info on contrast and movement (color insensitive) Carry input from �A� retinal ganglion (Y type) cells Parvocellular system Cells from retina terminate in LGN layers 3-6 carry info on fine detail, and color Carry input from �B� retinal ganglion cells (X type)

31. LGN Receptive Fields LGN shows circular receptive fields Size: larger for magnocellular than parvocellular Color sensitivity: only for parvocellular Color receptive fields (circular: on-off) e.g. red-on, green-off

32. Cortical Receptive Fields Hubel and Wiesel were interested in the visual stimuli that would excite a nerve cell in area 17 Anesthetized a cat, inserted microelectrode into area 17, recorded AP pattern during presentation on stimuli to retina Cells responded to features of visual field Shape Orientation Movement

34. Complexities of Visual Cortex Columnar Organization (input from same part of retina) Ocular dominance: cells respond to only one eye Columns for left and right alternate Orientation columns: Cells respond to same orientation, adjacent cells are shifted by 10 degrees Are at right angle to ocular dominance column Do a 180 degrees in 1 mm Color �blobs�-stained for cytochrome oxidase -Show up every 0.5 mm (one blob for each eye) Removal of one eye - alternate rows of blobs disappear

37. Spatial Frequency? Visual neurons respond to a sine wave grating: Alternating patches of light and dark Low f: large areas of light and dark High f: fine details

39. Visual Association Cortex From striate cortex (V1): see 2 streams Dorsal: �where� an object is Projects to post. parietal association cortex Ventral: �what� an object is (analysis of form) Projects to extrastriate cortex (V2, V3, V4, V5) and to inferior temporal cortex (TEO, TE, STS) Dorsal: mostly magnocellular input Ventral: equal mix of magnocellular and parvocellular input

41. Summary of Visual Cortex V1: responds to color, orientation, eye dominance V4: responds to color constancy (and form perception) Lesions impair color constancy V5: responds to movement Inf. temporal cortex TEO: coding of object features (2-d patterns, color) TE: recognition of objects (a face or a hand)

59. COLOR VISION REQUIRES: At least 2 photoreceptor types A way to compare their responses Different wavelengths of light

63. Genes-> Photopigments-> Color Vision

70. THE MAGICIANS AND PSYCHOLGY Inattentional blindness: if we don�t attend to something we won�t see it. Instead of a complete, detailed scene, we only see a small part which we are attending to! This is how magicians make things (dis)appear

71. Pick a card

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