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Chapter 4

chap 4 textbook notes

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University of Toronto St. George
Kristie Dukewich

CH4 – The Visual Cortex and Beyond o Both eyes receives from contralateral and ipsilateral visual field Following the Signals from the Retina to Cortex  TEMPORAL HEMI-RETINA – retina half closer to temple (outer)  The Visual System  Process nasal visual field (inner areas) o Overview of visual system, depicts pathways neural signals follow  NASAL HEMI-RETINA – retina half closer to nose (inner) once left retina:  Processes temporal visual field (outer areas)  Left half’s of both retina goes to left LGN & Striate Cortex  Right half’s of both retina goes to right LGN & Striate Cortex o Retinal Ganglion Cells: 2 types  MIDGET RGCs – small, compact dendritic field, 90% RGCs  Concentrated w/i fovea, small receptive field  Feature: sustains after stimulation (Stays on)  Biased towards processing central info (orientated)  PARASOL RGCs – large, umbrella like dendritic field, 10% RGCs  Large receptive field, photoreceptors span huge area  Feature: quick return to spontaneous firing  Biased towards giving periphery info (orientated) o OPTIC CHASM – half the wires cross from retina to LGN  Cut  BI-TEMPORAL BLINDNESS (HEMIANOPSIA) o LATERAL GENICULATE NUCLEUS (LGN) – within thalamus  most retina signals travel out the eye (in optic nerve) to LGN o PRIMARY VISUAL RECEIVING area – receives signal from LGN  In occipital lobe of cortex  AKA: STRIATE COTEX, b/c white stripes created within this area by nerve fibers running through it o From striate cortex 2 pathways for signals:  1. Temporal lobe (bot blue arrow)  2. Parietal lobe (top blue arrow)  Also reach frontal lobe of the brain o SUPERIOR COLLICULUS – area involved w/ controlling eye movements and other visual behaviors  Receives 10% fibers from optic nerve o Note: signals from half of each retina cross over to opp. brain  Processing the Lateral Geniculate Nucleus o Receptive fields of LGN Neurons  LGN neurons have same center-surround configuration as retinal ganglion cells  Neurons in LGN respond best to small spots of light on retina  like neurons in the optic nerve  If just consider receptive fields of LGN, appears LGN no function o Information flow in the LGN  LGN doesn’t simply receive signal from retina, transmit to cortex  90% optic nerve fibers enter LGN, 10% to superior colliculus  Also receives signals from the cortex, brain stem, other neurons in thalamus, other neurons in LGN  Amount of flow:  1. More input back from cortex than from retina  2. Smallest signal from is output to cortex  Every 10 nerve impulses LGN receives from retina, sends 4 to cortex suggestion information regulation o LGN function: regulates neural info from retina  visual cortex  Organizes input signals, sort by:  which eye, which receptors generated them  Type of environmental information represented in them  Not to create new receptive field properties o Organization by Left/Right eyes  Retinotopic map on LGN determined by recording from neurons  LGN bilateral structure  one in LGN w/ electrode penetrating LGN obliquely (at small angle) both left & right hemisphere, on  Determine location by simulating diff. place on retina till the side of 2 respective thalamus recording spots in LGN respond  1 LG nucleus = 6 layers (view in  Correspondence btwn location on retina and locations on LGN cross section) implies fibers inbtwn connect specific LGN to specific retina spot  Each layer receives signal  Neighboring location on LGN correspond w/ neighboring from only one eye locations retinas  Layers 2, 3, 5 (red layers) receives  Hence, receptive fields of neurons are near each other in from ipsilateral eye LGN or adjacent  IPSILATERAL EYE – eye on  Note: Retinotopic maps overlaps in each of LGN’s 6 layers same side of body as LGN  All neurons perpendicularly inserted in LGN = same  Information received are receptive fields on the retina from temporal half of retina  1 million ganglion cell fibers to each LGN,  Layers 1, 4, 6 (blue layers) receives from contralateral eye  There each fibers connects w/ correct LGN layer  CONTRALATERAL EYE – eye on opp. side of body as LGN  Fibers remap themselves on LGN as they are on retina  Information received are from nasal half of retina  Receptive Fields of Neurons in the STRIATE CORTEX (V1) (area 17)  Each eye’s info sorted into diff. layers, kept separated in LGN o 80% of cortex respond to visual stimuli  PARVOCELLULAR LAYERS – smaller receptive fields, mostly o Striate Cortex research pioneer by David Hubel & Thorsten Wiesel biased towards midget RGCs (not 1-1); Outer layers  Found cells in Striate Cortex with receptive fields similar w/  Process information at fovea, from cones center-surround receptive fields of neurons in retina & LGN  Biased towards processing color/stationary objects  Arranged side by side, not center-surround configuration  MAGNOCELLULAR LAYERS – larger receptive fields, mostly  SIMPLE CORTICAL CELLS – cells w/ side-by-side receptive fields biased towards parasol RGCs (not 1-1); Inner 2 layers  Responds to spots of light or to stationary stimuli  Process information from rods, not near fovea  Diff. simple cells respond to diff. orientations  Biased towards moving objects, sensitive to stimulus  Ex. (A) corresponds best to vertical bars, max excitation o Organization as a Spatial Map  ORIENTATION TUNING CURVE – property of simple cortical cells  RETINOTOPIC MAP on LGN – each point on LGN corresponds to  Dictates relationship btwn orientation and firing point on the retina  found by measuring response of a cell to diff. orientation  ex. Points A, B, C on cup received at A, B, C on retina, correspond to specific place on LGN  COMPLEX CELLS – cortical neurons respond best to moving bar of particular orientation moving across the entire receptive field  Also respond best to particular direction of movement  Their receptive fields not ++/--; instead by which area, when stimulated elicits response in neuron; see (a)  END-STOPPED CELLS – fire to moving lines of specific length or moving corners or angles  Ex. diagram (b): light corner stimulus being moved up/down across retina, neuron’s response increases as corner-shaped stimulus gets longer, stops when too long o Hubel & Wiesel research implication:  Experiment procedures:  Cortical neurons respond to oriented lines indicates neurons in cortex responds to selective patterns of light  not simply respond to any light  allows better perception of objects in environment  almost all can be represented by various orientated lines o Feature Detectors – simple, complex, end stopped cells o Summary Table:  Results: show adaptation selectively affects orientations  Adapting w/ vertical grating cause large increase in contrast threshold for vertically oriented test grating  Psychophysically determined selective adaptation curve (diagram a) very similar w/ orientation tuning for Simple Cortical neuron (diagram b) Do Feature Detectors play a role in perception?  Selective Rearing and Feature detectors  Selective Adaptation and Feature Detectors o SELECTIVE ADAPTATION – if neurons fire for long enough, they o SELECTIVE REARING – animal reared in environment that contains only certain types of stimuli, neurons respond to these stimuli become fatigued (adapted), 2 physiological effects: become more prevalent; follows:  1. Neuron firing rates decrease  NEURAL PLASTICITY (Experience-Dependent Plasticity) –  2. Neuron fires less when stimulus immediately represented  Selective b/c only neurons which respond to particular stimuli response properties of neurons can be shaped be perceptual adapt, other do not experience o Experiment by Colin Blakemore and Grahame Cooper (1970) o Neutral process endows neurons w/ properties making them feature  Placed kittens in striped (horizontal or vertical) tubes detectors  respond best to specific type of stimulus  But neurons RESPONDING to specific stimuli != they help w/
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