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This 6 page Class Notes was uploaded by Caoimhe Notetaker on Tuesday October 13, 2015. The Class Notes belongs to Nsci3300 at Tulane University taught by Paul Colombo in Summer 2015. Since its upload, it has received 61 views. For similar materials see Brian and behavior in Psychlogy at Tulane University.
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Date Created: 10/13/15
Brain and behavior 10052015 Chapter 6 The Visual System Vision is light re ected into your eyes from objects around you So NOTHING can see in the dark Light can be thought of as photons discrete particles of energy or waves Wavelength involved in perception of color Intensity involved in perception of brightness lrises regulate amount of light reaching the retinas give eyes its color 0 Light enters through the pupil hole in the iris Adjustment in pupil size in response to change in illumination compromise between sensitivity and acuty 0 When the pupil is constricted high illumination sharper more focused image 0 When the pupil is dilated to let in more light sacri ces acuty Behind pupil is the lens which focuses light 0 Accommodation process of adjusting con guration of lens to focus image Why two eyes 0 Vertebrates have two eyes bc Two sides they re both in front sacri cing ability to see behind us so we can perceive depth a difference between 3d and 2d objects 0 Binocular disparity the difference in position of same image on the two retinas Greater for close objects After passing through pupil and lens light reaches the retina Retina converts light to neural signals conducts signals towards CNS and participates in processing of the signal 0 Composed of 5 layers Receptors Horizontal cells Bipolar cells Amacrine cells Retinal ganglion cells 0 Communicates chemically via synapses and electrically via gapjuncUons Light reaches receptor layer after passing through other 4 Once receptors are activated neural message is transmitted back out through retinal layer to ganglion where axon projects across inside of retina before gathering in a bundle and exiting eye 2 problem with inside out arrangement of eye 0 Incoming light is distorted Fovea works to decrease this issue a Specialized for high acuity n Thinning of ganglion cell layer Fovea decreases distortion of incoming light 0 Blind spots exist Completion works to combat this issue a Visual system lls gap n Completion used commonly in visual system 0 Surface interpolation Extract key info edges and their locations and infers rest Receptors 2 types 0 Rods and cones l duplexity theory Species active only during day time cone only retinas Species active only at night rod only retinas Photopic vision cone mediated 0 With good lighting high acuity colored perceptions 0 Only a few cones converge to receive information from a few cones Scotopic vision rodmediated o More sensitive in dim light lacks details and color 0 Output of several 100 rods converge on a single ganglion cell No rods at Fovea more rods in nasal hemiretina than temporal Humans have 2 spectral sensitivity curves 0 Purkinje effect interesting visual effect in transition between sensitivities what is brightest changes 0 Test photopic curve with light at retina 0 Test scotopic curve with light at peripherals Eye movement Eyes continuously moving enables us to see during xation by keeping image moving in retina otherwise it would fade 0 Types of involuntary xational eye movement Tremors Drifts Saccades Visual transduction convergence of light to neural signals Rhodopsin absorptions of light 1st step in rodmediated vision Rhodopsin Gprotein coupled receptor that responds to light In dark rods open sodium channels keeping rods depolarized glutamate Light bleaches receptors closing channels hyper polarizes rods decreases glutamate release Retina Geniculate striate Pathways Conduct signals from each retina to the primary visual cortex striate via lateral geniculate nuclei of the thalamus Visual information Signals from left visual eld reach the right primary visual cortex either ipsilaterally from temporal hemiretina of right eye or contralaterally from nasal hemiretina of left eye When info enters layer of lateral geniculate stays segregated so visual system knows where info came from The retinageniculate striate systems is retinotopic each level of the system is organized like a map of the retina At least 2 parallel channels of communication ow through each lateral geniculate nucleus 0 Parvocellular layers top 4 cones responsive to colors details slow still objects 0 Magnocellular layers bottom 2 rods responsive to movement Contrast enhancement highlights edges Receptive eld of visual neurons area of visual eld within which it is possible for a visual stimulus to in uence the ring of the neuron Record the responses of neurons to various simple stimuli with in receptive eld in order to characterize the types of stimuli that most in uences activity Hubel and Wiesel Found 0 At each level the receptive elds in foveal area less than peripheral area 0 Consistent with the fact that fovea mediates high acuity vision 0 All neurons had circular receptive elds 0 All neurons were monocular Many neurons at each level had receptive elds that comprised an excitatory area AND an inhibitory area separated by a circular boundary On center cells respond to light shone in the central region of receptive eld with quotonquot ring and light shone in peripherals with quotoffquot Off center opposite reaction 0 Both respond best to contrast Increase contrast increase in uence of ring rate Complex cells differ from simple cells 0 Complex cells 0 Larger receptive elds 0 Binocular o Cant divide receptive elds into onoff regions Hubel and Weisel important conclusions about organization of visual cortex 0 Functional vertical columns right angles to cortical layer 0 Location is in uenced by location on retina of columns visual eld 0 Preferences of neurons increase complexity from retinal thalamus lower level lV simple cortical cells complex cells because neurons with simple preferences converged on neurons with more complex preferences Seeing color Component trichromatic theory Thomas young 1802 o 3 cones with different spectral sensitivity Opponent process theory Ewald Hering 1878 o 2 classes of cells for color and brightness o Hyperpolarized hypopolarized 0 Red or green 0 Blue or yellow Complementary colors ie redgreen produce white gray when combined equally Neither theory can account for color constancy o The tendency of an object to stay the same color despite major changes in wavelengths of light that it re ects Retinex theory of color vision the color of an object is determined by re ection Primary visual cortex located in posterior region of occipital lobe o Damage produces scotoma area of blindness Most visual info enters primary visual cortex via lateral geniculate nuclei The info from two lateral geniculate nuclei is received in primary visual cortex combined then segregated into multiple pathways Dorsal Stream ows from primary visual cortex to dorsal prestriate cortex to posterior panetal cortex 0 Respond to spatial stimuli Ventral stream ows from primary visual cortex to ventral prestriate cortex to inferotemporal cortex 0 Respond to characteristics of an object Damage to some areas of cortex may abolish certain aspects of vision while leaving other unaffected where vs what theory Prosopagnosia visual agnosia for faces 0 Recognition de cit are not restricted to faces Akinetopsia de ciency in ability to see movement progress in a normal smooth fashion associated with MT area
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