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Chapter 9

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McGill University
PSYC 212
Remy Allard

Chapter 9The Visual System Retinal Processing and Early VisionRetinathin film of cells that lines inside back wall of the eyeballA The photoreceptor arrayOptical eye designed to cast visual image exactly on the retinaoimage actually projected on very outer margins of retina within inside wall of eyeoalong margin where photoreceptors foundphotoreceptorsembedded within fibrous matrix that holds eyeball togetheroserves as lightabsorbing elements of retinafirst events of vision occuroabsorption of light produces neural signalophotoreceptors function as transducing elements of the visual system converting energy in light distributed across retinal image into a series of signals that are processed by other neurons in retinaphotoreceptors lie immediately in front of a layer of cells known as pigment epitheliumocontain pigmented material that absorbs stray light light that isnt captured by photoreceptorsa certain amount of incident light bypasses the photoreceptorsis then absorbed by pigment epitheliumvisual function can be impaired in cases where pigment is reducedabsent in cells because unabsorbed stray light interferes with normal processing of visual infoalbinoslack individuals who generally lack pigmented material in the pigment epitheliumskin tissue1 Rods and Cones2 types of photoreceptor cellsrods and conesrodsmore numerous 120 millionconesless 6 millionnames given because of shaperod and cone photoreceptors mediate different visual functionsoboth serve as front end of two different visual systemsrodsnight visionconesday visionowe must be able to function across wide range of light conditionsovisual system automatically changes from one mechanism to another when we go from daylight to nighttime conditionsphotoreceptor distribution across the retinaodensities of rod and cone photoreceptors change from one retina to the otheroretina divison2 halves nasal towards the nose temporal towards the templeomidpoint between the retina contains small point foveaooptic diskpoint in nasal retina through which retinal nerve fibres exit the eyeballno photoreceptors rodscones herebecause nerve fibres from retina exit the eyeball at this locationno light absorption heresmall part of nasal retina is blind aka blind spotodensities of cone photoreceptors quite low in peripheral parts of retina on both nasal and temporal sidesshoots upreaches maximum value at centre of fovea22differencedramatic7000 conesmm vs 150000 conesmmday vision mediated throughout all parts of the retinaorods more numerous in peripheryreach peak at 5 mm on either side of the fovea numbers drop near the centre of the ratinano rods in foveaall night vision functions occur through peripheral parts of the retina starting from outside foveafoveal retina specialized for detailed visionowhenever we scrutinize something in detail we direct our eyes so that its image falls on the foveaolarge concentration of cones in fovea dedicated to processing image detail primarily because of the way cones are connected to other neurons on the retinaoalthough day vision serves all parts of the retina fovea is further specialized for scrutinizing the details of a given objectperipheral retina is specialized for low light detectionobecause of absence of rods in the fovea it is severely handicapped under very low light conditions or when objects being viewed are dimoperipheral retina compensates for loss through large concentration of rod photoreceptorsovisual function under low light conditions mediated by peripheral retina2 Visual Transductiontransductory system unique for each sensory systemfor visual systemfirst step is transforming energy in the stimulus light into neural activity within those neurons photoreceptors located in first stage of visual pathwayophototransductionphotoreceptors contain photopigment materialofirst event in phototransductionocapture of light photons by photoreceptors to generate an electrical signaloacoomplished by photopigmentin high concentration in rods and conesphotopigment in all mammalian photoreceptorsrhodopsinorodcone photoreceptors can be divided into 2 segmentsinner segment
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