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Psychology 2220A/B
Jennifer Hoshooley

Chapter 6Light waves of electromagnetic energy that are between 380 and 760 nanometres in length Rattlesnakes can see infrared waves which we cannot and can see warmblooded prey in complete darknessWavelength plays an important role in the perception of colourIntensity plays an important role in the perception of brightnessDepth of focus a greater range of depths are simultaneously kept in focus on the retinasThe retina is composed of five layers of different types of neuronsReceptorsHorizontal cells specialized for lateral communicationBipolar cellsAmacrine cells specialized for lateral communicationRetinal ganglion cellsLateral communication communication across the major channels of sensory inputTwo visual problemsIncoming light is distorted by the retinal tissue through which it must pass before reaching the receptorsBlind spotBlind spot a gap in the receptor layer in order for the bundle of retinal ganglion cell axons to leave the eyeFovea an indentation 033 cm in diameter at the center of the retina that is specialized for highacuity vision for seeing fine details Thinning of retinal ganglion cell layer at the fovea reduces the distortion of incoming lightCompletion when the visual system detects a straight bar going into one side of the blind spot and another straight bar leaving the other side it fills in the missing bit for youSurface interpolation the process by which we perceive surfaces the visual system extracts information about edges and from it infers the appearance of large surfacesCones coneshaped receptors responsible for colour when an abundant amount of light is presentRods rodshaped receptors responsible for light sensitivity under dim lightDuplexity theory of vision the theory that cones and rods mediate different kinds of visionPhotopic vision conemediated vision that predominates in good lighting and provides highacuity coloured perceptions of the world Low sensitivity high acuityScotopic vision in dim illuminations that is rodmediated that lacks both detail and colour of photopic vision High sensitivity low acuityIn the scotopic system the output of several hundred rods converge on a single retinal ganglion cell where in the photopic system only a few cones converge on reach retinal ganglion cell to receives inputOnly cones are present in the foveaMore rods are in the nasal hemiretina the half of each retina next to the nose than in the temporal hemiretina the half next to the templesSpectral sensitivity curve a graph of the relative brightness of lights of the same intensity presented at different wavelengths Photopic spectral sensitivity curve can be determined by having subjects judge the relative brightness of different wavelengths of light shone on the foveaScotopic spectral sensitivity curve can be determined by asking subjects to judge the relative brightness of different wavelengths of light shone on the periphery of the retina at an intensity too low to activate the conesPurkinje effect the shift in relative brightness according to the photopic and scotopic spectral sensitivity curveVisual field the entire area that you can see at a particular momentTemporal integration the eyes continually scanning a visual field and our visual perception at any instant is a summation of recent visual informationSaccades small and quick jerky movements from our eyes Fixational eye movements tremor drifts and saccadesTransduction the conversion of one form of energy to another Visual transduction is the conversion of light to neural signals by the visual receptorsPigment any substance that absorbs lightRhodopsin a pigment exposed to intense light losing its colour but returned in the dark used in rodmediated visionRetinageniculate striate pathways conducts signals from each retina to the primary visual cortex or striate cortex via the lateral geniculate nuclei of the thalamusAll signals of the left visual field reach the right primary visual cortex either ipsilaterally from the temporal hemiretina of the right eye or contralaterally via the optic chiasm from the nasal hemiretina of the left eye Opposite is true of all signals from the right visual field Retinotopic each level of the system is organized like a map of the retinaParvocellular layers they are composed of neurons with small cell bodies Responsive to fine pattern details and to stationary or slowly moving objectsMagnocellular layers they are composed neurons with large cell bodies Responsive to movementMach bands they enhance the contrast at each edge and make the edge easier to seeContrast enhancement Every edge we look at is highlighted for us by mechanisms of our nervous systemsLateral inhibition example of where firing is most intense by intense light and the least where it is right beside dim lightReceptive field the area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuronHubel and Wiesel stated thatAt each level the receptive fields in the foveal area of the retina were smaller than those at the periphery this is consistent with the fact that the fovea mediates highacuity visionAll the neurons had receptive fields that were circularAll the neurons were monocular each neuron had a receptive field in one eye but not the otherMany neurons at each of the three levels of the retinageniculate0striate system had receptive fields that comprised an excitatory area and an inhibitory area separated by a circular boundaryOncenter cells respond to lights shone in the central region of their receptive fields with on firing and to lights shone in the periphery of their receptive fields with inhibition followed by off firing when turned offOffcenter cells opposite pattern respond with inhibition and off firing in response to lights in the center of their receptive fields and with on firing to lights in the periphery of their receptive fields Simple cells have receptive fields that can be divided into antagonistic on and off regions and are thus unresponsive to diffuse lightComplex cells are more numerous and have rectangular receptive fields respond best to straightline stimuli in a specific orientation and are unresponsive to diffuse light They differ because they have larger receptive fields into static on and off regions It also responds to a particular straightedge stimulus of a particular orientation regardless of its position within the receptive field of that cells Complex cells are also binocular and respond to stimulation of either eye Ocular dominance they respond more robustly to stimulation of one eye than with the same stimulation of the otherIn a given column all monocular neurons and binocular neurons that display dominance are dominated by the same eye As the electrode advances the tip moves alternately through columns of right and left eye dominance
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