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Chapter 12

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PSYC 212
Remy Allard

Chapter 12The Visual System Depth Perception and StereopsisBinocular visionvision with two eyesCyclopean perceptionthe perception of a unitary reality such as the singleness of visionA Extrapersonal SpaceTwo parameters that have spatial relationship with usobject direciotn and object distanceSpatial relationship dynamic because were not stationary creaturesEvery time we move body spatial relationships change with respect of all external objectsAbsolute depththe actual distance between an external object and the perceiver1 Spatial Frames of ReferenceFrame of referenceset of axes by which the positionplacement of an object is describedTwo ways to classify spatial location of objectsoSpecify objects location independent of viewerallocentric a frame of reference that specifies object locations independent of the vieweraka exocentricSpatial positionsdirections defined in external mannerAdvantagesprovide spatialdirectional information in unambiguous terms ie two cars maintain the same spatial relationship in environment regardless of whose watching themDrawback space perception in humans is actually viewercenteredwe perceive our extrapersonal space only in relation to our own bodyselfSpecifying direction of an object with respect to the viewer is more meaningfuloViewerdependent systemegocentric a frame of reference that specifies object locations relative to a viewers current positionSpecified in the egocentric directionImplies we must have a reference point on our body for judging directions of objectsVisual egocentrethe visual point of reference for egocentric directional judgmentVisually fixated objects generally perceived to be projected to a point midway between two eyesVisual egocentre called cyclopean eye serves as common point of reference when two eyes operate togetherThe direction of an object is judged as if seen through single eye at midpoint between two2 Head and body movement in spaceIf directional judgments are made egocentric frame of reference if we turn head or body cyclopean eye moves with body changing visual perspectiveraising possibility that world would look to move in opposite directionOur impression of the world is stable in spite of frequent headbody movementsAlways have accurate sense of object direction with respect to ourselves regardless of head turns tilts slantsBrains register selfmotion then take into account for brain to produce stable space perceptionHuman dynamics oWith directional judgments any movement of the body is defined within particular frame of referenceoAccepted conventiondefine principal axes of body with respect to gravity in upright stanceoMovement of headbody can be defined with respect to these axesoPrinciple axes X Y and Z defined as being contained within traverse coronal median placesoTransverse plane containing Xaxishorizontally orienteddivides body into 2 halve upper and lowerHead and body rotations around this axis produce tilting movement rolloCoronal planevertically oriented planeseparates body into front and back halfcontains Y axisHeadbody rotations around axis produce forwardbackward slanting movements pitchoMedian plane separates left and right sides of bodycontains Zaxis Rotations around this axis produce headbody turns yawProprioceptive and efferent signalsoSome movement information provided by receptors located in jointsmuscles locating local movementoPrioprioceptive systemthe receptors in the highly accurate system of detectors or muscle contraction and movement around a jointoPriprioceptive perception or proprioceptionkey aspect of somatosensory systemoProprioception is accompanied by another mechanism that provides information on body position and movementoWhenever we consciously move a part of our body a command issued from motor centres of brain to appropriate muscles oPerceptual centres of brain receive copy of the commandefference copyacopy of the motor command that is sent to various parts of the brain including those engaged in perceptual function or corollary discharge a copy of the motor command that is sent to perceptual centreswhen the motor centres of the brain produce a voluntary movement a copy of the neural command to the muscles is made available to the perceptual centres of the brain so that it knows what limb movements have just been initiatedThe vestibular system
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