Chapter 12

Chapter 12 - Chapter 12- The Visual System: Depth...

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Chapter 12- The Visual System: Depth Perception and Stereopsis A. Extrapersonal Space Why binocular vision? Many body organs are paired; better reason- survival advantage, better sensory functions such as light detection/discrimination, depth perception, eye-hand coordination, visual resolution, object detection, and attention to fixated objects. - Cyclopean perception : a unified vision of the world built on input from the two eyes. - There are 2 parameters that define the spatial relationship btw us + objects in the external world: object direction and object distance . - Absolute depth : actual distance btw an external object and the perceiver. - What types of mechanisms are at our disposal for perceiving absolute depth? Spatial frames of reference - Frame of reference= set of axes by which the position/placement of an object is described. - 2 general ways of classifying the spatial location of objects: o Allocentric : viewer-independent system; objects are represented in relation only to each other; they maintain spatial relationships regardless of the viewer’s position. o Egocentric : viewer-dependent system; object positions in this frame of reference are specified in terms of egocentric direction. - Egocentric FOR implies that we need a reference point on our body for judging the directions of objects; this visual point = the visual egocenter (aka cyclopean eye ) . - Psychophysical tests show on directional judgement show that visually fixated objects are generally perceived to be projected to a point midway btw the two eyes; i.e. object direction is judged as if seen through a single cyclopean eye located at the midpoint btw the two. Head and body movements in space - Presumably, the cyclopean eye/visual egocenter moves in accordance w/ our head/body. o However, we do not see the visual world as moving in the opposite direction as us (as though panning a camcorder in one direction). o Thus, our brain must register self-motion and produce stable space perception. Human dynamics: - As w/ directional judgements, body movements must be defined w/in a frame of reference. - Generally, the principal axes of the upright body w/ respect to gravity are used as a FOR: o X-axis: contained w/in the transverse plane ; horizontally oriented; divides body into upper/lower halves; head/body rotations around this axis produce a tilting movement called roll. o Y-axis: contained w/in the coronal plane ; vertically oriented; divides body into front/back halves; head/body rotations around this axis produce forward/backward slanting movements called pitch .
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o Z-axis: contained w/in the median plane ; vertical; separates body into left/right halves; rotations along this axis produce head/body turns called yaw . Proprioceptive and efferent signals:
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Chapter 12 - Chapter 12- The Visual System: Depth...

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