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Chapter 7

Chapter 7 - PSYB51

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Mathias Niemier

Chapter 7 – Motion Perception  Motion after effect (MAE): illusion of motion of stationary object that occurs after prolonged exposure to moving object. After viewing motion in constant direction for sustained period of time (at least 15 seconds) see any stationary objects that we view subsequently as moving in opposite direction. o Caused by opponent processes for motor detection Computation of Visual Motion Apparent Motion  Apparent motion: illusory impression of smooth motion resulting from rapid alternation of objects that appear in different locations in rapid succession. Demonstrated by Sigmund Exner  Exner set up contraption that would generate electrical sparks separated from each other by very short distance in space and time o Even though there were 2 separate sparks – observes said they saw single spark Correspondence and Aperture Problems  Although apparent motion turns out to be win for motion detection circuit, 2 more of classic perceptual “problems” that we’ve discussed  Aperture: opening that allows only partial view of object  Correspondence problem: problem faced by motion detection system of knowing which feature in frame 2 corresponds to particular feature in frame 1  Difficulty for motion detection system is how does it know which circles in frame 2 correspond to which circles in frame 1 o Because we have motion detectors for all directions, one detector will sense diagonal motion implied by matching  Aperture problem: fact that when moving objects is viewed through aperture (or receptive field), direction of motion of local feature or part of object may be ambiguous  Every V1 cell sees world through small aperture o None of V1 cells can tell with certainty which visual elements correspond to one another when object moves, even when no mask is present o Solution to problem is to have another set of neurons listen to V1 neurons and integrate potentially conflicting signals Detection of Global Motion in Area MT  Lesions to magnocellular layers of LGN impair perception of large, rapidly moving objects  Info from magnocellular neurons feeds into V1 and passed onto middle temporal lobe/MT  Middle temporal lobe (MT): area of brain thought to be important in perception of motion o Vast majority of neurons in MT are selective for motion in one particular direction, but show little selectivity for form or color o MT cells do correspond to orange neuron  Newsome and Pare trained group of monkeys to respond to correlated dot motion displays o No single dot in diagrams is sufficient to determine overall direction of correlated direction, neuron must integrate info from many local motion detectors  Monkeys in Newsome and Pare’s study could recognize correlated motion direction when only 2-3% of dots were moving in direction o Lesioned MT areas and monkeys needed about 10 times as many correlated dots to identify direction of motion o Ability to discriminate wasn’t impaired  Lesion studies are central to understanding of specificity of brain areas.  To test involvement of MT neurons in global motion perception more directly, Newsome trained new group of monkeys to discriminate correlated motion directions, then poked around in monkey’s MT areas to find groups of neurons that responded to one particular direction o Once found new group of neurons that responded, showed monkey new set of stimuli and electrically stimulated identified MT neurons Motion Aftereffects Revisited  Existence of MAE implies opponent process system much like in color vision  When we view stationary object, responses of neurons tuned to different directions of motion are normally balanced – neurons sensitive to upward motion fire at same rate as neurons sensitive to downward motion, so signals cancel out and no motion is perceived o When looking at waterfall for long time, detectors sensitive to downward motion become fatigued - when switch gaze to stationary object, neurons sensitive to upward motion fire faster than tired downward-sensitive neurons, and perceive rocks as drifting up  Interocular transfer: transfer of an effect (such as adaption) from one eye to other  Fact that strong MAE is obtained when one eye is adapted and other tested means that effect must be reflecting activities of neurons in part of visual system where info collected from 2 eyes is combined  Evidence suggests that MAE in humans is caused by same brain region shown to responsible for global-motion detection in monkeys – cortical area MT  First order motion: mot
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