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Chapter 8

Sensation and Perception Psych 367 Chapter 8.docx
Sensation and Perception Psych 367 Chapter 8.docx

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University of Alberta
Douglas Wylie

Chapter 8 Perceiving motion Functions of motion Motion helps us understand events in our environment - Motion perception Is also essential for our ability to move through the environment - This movement is called optic flow and it provides information about the walkers direction and speed - Motion agnosia: lost the ability to perceive motion o Difficult to follow dialogue because she couldn’t see motions of a speaker face or mouth o Sudden appearance of people or objects Motion attracts attention - attention capture: The ability of motion attracts attention - This effect occurs when you are consciously looking for something, also while you are paying attention to something else Motion provides information about objects - Soon as all the elements of the bird begin moving in the same direction, the bird becomes visible - Moving relative to an objects have help perceive its shape more accurately - Motion relative to objects is constantly adding to the information we have about the object Studying motion perception When do we perceive motion - Real motion: actual motion of an object - Illusory motion: perception of motion when there is none o Apparent motion study - Induced motion occurs when motion of one object usually a large one, causes a nearby stationary object (usually smaller) to appear to move (clouds moving makes the moon seem to move) - Motion after effects: occur after viewing a moving stimulus for 30 – 60 seconds and then viewing a stationary stimulus, when appear to move o Waterfall illusion Comparing real and apparent motion - These two type of motion have much in common - Larsen: o Presented three types of displays to a person in an fMRI scanner o Control condition: 2 dots in slightly different positions were flashed at same time o Real motion display: small dot moved back and forth o Apparent motion display: dots were flashed one after the other so they appeared to move back and forth o Activation associated with apparent motion was similar to real motion Motion perception: information in the environment - Motion perception cant be explained by considering just what is happening on the retina - Gibsons approach involves looking for information in the environment that provides information for perception - Optic array: the structure created by the surfaces, textures, and contours of the environment o He focused on how movement of the observer causes changes in the optic array - Local disturbances in the optic array: occurs when one object moves relative to the environment , covering and uncovering the stationary background - Jermy’s image is stationary on the retina, the same local disturbance information that was available when Maria was keeping her eyes still remains available when she is moving her eyes, and this local disturbance information indicates that Jeremy is moving. - This local disturbance in the optic array provides information that Jeremy is moving relative to the environment - The fact that everything moves at once is called global optic flow, this signals that maria is moving but that the environment is not Neural firing to motion across the retina Motion of a stimulus across the retina: the aperture problem - As the stimulus sweeps across the retina, it activates directionally selective neurons in the cortex that respond to oriented bars that are moving in a specific direction - The response of single directionally selective neurons does not provide sufficient information to indicate the direction in which an object is moving - As the pole moves to the right, to moves across the receptive field in the direction indicated by the red arrows and the neuron fires - If you were able to focus only on what was happening inside the aperture, you probably noticed that the direction that the front edge of the pencil was moving appeared the same whether the pencil was moving horizontal to the right or up and to the right - Movement of an edge across an aperture occurs perpendicular to the direction in which the edge is oriented ( pencil was vertical, the movement was horizontal) - So the activity of this neuron would not provide accurate information about the direction of the pencils motion - Aperture problem: The fact that viewing only a small portion of a larger stimulus can result in misleading information about the direction in which the stimulus is moving - The visual system appears to solve the aperture problem by pooling the responses of a number of neurons like our complex cell - This occurs in the medial temporal cortex, a nucleus in the dorsal stream (where) which contains a large number of directionally selective neurons and which we will see is important for movement perception - Pack and Born: determined how neurons in the monkeys MT cortex responded to moving oriented lines like the poll or pencil o They found that the MT neurons initial response to the stimulus, at about 70 msec after was determined by the orientation of the bar o 140 msec after presentation of the moving bars, the neurons began responding to the actual direction in which the bars were moving o MT neurons receive signals from a number of neurons in the striate cortex and then combine these signals to determine the actual direction of motion - A neuron could use information about the end of a moving object to determine its direction of motion - Neurons that could signal this information, because they respond to the end of moving objects, are found in the striate cortex - The visual system apparently can solve this problem by o Using information for neurons in the MT cortex that pool the responses of a number of directionally selective neurons, o Using information from neurons in the striate cortex that respond to the movement of the ends of objects Motion of arrays of dots on the retina Neural firing and the perception of moving dot stimuli - Newsome: created moving dot displays in which the direction of motion of individual dots can be varied - Coherence: to indicate the degree to which the dots move in the same direction - Newsome o Determined the relationship between a monkeys ability to judge the direction in which dots were moving o The response of a neuron in the monkey MT cortex o As dots coherence increased  The monkey judged the direction of motion more accurately  The MT neurons fired more rapidly o The MT neurons always fired faster than its baseline rate - This experiment is important because it directly measured the relationship between physiological processes and experience and action which is usually inferred Effect of lesioning and micro stimulation - Mt cortex is important for motion perception - A monkey with intact MT can begin detecting the direction of dots are moving when coherence is as low as 1-2 % - After MT lesion, its 10-20% - Micro-stimulation o Achieved by lowering a small wire electrode into the cortex and passing a weak electrical charge through the tip of the electrode o This weak shock stimulates neurons that are near the electrode to and causes them to fire - Neurons are organized in orientation columns in the cortex, with neurons in the same column responding best to
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