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PSYC2030 - Sensation and Perception Chapter 7

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York University
PSYC 2220
Jennifer Steeves

Lecture 7 Motion Perception *Check out demos on the books website Detecting Motion • Why do we process vision • How good is the Human visual System at detecting motion? o Velocity  If something moves very slowly it is usually not perceptible (water slowly falling down a glass)  It’s the same if something moves too fast. If something moves too fast, there will not be a percept of motion (jumping) o Distance  Fovea versus peripheral visual field o Luminance  On a plain background you can notice motion easier than a busy distracting background o Duration  About more than 5 changes per minute o Relative versus Absolute motion  The more stuff you have in a scene, the easier it is to determine movement relative to each object Colour is processed by the retina, goes through the LGN, primary cortex, and then the “what” pathway. The dorsal pathway is the parallel pathway and is the “where” cortex. There is evidence for motion activity in both pathways. The first area to receive feed is V1 (there are motion sensitive cells in columns, just like colour). The other area that is indicated is the Medial Temporal (MT). If it is damaged, then patients report problems to their motion processing. Next is the MST, responsible for taking care of visual stimuli getting bigger, small, and orientation. E.g. looking at something, walking towards it, and everything moving around your visual field other than the object of focus. Computation of Visual Motion • Apparent motion o The (illusionary impression of smooth motion resulting form the rapid alternation of objects appearing in different locations in rapid succession o Motion detector circuit does not need real motion in order to fire o There is ambiguity, thus making it difficult to determine the actual movement (only being able to see a part/bit of the object) • Correspondence problem (motion): the problem faced by the motion detection system of knowing which feature in frame 2 corresponds to a particular feature in frame 1 • Aperture Problem: The fact that when a moving object is viewed through an aperture (or a receptive field), the direction of motion of a local feature or part of the object may be ambiguous • Global-motion detectors are found in: o Lesions in magnocellular layers of LGN impair perception of large, rapidly moving objects o Cells in V1 are direction-selective o Middle temporal lobe (MT) plays an important role in motion perception  The vast majority of neurons in MT are selective for motion in a particular direction  Activity of MT cells in non-human primates reflects conscious percept o For moving lines both V1 and MT fire for the perceived direction of motion. o It is possible to have two different responses between the V1 and MT o Plaid Stimuli: One perceives motion in different direction than when the moving lines are shown one at a time  E.g. Two perpendicular sets of lines o V1 cells only care about basic directions of movement (up, down, left, right) and MT cells can compensate for basic and more complicated directions (diagonal movement). • Newsome and Pare (1988) conducted a study on motion perception in monkeys o Trained monkey to respond to correlated dot motion displays o The MT area of the monkeys was lesioned o Result: Monkeys needed about ten times as many dots to correctly identify direction of motion • Motion aftereffect (MAE): The illusion of motion of a stationary object that occurs after prolonged exposure to a moving object o Existence of MAE implies an opponent process system, like that of colour vision o MAE exhibits intraocular transfer: The transfer of an effect (such as adaptation) from one eye to another  Therefore, MAE must occur in neurons that respond to both eyes • Input from both eyes is co
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