7Motion Perception Chapter 7 7 Motion Perception Computation of Visual Motion Eye Movements Using Motion Information The Man Who Couldn t See Motion 7 Computation of Visual Motion How would you build a motion detector? Involves a change in position over time. So start with two adjacent receptors separated by fixed distance.
7 A Neural Circuit for Detection of Rightward Motion (Part 2) 7 Computation of Visual Motion Apparent motion: The illusory impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession. Apparent motion was first demonstrated by Sigmund Exner (1875). Neural circuit does not need real motion in order to fire. 7 Still Images from an Animation
7 Computation of Visual Motion Aperture: An opening that allows only a partial view of an 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 (receptive field), the direction of motion of a local feature or part of the object may be ambiguous. 7 Global Motion Detector 7 Computation of Visual Motion We can say something about where the globalmotion detectors are: Lesions in magnocellular layers of LGN impair perception of large, rapidly moving objects. Middle temporal lobe: Plays important role in motion perception. Vast majority of neurons in MT are selective for motion in particular direction.
7 The Medial Temporal Lobe 7 Computation of Visual Motion Experiment with monkeys (Newsome and Pare, 1988) Trained monkeys to respond to correlated dot motion displays. MT areas of monkeys were lesioned. Results: Monkeys needed about 10 times as many dots to correctly identify direction of motion. 7 The Newsome and Pare Paradigm
7 Computation of Visual Motion Disadvantages of using lesion studies to study motion: Invasive. Lesions may be incomplete or may influence other structures. 7 Computation of Visual Motion Motion aftereffect: The illusion of a stationary object that occurs after prolonged exposure to a moving object. Existence of this effect implies an opponent-process system, like that of color vision. Interocular transfer: The transfer of an effect (e.g., adaptation) from one eye to the other. 7 Computation of Visual Motion What does this interocular transfer tell us about the locus of the MAE in the visual system? Result of activities of neurons in a part of the visual system where information collected from two eyes is combined. Input from both eyes is combined in area V1. Recent studies: Locate site of motion aftereffects more precisely.
7 Computation of Visual Motion First-order motion: Motion of an object that is defined by changes in luminance. Second order motion: Motion of an object that is defined by changes in contrast or texture, but not by luminance. 7 Second-Order Motion (Part 1) 7 Eye Movements Eye movements: Smooth pursuit: Eyes move smoothly to follow moving object. Saccade: Rapid movement of eyes that change fixation from one object or location to another. Superior colliculus: Structure in midbrain that plays important role in initiating and guiding eye movements.
7 Smooth Pursuit 7 Eye Movements Why do we perceive the pencil to be in motion in the first case, but perceive the dot to be stationary? Because in the first case there is an eye movement. 7 Eye Movements Six muscles are attached to each eye and are arranged in three pairs: Controlled by extensive network of structures in the brain. When stimulated with electrical signals, eye movements can be observed.
7 Physiology of the Eye (Part 2) 7 Eye Movements Vergence eye movements: Type of eye movement in which two eyes move in opposite directions, done deliberately Saccades: Done deliberately as well 7 Eye Movements Tricky problem of discriminating motion across the retina that is due to eye movements vs. object movements Demonstrate this. Why does this happen? Saccadic suppression: Reduction of visual sensitivity that occurs when one makes a saccadic eye movement; eliminates smear from retinal image motion during an eye movement.
7 Eye Movements Motor system solves problem of why an object in motion may appear stationary by sending out two copies of each in order to move eyes One copy goes to eye muscles Another ( efference copy ) goes to an area of visual system that has been dubbed comparator. Comparator compensates for image changed caused by eye movement, inhibiting any attempts by other parts of the visual system to interpret changes as object motion. 7 The Comparator 7 Using Motion Information How do we use motion information to navigate? Optic array: Describes collection of light rays that interact with objects of the world in front of viewer Optic flow: Changing angular position of points in perspective image that you experience as you move through the world Example of pilot landing her plane: Radial expansion
7 Optic Flow 7 Using Motion Information Are humans actually able to make use of optic flow information? Computer-generated displays of moving dots and lines to stimulate optic flow information 7 Using Motion Information Biological motion: The pattern of movement of living things (i.e., humans, animals).
7 Using Motion Information Avoiding imminent collision: how do we estimate the time to collision (TTC) of an approaching object? Information source: Tau; it relies on information directly from retinal image need to track visual angle subtended by approaching object as it approaches the eye. Not clear if we actually make use of tau. 7 The Man Who Couldn t See Motion Akinetopsia: A rare neuropsychological disorder in which the affected individual has no perception of motion. 7 The Man Who Couldn t See Motion A 47-year-old man reported seeing streams of multiple, frozen images trailing in the wake of moving objects. As soon as motion ceased, the images collapsed into each other. If nothing was in motion and he held perfectly still, his vision was entirely normal.
7 The Man Who Couldn t See Motion The moment anything moved, however, it left a stream of static copies in its path. For example, while out for an evening stroll, he saw a pack of identical dogs lined up behind his West Highland terrier. Akinetopsia is caused by disruptions to cortical area of MT.