Chapter 7

Chapter 7 Key Terms Akinetopsia: A rare neuropsychological disorder in which the affected individual has no perception of motion. Aperture:An opening that allows only a partial view of the object. 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. apparent motion: The (illusory) impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession. biological motion: The pattern of movement of living beings (humans and animals). Comparator: An area of the visual system that receives one copy of the order issued by the motor system when the eyes move (the other copy goes to the eye muscles). The comparator can compensate for the image changes caused by the eye movement. 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. first-order motion:The motion of an object that is defined by changes in luminance. focus of expansion (FOE): The point in the center of the horizon from which, when we are in motion (e.g., driving on the highway), all points in the perspective image seem to emanate. The focus of expansion is one aspect of optic flow. interocular transfer: The transfer of an effect (such as adaptation) from one eye to the other. luminance-defined object: An object that is delineated by changes in reflected light. middle temporal lobe (MT): An area of the brain thought to be important in the perception of motion. motion aftereffect (MAE): The illusion of motion of a stationary object that occurs after prolonged exposure to a moving object. optic array:The collection of light rays that interact with objects in the world in front of a viewer. Term coined by J. J. Gibson. optic flow:The changing angular positions of points in a perspective image that we experience as we move through the world. reflexive eye movement: A movement of the eye that is automatic and involuntary. Saccade:A rapid movement of the eyes that changes fixation from one object or location to another. saccadic suppression: The reduction of visual sensitivity that occurs when we make saccadic eye movements. Saccadic suppression eliminates the smear from retinal image motion during an eye movement. second-order motion: The motion of an object that is defined by changes in contrast or texture, but not by luminance. smooth pursuit: A type of eye movement in which the eyes move smoothly to follow a moving object. superior colliculus: A structure in the midbrain that is important in initiating and guiding eye movements. tau (?): Information in the optic flow that could signal TTC (time to collision) without the necessity of estimating either absolute distances or rates. The ratio of the retinal image size at any moment to the rate at which the image is expanding is tau, and TTC is proportional to tau. texture-defined (contrast-defined) object: An object that is defined by changes in contrast or texture, but not by luminance. time to collision (TTC): The time required for a moving object (such as a cricket ball) to hit a stationary object (such as a batsman?s head). TTC = distance/rate. Vergence: A type of eye movement in which the two eyes move in opposite directions?for example, both eyes turn toward the nose (convergence) or away from the nose (divergence). Summary Like color or orientation, motion is a primary perceptual dimension, which is coded at various levels in the brain. Motion information is used to determine where objects are going and when they are likely to get there, and to help us move through our environment without being hit in the head by flying objects. We can build a simple motion-detecting circuit by using linear filters that delay and sum information (and are followed by nonlinearities). V1 neurons view the world through a small window, leading to the well-known ? aperture problem? (that is, a V1 neuron is unable to tell which elements correspond with one another when an object moves through its receptive field). There is strong physiological and behavioral evidence that the middle temporal area (MT) is involved in the perception of global motion. Aftereffects for motion, like those for orientation or color, can provide important insights into the underlying mechanisms in humans. There appear to be separate systems for analyzing luminance-defined (first-order) motion and contrast- or texture-defined (second-order) motion. The brain has to figure out which retinal motion arises in the world, and which arises because of eye movements. Moreover, the brain must suppress the motion signals generated by our eye movements, or the world will be pretty ?smeared.? Motion information is critically important to us for navigating around our world, avoiding imminent collision, and recognizing the movement of animals and people.