Chapter 8. Perceiving Motion

akinetopsia

a condition in which damage to an area of the cortex involved in motion perception causes blindness to motion

aperture problem

occurs when only a portion of a moving stimulus can be seen, as when the stimulus is viewed through a narrow aperature or through the “field of view” of a neuron’s receptive field

• can result in misleading information about the direction in which the stimulus is moving

apparent motion

an illusion of movement that occurs when two objects separated in space are presented rapidly, one after another, separated by a brief time interval

biological motion

motion produced by biological organisms

• experiments used walking humans with lights attached to their joints and limbs as stimuli

coherence

in research on movement perception in which arrays of moving dots are used as stimuli, the degree of correlation between the direction of the moving dots

• 0% means all dots move independently

• 100% means all dots move in the same direction

comparator

a structure hypothesized by the corollary discharge theory of movement perception

• corollary discharge signal and the sensory movement signal meet at the ________ to determine whether movement will be perceived

corollary discharge signals (CDS)

a copy of the motor signal that is sent to the eye muscles to cause movement of the eye

• copy is sent to the hypothetical comparator of the corollary discharge theory

corollary discharge theory

the theory that explains motion perception as being determined both by movement of the image on the retina and by signals that indicate movement of the eyes

delay unit

a component of the Reichardt detector proposed to explain how neural firing occurs to different directions of movement

• delays transmission of nerve impulses as they travel from the receptors toward the brain

event

a segment of time at a particular location that is perceived by observers to have a beginning and an ending

event boundary

the point in time when one event ends and another begins

global optic flow

information for movement that occurs when all elements in a scene move

• indicates the observer is moving, not the scene

illusory motion

perception of motion when there actually is none

image displacement signal (IDS)

in corollary discharge theory, the signal that occurs when an image moves across the visual receptors

implied motion

when a still picture depicts an action that involves motion, so that an observer could potentially extend the action depicted in the picture in his or her mind based on what will most likely happen next

induced motion

the illusory movement of one object that is caused by the movement of another object that is nearby

local disturbance in the optic array

occurs when one object moves relative to the environment, so that the stationary background is covered and uncovered by the moving object

• indicates the object is moving relative to the environment

microstimulation

a procedure in which a small electrode is inserted into the cortex and an electrical current passed through the electrode activates neurons near the tip of the electrode

• used to determine how activating specific groups of neurons affects perception

middle temporal (MT) area

brain region in the temporal lobe that contains many directionally selective neurons

motion aftereffect

an illusion that occurs after a person views a moving stimulus and then sees movement in the opposite direction when viewing a stationary stimulus immediately afterward

motor signal (MS)

in corollary discharge theory, the signal that is sent to the eye muscles when the observer moves or tries to move his or her eyes

optic array

the structured pattern of light created by the presence of objects, surfaces, and textures in the environment

output unit

a component of the Reichardt detector that compares signals received from two or more neurons

• according to Reichardt’s model, activity here is necessary for motion perception

point-light walker

a biological motion stimulus created by placing lights at a number of places on a person’s body and having an observer view the moving-light stimulus that results as the person moves in the dark

real motion

the physical movement of a stimulus

• contrasts with apparent motion

reichardt detector

a neural circuit proposed by Werner Reichardt, in which signals caused by movement of a stimulus across the receptors are processed by a delay unit and an output unit so that signals are generated by movement in one direction but not in the opposite direction

representational momentum

occurs when motion depicted in a still picture continues in an observer’s mind

shortest path constraint

in the perception of apparent motion, the principle that apparent movement tends to occur along the shortest path between two stimuli

transcranial magnetic stimulation (TMS)

presenting a strong magnetic field to the head that temporarily disrupts the functioning of a specific area of the brain

waterfall illusion

an aftereffect of movement that occurs after viewing a stimulus moving in one direction, such as a waterfall

• viewing the waterfall makes other objects appear to move in the opposite direction