motion

saccadic eye movements

rapid shifts from one point to another

smooth pursuit

continuous smooth movement of eyes to track a moving object

corollary discharge signal (CDS)

when the brain sends a signal to move the eyes, it also sends a copy to visual motion areas

motion aftereffect

a stationary scene appears to move in the opposite direction after prolonged viewing of motion in one direction, caused by fatigue in motion-selective neurons

aperture problem

ambiguity in perceiving the true direction of a moving object when only a small portion of it is visible through an opening

area MT/V5

brain area critical for motion perception, nearly all neurons are direction-selective and have large RFs

apparent motion

the illusion of movement when static images are shown rapidly in sequence, the visual system links them to continuous motion

segmentation from motion

using motion cues to separate one object from another or from its background, even when color or texture stay constant

why motion perception is hard

both objects and our eyes move, so the brain must distinguish retinal image motion caused by world from that caused by eye movements

retinal image during eye movement

any motion makes the image move across the retina, brain must account for this to perceive stability

eye movement compensation

brain uses motor signals and eye muscle feedback to determine whether motion on the retina is due to the object or the eyes

real motion cells

neurons that respond when an object truly moves across their RF, not just when motion is caused by eye movement

anticipatory receptive fields

neurons that shift their RF just before a saccade, predicting where stimuli will appear after the eye moves

frontal eye fields (FEF)

cortical region that initiates eye movements and sends predictive signals to the superior culliculus

purpose of corollary discharge

distinguishes self-generated eye motion from motion in the world, keeping visual perception stable

motion detection requirements

a neuron needs input from at least two retinal positions and must be sensitive to both the order and the timing of their activation

direction coding

determined by the order in which RFs are stimulated

speed coding

determined by the delay and spatial distance between RFs that activate the same motion neuron

motion sensitive neurons

found as early as V1, tuned to specific motion directions and speeds

tuning curves

graphs showing a neuron’s firing rate as a function of motion direction or speed, peaking at its preferred stimulus

delay lines

neural circuits that delay one input so signals from two points coincide, allowing detection of a specific motion direction and velocity

how delay lines explain motion illusions

when sequential stimuli match a neuron’s preferred timing, apparent motion is perceived even without continuous movement

solution to aperture problem

higher level areas (MT/V5) combine many local motion signals to compute global object motion

coherent motion

a stimulus where a proportion of moving dots share the same direction