motion parallax

Meaning: Motion parallax is a monocular depth cue produced when the observer moves, or when the scene moves past a still observer. Nearby objects sweep quickly across the visual field, while distant objects shift slowly. The brain reads these differences in apparent speed as differences in distance. From a moving car, roadside posts blur past while mountains seem nearly still. Relative to a fixated point, nearer objects appear to move opposite the observer's direction and farther ones with it.

On the MCAT: It is listed among the monocular cues, with linear perspective, relative size, interposition, and texture gradient, in contrast to binocular cues such as retinal disparity and convergence. A typical item describes a passenger on a train and asks which cue explains judging distance, or which objects seem to move faster.

Common confusion: Students mix it up with retinal disparity. Disparity compares the two eyes' images at one moment and needs both eyes. Motion parallax compares images over time as the viewer moves and works with one eye. Also, near objects are not actually moving faster; their apparent speed across the retina is what is greater.