Optical Illusion Terms Glossary

A working glossary of the perception and vision-science terms used across this site's illusion and science pages, in plain language first.

Afterimage

A visual sensation that persists, usually in inverted or complementary colors, after you stop looking at the image that caused it. Caused by temporary photoreceptor fatigue and the opponent-process color channels in the visual system. See the negative afterimage illusion.

Ambiguous figure / multistable image

An image that supports two or more valid interpretations, between which perception involuntarily alternates, rather than settling on one reading permanently. The Necker cube and Rubin's vase are classic examples.

Bistable perception

The specific case of multistable perception where exactly two interpretations compete, as with the Necker cube's two possible orientations.

Color constancy

The visual system's tendency to perceive an object's color as stable even as the color of the light illuminating it changes - part of why "the dress" split viewers so sharply, since people made different unconscious assumptions about the ambient lighting in the photo.

Depth cue

Any piece of visual information the brain uses to judge distance and three-dimensional structure from a two-dimensional retinal image - including linear perspective, occlusion, relative size, shading, motion parallax, and binocular disparity.

Figure-ground organization

A Gestalt principle describing how the visual system separates a scene into a foreground "figure" and a background "ground." Illusions like Rubin's vase work by making the figure-ground assignment genuinely ambiguous.

Gestalt principles

A set of rules describing how the visual system groups individual elements into organized wholes - including proximity, similarity, closure, continuity, and figure-ground. See Gestalt principles of perception.

Illusory contour

An edge or boundary that's perceived even though no physical line is drawn there, produced by the visual system's assumption that partially occluded shapes continue behind whatever's in front of them. The Kanizsa triangle is the standard example.

Inattentional blindness

The failure to notice a fully visible but unexpected object or event because attention is engaged elsewhere - demonstrated most famously by the invisible gorilla experiment.

Lateral inhibition

A property of visual neurons where activity in one cell suppresses the activity of its neighbors, which sharpens edges and contrast boundaries. A contributing factor in illusions like the Hermann grid.

Lightness constancy

The visual system's tendency to perceive a surface's relative brightness (lightness) as stable regardless of how much light is actually falling on it - the mechanism exploited by the checker shadow illusion.

Linear perspective

The visual convention by which parallel lines appear to converge as they recede into the distance - the depth cue exploited directly by the Ponzo illusion.

Motion aftereffect

A perceived illusory motion in a stationary scene after prolonged viewing of real motion in one direction, caused by adaptation in motion-detecting neurons. See the motion aftereffect (waterfall) illusion.

Multistable perception

See Ambiguous figure, above.

Peripheral vision

Vision outside the small, high-acuity central region of the visual field (the fovea). Several illusions, including the Hermann grid and peripheral drift illusion, are strongest in peripheral vision and weaken or vanish when viewed directly.

Photosensitive epilepsy

A form of epilepsy in which seizures can be triggered by flashing lights or certain high-contrast repeating patterns, affecting a small percentage of people with epilepsy. See the illusion safety page for more detail.

Predictive processing

A framework describing perception as an active process of prediction and error-correction, where the brain continuously generates expectations about incoming sensory data and updates them based on mismatches, rather than passively receiving a complete picture. See predictive processing and the brain's shortcuts.

Receptive field

The specific region of visual space that, when stimulated, changes the firing rate of a given visual neuron. The size, shape, and internal structure of receptive fields (center-surround organization, in particular) explain several low-level illusions.

Size constancy

The visual system's tendency to perceive an object's true physical size as stable even as its distance - and therefore its retinal image size - changes. The mechanism behind the Ponzo illusion and the moon illusion.

Simultaneous contrast

The way a color or shade's perceived appearance shifts depending on the colors or shades immediately surrounding it. See the simultaneous contrast illusion.

Visual cortex

The region of the brain, located in the occipital lobe, primarily responsible for processing visual information, organized into a hierarchy of areas (V1 through V5 and beyond) that each specialize in different aspects of vision. See the role of the visual cortex.