afterimage

An afterimage is a visual impression that persists after the original stimulus has ended. Afterimages can involve color, motion, or form, and they arise from neural adaptation at retinal or cortical levels. They are commonly described as chromatic (color) afterimages, motion afterimages, and orientation or pattern aftereffects.

Chromatic afterimages result from selective adaptation of the cone photoreceptors and the opponent-process pathways in the retina and brain. Prolonged exposure to a color fatigues the corresponding channels, and when the stimulus is removed, the complementary color is perceived. For example, staring at a bright red image may produce a greenish afterimage on a neutral surface. Blue-yellow afterimages can follow exposure to certain greenish-yellow stimuli. These afterimages are typically brief, fading in seconds to a couple of minutes.

Motion afterimages arise from adaptation of motion-sensitive neurons in the visual cortex. After viewing motion in one direction, a stationary scene can appear to move in the opposite direction. The motion aftereffect is often demonstrated with a waterfall or moving stripes. Pattern- or orientation-related aftereffects, such as the tilt aftereffect, occur when exposure to a tilted pattern shifts the perceived orientation of subsequently viewed lines, due to orientation-selective neurons in the visual cortex.

Duration and individual variation: the strength and duration of afterimages depend on stimulus intensity, duration of exposure, and observer differences. In healthy individuals, afterimages are common and benign, typically resolving without intervention. Persistent or unusual afterimages can sometimes indicate underlying visual or neurological conditions and may warrant clinical evaluation.