efterbild

Efterbild, or afterimage, is a visual percept that continues to appear after the original stimulus has been removed from view. The term is used in Swedish as etterbild? No, korrekt Swedish is efterbild and corresponds to the English term afterimage. Afterimages arise when the visual system adapts to a sustained stimulus, typically through the bleaching of photopigments in the retina and the processing of color by opponent-color pathways.

Two main types are commonly described: positive afterimage, which preserves the hue and relative brightness of the original stimulus but fades with time, and negative afterimage, where the colors appear inverted or complementary to the original colors on a neutral background. Negative afterimages are often explained by opponent-process theory, where cells sensitive to color pairs such as red-green and blue-yellow respond in opposite directions after adaptation.

Causes and mechanisms: Prolonged exposure to a bright or highly saturated color reduces the responsiveness of the corresponding cone photoreceptors. When the stimulus is removed, the rebound activity in the opponent pathways can produce a persisting image with altered color. Some afterimages involve higher-level cortical processing and can reflect complex patterns or motion.

Demonstrations: A classic experiment is to stare at a bright colored patch for several seconds and then look at a white surface; the perceived color is often the complementary hue. Factors such as stimulus brightness, duration, and ambient lighting influence the strength and duration of an afterimage.

Duration and variation: Afterimages typically last from a fraction of a second to several seconds; in some cases they persist longer, especially for intense stimuli.