Nachbild

Nachbild, literally “afterimage” in German, is a visual perceptual phenomenon in which an image continues to appear after the original stimulus has been removed. Afterimages can arise when a person fixates on a bright or high-contrast scene, a colored pattern, or any stimulus that produces sustained adaptation in the visual system.

There are two main types. Positive afterimages preserve the colors of the original stimulus and are typically short-lived, resulting from temporary bleaching of photoreceptors that slowly recovers. Negative afterimages, by contrast, show colors that are complementary to the originals and reflect the activity of the retina and brain’s color-opponent pathways. The latter often emerges after longer or more intense stimulation and can persist slightly longer than positive afterimages.

Mechanisms involve both retinal processes and higher-level processing. Retinal adaptation reduces the responsiveness of photoreceptors to continued stimulation, while cortical processing in the visual pathways maintains residual activity that outlasts the stimulus. The duration of an afterimage varies with luminance, exposure duration, and individual differences, generally from fractions of a second to several seconds, though stronger stimuli can produce longer persistence.

Context and applications: afterimages are a common laboratory and everyday phenomenon, useful for studying color vision and sensory adaptation. In addition to perceptual afterimages, the term can describe residual image effects on displays, such as phosphor persistence on older CRT screens or transient image persistence on modern monitors under certain conditions. Nachbild illustrates how the visual system encodes and briefly stores information about recent stimuli, revealing the dynamics of adaptation and color processing.