Jälkivärjäytymät

Jälkivärjäytymät, known in English as afterimages, are a visual phenomenon that occurs after a bright light source is removed from view. They are perceived as an image that persists in the visual field, often with inverted colors and luminosity compared to the original stimulus. This phenomenon is a result of the physiological processes in the eye and brain's response to light stimulation.

When light enters the eye, it stimulates photoreceptor cells, rods and cones, in the retina. Prolonged exposure to a bright light causes these cells to become fatigued or adapted to the stimulation. When the light source is suddenly removed, the adapted photoreceptor cells respond less vigorously than the surrounding unadapted cells. This difference in neural activity is interpreted by the brain as an image.

The color inversion of afterimages, known as negative afterimages, is a classic example of opponent-process theory of color vision. If one stares at a red object, the red-sensitive cone cells become less responsive. When the gaze shifts to a neutral background, these less responsive red cones will signal a deficit of red, leading to the perception of its complementary color, green. Similarly, staring at a bright object leads to a dark afterimage, and staring at a dark object can produce a bright afterimage.

Afterimages are transient and their duration depends on factors such as the intensity and duration of the initial stimulus, as well as individual differences in visual perception. They are a common and harmless visual experience, often demonstrated in optical illusions and psychological experiments. Understanding afterimages provides insight into the complex mechanisms of human vision and neural adaptation.