An afterimage is an image that continues to be perceived after the physical stimulus that it originated from disappears from the observer’s visual field. The afterimage can be a result of an exposition to a grayscale pattern, a colored pattern, or even to a motion stimulus. There are different kinds of afterimages, for example, the positive one will be the same with the physical stimulus, while the negative one will result in the opposite in terms of luminance, colors, or direction of the physical stimulus that generates it.
In everyday experience it is not so uncommon to experience several types of aftereffects. The observation done by Addams  while watching the Falls of Foyers, near to the Loch Ness in Scotland, is probably the most famous description of the motion aftereffect outside a scientific laboratory. The motion aftereffect was already observed by Aristotle and Lucretius in the ancient times, but the description done by Addams remains the most cited example: if an observer watches a fall for about 60 s and, after that, she/he moves their gaze to the wood nearby, everything will seem to move upward. Now it is known that motion aftereffect can be so powerful that it can influence the direction of the illusory motion produced by static patterns  or even followed after only milliseconds of adaptation .
However, even if low-level cortical processes seemed perfectly appropriated in order to explain the afterimage, the discovery that the top-down processes are able to influence radically both the primary visual cortex and also some subcortical “stations” of the visual system, as the lateral geniculate nucleus (LGN), together with some new types of aftereffects that showed the influence of attention [6, 7],consciousness , or an effect that lasts for days, weeks, or even months, seemed to clearly reject an explanation in terms of the short-lived adaptation reported typically in single cells of V1. These effects all suggested that the final percept of the afterimages is probably a complex result of the interaction and the adaptation of several districts throughout the visual system from the retina to the high-level cortical areas. Recently it was shown that the color afterimage signals are generated in the retina, but they are modified by cortical processes . In sum the retinal afterimages face the similar destiny of all retinal signals that in humans are virtually always heavily modulated by the cortex.
An interesting question that can be posed, however, is that whether the afterimages have a function or if they are just a side effect incurred as a minor consequence of having evolved such a complex and superb machinery known as the human visual system. The answer, as it was probably expected, seems to be that the afterimages and the neural adaptation in general are part of complex neural strategies evolved during thousands of years in order to improve our world perception (e.g. [10, 11]).
Breathing Light Illusion
In summary the afterimages are interesting phenomena that helped to understand our brain mechanism, they are able to produce striking illusory effects, and the underlying mechanisms seem to be relevant in producing a more efficient perception of the world.
- 1.Addams, R.: An account of a peculiar optical phenomenon seen after having looked at a moving body etc. London Edinburgh Philosoph. Magaz. J. Sci. 3rd series. 5, 373–374 (1834)Google Scholar
- 9.Zaidi, Q., Ennis, R., Cao, D., Lee, B.: Neural locus of color afterimages. Curr. Biol. 22(3), 220–224 (2012)Google Scholar
- 11.Barlow, H.B., Földiák, P.: Adaptation and decorrelation in the cortex. In: Darbin, R., Miall, C., Mitchison, G. (eds.) The Computing Neuron, pp. 54–72. Wesley Publishers, Reading (1989)Google Scholar
- 18.Gori, S., Mascheretti, S., Giora, E., Ronconi, L., Ruffino, M., Quadrelli, E., Facoetti, A., Marino, C.: The DCDC2 intron 2 deletion impairs illusory motion perception unveiling the selective role of magnocellular-dorsal stream in reading (dis)ability. Cerebral Cortex. 25(6), 1685–1695 (2015)Google Scholar