afterimage

   Also referred to as aftersensation, afterimagery, and post-image. All three terms are used to denote a group of visual percepts that occur in response to a primary light stimulus to the eye, typically noticed after the stimulus has been shifted or removed. Afterimages tend to appear in a temporal sequence that can be subdivided into various stages. Under laboratory conditions, where brief primary stimuli of a relatively high intensity can be used, seven or more stages have been distinguished. The classical literature, however, which involves afterimages that occur under natural circumstances, confines itself to three stages. The first of these stages involves a very brief * positive afterimage that is referred to as * Hering's afterimage. These positive afterimages have the same relative brightness relations as the primary stimulus. The second stage involves a * negative afterimage known as the * Purkinje afterimage. These afterimages display

   Fig. 2 Afterimages. From left to right: original optical stimulus, Hering's afterimage, Purkinje's afterimage, Hess afterimage. Illustration by JDB colours and brightness relations opposite to those of the primary stimulus. In addition, they tend to appear smaller than the objects or stimuli from which they derive, to last for seconds to minutes after the primary stimulus is removed, and to change slightly in size and shape before fading away. The third stage involves another positive afterimage, called the * Hess afterimage. In his classic book chapter on afterimages, the American psychophysicist John Lott Brown provides the following additional nomenclature for various types of afterimages. The term *homochromatic afterimage is used to denote an afterimage in which the distribution of hues is the same as that of the original stimulation field (as in positive afterimages). The term * complementary afterimage is used to denote an afterimage in which the hues are approximately the complements of those in the original stimulating field (as in negative afterimages). And the term * original afterimage is used to denote an afterimage seen in complete darkness after exposure of the eye to a primary stimulus. Negative afterimages have traditionally been classified as *entoptic phenomena. They are believed to be mediated primarily by the bleaching of photochemical pigments, and/or neural adaptation of the retina, whereas the changes in the apparent size of these afterimages have traditionally been attributed to convergence and divergence movements of the eye. However, there is emerging evidence that both peripheral and central mechanisms may be responsible for the mediation of certain aspects of all types of afterimages. In hallucinogen-induced states such as LSD and mescaline intoxication, the hues of afterimages can be exceptionally strong, and the images themselves can last longer than usual. They may even appear more real and material than the * visual hallucinations that occur during such states. The occurrence of positive afterimages (other than the afterimages identified by Hering and Hess) is rare, especially under physiological circumstances. They have been reported to occur in drug-induced states, as well as in drug-induced * flashback phenomena and * hallucinogen-induced persistent perception disorder (HPPD). Phenomeno-logically as well as conceptually, positive afterimages would seem to be more akin to central phenomena such as *palinopsia, *polyopia, and the * trailing phenomenon than to negative afterimages. Afterimages are commonly classified as *physiological illusions. Because of their lack of a tangible substratum in the extracorporeal world they are also classified as *fiction illusions.

   References

   Brown, J.L. (1965). Afterimages.In: Vision and visual perception. Edited by Graham, C.H. New York, NY: John Wiley & Sons. Shimojo, S., Kamitani, Y., Nishida, S. (2001). Afterimage of perceptually filled-in surface.

   Science, 293, 1677-1680. Roeckelein, J.E. (2004). Imagery in psychology: A reference guide. Westport, CT: Praeger.