An experience of an afterimage is caused by a previously seen stimulus, when that stimulus itself is no longer present.
Positive afterimages are the same colour as the previously seen stimulus. They often occur when there is no stimulation—for example because the lights have gone out, or because your eyes are closed and your hands are in front of them to block all light. In these conditions they occur when some cells (the cones) on the retina keep transmiting signals to the brain for a little while after they have been stimulated. But they can also happen in other conditions, such as when presented with a previously seen outline of a shape, as occurs in the Colour Dove Illusion.
(Negative afterimages exhibit inverted lightness levels, or colours complementary to, those of the stimulus. They are usually induced by prolonged viewing of a stimulus and then best seen against a brightly light background. They occur (as least in part) becasue some cells (the cones) on the retina do not respond to the present stimulation because they have been desensitised by looking at a prior stimulus.)
Many philosophers of perception seek to analyse afterimages as pathological cases of visual experience. Many philosophers think that in order for an experience as of seeing an object to amount to genuine visual perception, that object must exist and one’s visual experience must be appropriately caused by that object. Now, visual illusions are usually analysed as cases in which one perceives the objects of the public, external world, but that perceptual experience is somehow inaccurate or non-veridical. In the case of an afterimage, one may have a lasting visual experience as of a green square on a white wall, but no such green square, or indeed a square of any sort, need exists independently of one’s own nervous system in front of one at that time (and, in the case of negative afterimages, no green square need to have been previosuly in front of one). Afterimages are not ordinary public objects, but rather they arise as artefacts of individual perceptual systems. This has led many philosophers to suggest that the visual experience of an afterimage is a failure of perception, and hence afterimages are best characterised as a type of hallucination.
However, this line of thought can be resisted. One might try to argue that one is seeing the square inducing figure—albeit seeing its colour incorrently, and one is also seeing it at a later time that it was present in front of one. Such delayed perception might seem odd, but, in its defence, one could point to the fact that there are other cases of delayed perception. For example, when we see the stars in the night sky, we are looking at the stars as they were years ago. Indeed, a star that we may happily say that we presently see may in fact no longer exist. It may have blown up in a supernova, the light from which has not yet reached us.
Philosophers disagree as to how we should best explain illusions and hallucinations, and some theories of perception may accommodate one phenomenon better than they do the other. See Macpherson (2013) for a detailed overview of various philosophical approaches to hallucinatory perceptual experiences. Afterimages figure in debates as to whether we are directly aware of physical objects or, rather, internal (mental, private) objects called sense-data. Those who think that afterimages are hallucinations and who hold that, when we have a visual experience as of an object, we must be aware of an object, typically hold that the experience of an afterimage involves the experience of a mental object (a sense-datum), as there is no such physical object to be seen (see Robinson 1994; also see Crane and French 2015 for discussion).
Many philosophers, particularly physicalists, reject this conclusion and argue that experiences of afterimages can be given other explanations. Jack Smart’s famous paper in which he argues for physicalism contains a discussion of the experience of an afterimage (Smart 1959). Smart claims that when we experience an afterimage we only seem to be aware of an object, but we are not. He says we can explain away those appearances as follows:
'When a person says ‘I see a yellowish-orange after-image’ he is saying something like this: "There is something going on which is like what is going on when I have my eyes open, am awake, and there is an orange illuminated in good light in front of me"' (p. 150)
Many philosophers, however, have claimed that this analysis is an insufficient account of what is going on in the experience of afterimages. See, for example, David Chalmers (1996, p. 360).