Biology 202

2006 Second Web Paper

On Serendip

The common explanation given for an afterimage is that the photoreceptors (rods and cones) in the eye become "fatigued", and do not work as well as the those photoreceptors that were not affected (the "fatigue" is actually caused by the temporary bleaching of the light sensitive pigments contained within the photoreceptors)(1, 2, 4). This results in the information that is provided by the photoreceptors not being in balance, causing the afterimages to appear. As the photoreceptors become less "fatigued", which takes between ten and thirty seconds, the balance is recovered, resulting in the afterimage disappearing (2).

While the explanation above may explain some of the afterimages, it should not be used to explain all afterimages. If the brain is the main way through which we view our world, then wouldn't it be just as responsible for afterimages as "fatigued" photoreceptors? Perhaps instead of just a photoreceptor that has been temporarily bleached, causing it to not be able to react as quickly as the other receptors around it, the portion of the brain that is responsible for vision had adjusted to the image that was being focused upon, and was expecting the image to not change, so that when the change did occur, for the first few seconds the old image was still expected, and thus the afterimage was formed (3, 6).
The involvement of the brain in the creation of afterimages would explain why there is such a variety in the images reported; both positive and negative afterimages, afterimages that occur in a sequence (an alternation between positive and negative afterimages from a single stimulus), afterimages that change color, and afterimages that occur without a visible stimulus (4, 6). It is the afterimages that occur without a visible stimulus that provide the most convincing argument for the brain's involvement in the creation of afterimages.

When a subject is exposed to light from the green range of the visible spectrum, red afterimages have been reported, even when the subject is unaware of the stimuli (such as when the subject has their eyes closed). This suggests the involvement of the portion of the brain responsible for vision in their creation, as the afterimages, when reported, are distinct, whereas the original stimulus was not distinct enough to be noticed by the subject (3, 5). Interestingly, when the experiment was carried out on subjects who were unable to perceive certain colors, such as those who were colorblind, a response from the pupil was generated for the aftereffect, in the form of contractions. The subjects, however, reported seeing neither the stimulus nor the aftereffect. This suggests that both the photoreceptors and the brain play a part in the generation of an aftereffect (3, 5).

Unfortunately, the process by which an aftereffect occurs is not fully understood. While the common explanation given for the phenomenon is that it is created by "fatigued" photoreceptors, there is building evidence that they are not the only part of the body involved in the creation of afterimages, as evidenced by the variety of types of afterimages that can occur from a single stimulus. While the involvement of the brain in the creation of the afterimages is suspected, the exact nature of the involvement is currently unknown. Hopefully further investigation into the nature of the brain's involvement in the creation of these images will occur, as it could lead to other breakthroughs in our understanding of how we perceive the world and ourselves.

Sources

1) "How Do We See Colors?" - an explanation of the different photoreceptors.

2) "Afterimages" - an example of a negative afterimage, as well as an explanation as to their occurrence.

3)"The unseen color aftereffect of an unseen stimulus: Insight from blindsight into mechanisms of color afterimages."

4) "Retinal Vision" - an explanation of different types of afterimages.

5)Sperling, G. "Negative Afterimage without Prior Positive Image." Science, New Series, Vol. 131, No. 3413. (May 27, 1960), pp. 1613-1614.

6) "Visual Illusions and Neurobiology" - explains how the brain may be responsible for certain afterimages.


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