After-images are generally experienced as indistinct patches of color that seem to "float" before the eyes. After-images are often caused by staring at bright lights or brightly-colored objects.
Of all the types of imagery, after-images are the easiest to classify as being sensations, rather than being like sensations. As Descartes noted, sensations are something that happen to us -- they are apparently due to outside causes that operate independently of our will. We can not consciously will to have or not to have a sensation.
The production of after-images is clearly automatic in the vast majority of cases. Staring at bright lights or bright colors will eventually "saturate" or overload the retinal and other nevous tissues involved and cause an after-image no matter what we think or imagine. After-images also fade at a rate that is apparently set by our physical constitution, rather than any conceptual activity. This is in contrast to other types of mental images, such as memory images, that come and go according our will and/or conceptual activity. After-images also have some properties in common with eidetic images, but also some important differences: after-images tend to appear to move in space when the eyes move, while eidetic images appear to be static in space.
Do we literally see or perceive after-images? This is a debatable question. According to the definition of mental images in traditional psychology, the answer is technically "no." After-images are quasi-perceptual experiences that occur in the absense of a genuine stimulus object (e.g., a yellow-colored light) that would produce a similar experience. Also, after-images fade, appear to have an indefinate location in space, and have other properties that distinguish them from ordinary vision. Nevertheless, it is tempting to count "I see an after-image" as being a legitimately meaningful phase. One of the sources for the common-sense meaning of this phrase, I suggest, is that in describing after-image experiences, we apparently refer to
A SPECIFIC TRANSITORY OBJECT OF SENSATION WITH A SIZE, COLOR, AND SHAPE.
Hence, the meaning of the phrase derives as much from the referece to the OBJECT and the implied assertion "there exists an object of my perception" as much as it does from the VERB ("to see") and the implied assertion "I am presently undergoing a visual experience." The fact that we can identify after-images as specific objects is one of the reasons for asserting that mental objects exist. In addition, the objective status of this type of mental image is supported by the fact that there is a sense in which after-images can be VISUALLY INSPECTED and described in a certain amount of detail. These factors are significant in the philosophic defense of mental images as a species of mental object, made elsewhere on these pages.
While it is clear that after-images can be inspected, some people (and even some philosophers) claim that memory images can be "inspected" in much the same way. Whether or not eidetic images and memory images can be inspected in the same manner has been hotly debated in empirical psychology and philosophy. In fact, much of the contemporary imagery debate revolves around the issue of "inspection" of memory images. That is why, again, that it is important to take an unambiguous case of "inspection" (as in the example of after-images) in order to establish what we mean, phenomenologically, by "inspecting" mental objects.
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