Positive afterimages have long been explained in terms of continuing photoreceptor or retinal neuron activity. However, our results suggest that positive afterimages are not caused by continuing activity at a peripheral level, but rather reflect the persistence of a visual representation in the absence of evidence that the world has changed.
METHOD AND RESULTS: A bleaching flash created temporary scotomas, making the rods in the corresponding area insensitive for a short time. Under scotopic conditions observers then viewed a uniform white or uniform black field. Observers reported that at first, the scotoma was visible as a hole in the otherwise uniform field, but after several seconds it was perceptually filled with the color of the surrounding field. When the uniform white or black field was then replaced with grey, an afterimage was experienced. If the scotoma had filled in with white, for a few seconds observers experienced the scotoma as bright (a positive afterimage) against the grey field. If the scotoma had filled in with black, the afterimage was dark (negative).
DISCUSSION: The afterimages cannot be explained by persisting photoreceptor activity. Instead, they can be understood as the persistence of a filled-in representation in the absence of contrary information. When the uniform field was replaced with grey, the insensitive scotoma area of the retina was unable to signal the change. Hence, the visual system likely assumed that nothing had changed at the scotoma, yielding the positive afterimage. The positive afterimage then persisted until the retinally stabilized scotoma was again filled by its surround. Consistent with this theory, if the afterimage of a central scotoma has faded against a white field bounded by darkness, transferring the gaze to the white-dark border reveals the afterimage as simply a protrusion of the white field color into the outer darkness. Other results indicate a cortical basis for the filled-in, persisting representation.
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Some general comments about afterimages: It should be emphasized that the mechanism inferred for the afterimages in our experiments cannot be the explanation for all positive afterimages. For example, some positive afterimages are almost certainly caused by persisting discharge at the photoreceptor level (see Adelson 1982, Vision Research and references therein). Another phenomenon classically called "visual persistence" causes even very briefly presented stimuli to appear to be present for up to 100 milliseconds. However, this experience does not fall under the technical term "afterimage" and is usually distinguished from afterimages by its shorter duration. The cause of visual persistence is unknown but may reflect the basic impulse response of the system. Some aspects of negative afterimages can be easily explained by adaptation by differential adaptation of the three different cone classes (sometimes known as red, green, and blue). In general, however, the relationship of the continued discharge of photoreceptors and the persistence of cortical representations to afterimages is likely to be complex, and will not be fully understood by scientists for some time.
