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Alex O. Holcombe, Donald I. A. MacLeod, Scott T. Mitten; Positive afterimages caused by a filled-in representation. Journal of Vision 2004;4(8):485. doi: 10.1167/4.8.485.
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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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