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Jeroen J.A. van Boxtel, Christof Koch; Boundary information and filling-in in afterimage perception. Journal of Vision 2010;10(7):1159. doi: 10.1167/10.7.1159.
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Afterimages are the result of retinal bleaching, and other neural adaptation processes. The mainstream idea is that afterimages disappear when the level of adaptation-modulated signal (the true afterimage) is reduced below a contrast-detection threshold (e.g. Leguire & Blake, 1982). Here we manipulated afterimage visibility by projecting a ring (i.e. a boundary) around the afterimage, without changing the visual stimulation at the position of the aftereffect itself, and thus without directly affecting adaptation-modulated signals. We show that the duration of the afterimage is lengthened by about 50% with a ring that exactly encompasses the afterimage. Rings that are equal or smaller in size than the afterimage increase afterimage duration relative to a condition without a ring, while boundaries larger than the afterimage decrease afterimage duration. We find furthermore that maximum modulation occurs for intermediate contrasts of the ring, making attentional capture (by large luminance changes) an unlikely cause of the effect. Finally, placing a ring around the position of an already faded afterimage, revives the afterimage. Our data show that boundary signals (i.e. the ring) are crucial in the determination of afterimage perception. When boundary signals are present, the area within the boundary is filled in with features that are present just within its circumference, which is true afterimage when the ring snugly fits the afterimage area. When the the ring is larger than the afterimage, or absent, the filled-in feature is the gray background, which effectively shortens afterimage duration, even though the true afterimage is still present. We suggest that an ‘active’ boundary and filling-in mechanism is involved in afterimage perception, similar to that proposed for peripheral fading and retinally stabalized images.
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