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Fading and Filling-in

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Since Troxler’s original observation in 1804, fading and filling-in phenomena have aroused the interest of researchers. However, the question of why stimuli imaged on the retina become invisible (fading) and – vice versa – why incomplete stimuli are perceived as complete (filling-in), has been systematically studied only during the last 30 years.

Fading is due to local adaptation and is comparable to a stimulus lowered to subthreshold. We now know that most targets, whether static or flickering, fade into the background. Strict prolonged fixation is crucial; as soon as the eye moves, the percept is refreshed and reappears. We assume that this restoration is achieved by the edge signal arising at the border of the target due to microsaccades. This signal if laterally propagated could revive the brightness or color of the interior. The time course of fading typically does not exceed 15 seconds. By comparison, an image artificially stabilized on the retina disappears almost instantaneously.

When a percept fades, the void may be replaced by properties from the surrounding background. This is called filling-in. Backgrounds need not be uniform, a textured background or dynamic visual noise field will be as effective. Filling-in ensures the perception of brightness, color, and texture in a artificial scotoma, where, in fact, there is no signal from the retina reaching the brain. Other examples are lesion scotomata and the optic disk (blind spot) which are filled-in with properties of the surround and therefore are invisible. We have found that filling-in of the blind spot requires relatively little surround information. A thin red ring hugging the boundary of the blind spot will fill in the “blind“ area uniformly and completely with color. Similarly, a thin chromatic double contour will induce a spread of “watercolor” over a large area. However, the watercolor effect requires no fixation.

Psychophysical and neurophysiological experiments suggest that filling-in is achieved by long-range horizontal interaction beyond the classical receptive field. Candidates are visual areas V1-3.

Literature: Luiz Pessoa & Peter De Weerd (eds.). Filling-In: From Perceptual Completion to Cortical Reorganization. Oxford University Press 2003

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