“School of Cognitive Sciences”
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| Paper IPM / Cognitive Sciences / 11237 |
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| Abstract: | |
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Contrary to belief that negative afterimage is the sole result of photoreceptor adaptation at retinal level, there is evidence for contribution of postretinal factors to the perception of afterimage. To establish whether afterimage size is affected by context information we measured the perceived afterimage size under conditions of the Ebbinghaus illusion of size contrast. In an adaptation phase, observers fixated on the centre of a red inducer circle (diameter = 1.55 deg) for 50 s. The inducer then disappeared and six light-green (Titchener) circles were displayed around the fixation point. The diameters of these peripheral circles were either small (0.46 deg) or large (3.1 deg) in different trials. Each subject reported his/her perceived afterimage size by adjusting the length of a line to match the diameter of perceived afterimage by means of mouse clicks. The results show that perceived afterimage size is significantly larger when afterimage is surrounded by small Titchener circles. Thus, negative afterimage is susceptible to Ebbinghaus illusion. We suggest that our perception of negative afterimages is modulated by context information. A neural correlate of size contrast illusions such as the Ebbinghaus illusion is believed to develop beyond the retina. Hence, the perception of negative afterimages is modulated by high-level cognitive factors.
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