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Scientist Reports on Conventional and 'Perceptual' Afterimages
In the 31 August 2001 issue of the international journal Science, Shinsuke Shimojo of the California Institute of Technology and NTT Communication Science Laboratory in Japan and colleagues report on experiments that reveal two distinct types of afterimages--one produced in the eye, and another that appears to be created in the brain. To observe this phenomenon for yourself, do the experiment below:
Figure 1A: The image on the left is the visual stimulus. Stare at the fixation point (the small white dot) for 10-20 seconds, and then shift your gaze to the second fixation point at the right. You may see a number of afterimages produced by the visual stimulus (details below).
Figure 1B: These are some of the afterimage types that are typically seen after the experiment. The left and center afterimages represent local, or conventional afterimages. They are probably formed bybleaching of photochemical pigments or neural adaptations in the eye's retina. The right afterimage is a global, or perceptually filled-in afterimage. Although the "square" in the middle of the red and white discs is not actually there in the picture (only partially represented by the four red corners), adaptation in the brain's cortical neural circuits creates an afterimage of the entire square that was "perceived" during the experiment.
-- Becky Ham