A modified Müller-Lyer illusion was used to test whether the adaptation of orientation-specific cells contribute to illusion decrement. Subjects either scanned the illusion configuration (a condition known to produce decrement), fixated one vertex of the illusion configuration, scanned a field of parallel lines set at the same angle from the horizontal as the wings of the illusion configuration (to adapt the orientation-specific receptors), or scanned a field of randomly spaced dots. Over the 5-min test period, those subjects who scanned the illusion configuration between judgments showed significantly more illusion decrement than did any of the other three groups. This suggests that active exploration of the illusion figure, rather than adaptation or fatigue of orientation-specific contour detectors in the cortex, is required for illusion decrement to occur.
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