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Tzvi Ganel, Eran Chajut; Sensitivity of visuomotor control to real and to illusory size. Journal of Vision 2008;8(6):619. doi: 10.1167/8.6.619.
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© ARVO (1962-2015); The Authors (2016-present)
Evidence from neuroanatomical and neuropsychological studies supports the notion that different systems mediate visual control of action and visual perception. Yet, behavioral studies, including those who have focused on visual illusions, show a diverse pattern of results. In a first series of experiments we have attempted to disentangle - within the same paradigm - the effects of the real size and the apparent size of objects on action and perception. To this end, two objects of different sizes were presented within a size-contrast illusion, so that in most experimental trials the object that was perceived as the larger one was actually physically smaller than the other object. This design allowed us to focus on trials in which subjects have overtly decided that one of the objects is the smaller (or larger) of the pair, and to test whether or not the opening between the fingers of the grasping hand goes in the direction of this erroneous decision. The results showed that although subjects were affected by the illusion when making size decisions, the opening of their fingers reflected the real size differences between the objects. Thus, the real and apparent differences in the sizes of the objects had opposite effects on action and perception. These findings also indicate that visuomotor control may have enhanced sensitivity to real size. In a second series of experiments we examined this idea by focusing on the psychophysical function that links visuomotor control with size. Results showed that in the absence of illusory context, action and perception showed similar psychophysical functions for size. However, JND's (Just Noticeable Differences) increased with object size for perceptual estimations, but remained constant across different sizes when grasping the same objects. These findings suggest that qualitative differences exist in the way size is represented for visuomotor control and for perception.
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