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Kate E. Merritt, Robert L. Whitwell, Gavin Buckingham, Philippe Chouinard, Melvyn A. Goodale; Dissociating Action and Perception Using a 3D Variant of the Sanders Illusion While Controlling for Visual and Haptic Feedback. Journal of Vision 2014;14(10):422. doi: https://doi.org/10.1167/14.10.422.
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According to the two visual systems hypothesis (TVSH), 'vision-for-action' and 'vision-for-perception' are mediated by two distinct anatomical cortical pathways. Supporting evidence for the TVSH has come from neuropsychological, neurophysiological, and neuroimaging studies of humans and non-human primates. One particularly contentious line of evidence, however, has come from studies that have shown stronger effects of pictorial illusions on perceptual estimation tasks than on grasping tasks. Critical re-appraisals of these studies have rendered the perception-action dissociation interpretation problematic, noting confounding task-differences in attention, stimulus-response functions, obstacle avoidance, and visual and haptic feedback. Here, we asked participants to either reach out and pick up (length-wise) target bars embedded in the Sanders parallelogram illusion or perceptually estimate their lengths. We removed online and offline sources of visual feedback by suppressing the participants' vision throughout their grasps. We controlled for haptic feedback by administering the grasping and perceptual estimation tasks in an alternating task schedule. Thus, participants had the same opportunity to touch the targets in the perceptual estimation task as they did in the grasping task. Furthermore, we administered each task in a more traditional manner by separating them into two blocks of trials. The results of our experiments were clear. In line with the TVSH, the illusory effect of the Sanders display was significantly weaker on grasps than on perceptual estimates when the tasks was blocked separately and when the perceptual estimation and grasping tasks were alternated from trial to trial. In addition to this key finding, there was no evidence to suggest systematic between-task differences in the response functions to target length. Not surprisingly, therefore, an analysis of the 'corrected' illusory effects supported our key findings. We conclude that the Sanders illusion reveals robust positive evidence for separate visual-perceptual and visuomotor systems in neurologically-intact populations.
Meeting abstract presented at VSS 2014
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