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Jacqueline Snow, Marlene Behrmann, Melvyn Goodale; Neuropsychological evidence for separate shape representations in vision and touch: a study using the Judd variant of the Muller-Lyer illusion.. Journal of Vision 2013;13(9):1076. doi: 10.1167/13.9.1076.
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The Judd variant of the Muller-Lyer figure consists of a horizontal shaft with an arrowhead at each end; one pointing inward with respect to the shaft (forming the "head" end) and the other pointing outward (forming the "tail"). The arrowhead inducers produce a visual illusion that consists of a displacement of the perceived midpoint towards the "tail" end of the horizontal shaft. The strength of the Muller-Lyer illusion in vision is known to be correlated with the fMRI responses in the lateral occipital area (LO), a region critically involved in representing shape cues in vision. Interestingly, a Judd illusion of similar magnitude is elicited when observers explore the stimulus via touch alone. Importantly, Mancini, Bolognini, Bricolo & Vallar (2011) reported that transcranial magnetic stimulation (TMS) over LO reduced the strength of the Judd illusion both in vision and in touch, an effect they interpreted to reflect the critical role of LO in both visual and haptic shape representation. In contrast to the subtle effects of TMS on cognition and behavior, unilateral damage to area LO in humans produces severe deficits in visual object recognition – a condition known as visual form agnosia. Here, we investigated the magnitude of the haptic Judd illusion in a patient with visual agnosia arising from circumscribed unilateral right hemisphere LO damage. If LO is critical for shape representation via touch (as it is in vision) then unilateral damage to LO should produce a reduction in the magnitude of the haptic Judd illusion. On the contrary, our patient showed a haptic Judd illusion, the magnitude of which was greater via touch that it was in vision. Moreover, despite showing visual object agnosia, his haptic shape recognition was normal. Taken together, our neuropsychological data indicate that LO is not critical for shape recognition via touch.
Meeting abstract presented at VSS 2013
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