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Charles Leek, R. Martyn Bracewell, John Hindle, Stephen Johnston; Functional specialization in Supplementary Motor Area (SMA): Evidence from visuo-spatial transformation deficits in Parkinson's disease. Journal of Vision 2010;10(7):903. doi: 10.1167/10.7.903.
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Leek & Johnston (2009, Nature Reviews Neuroscience, 10, 78-79) have suggested that one function of the anterior (pre-) SMA in humans is the computation of abstract visuo-spatial vector transformations. According to this hypothesis, pre-SMA should be involved in any visual task that requires the transformation or remapping of a spatial location (vector) regardless of whether there is a motor component to the task. We have previously examined this using functional brain imaging of pre-SMA during the performance of visuo spatial transformation (e.g., mental rotation, mental grid navigation) and non-transformational (VSTM for static spatial locations, non-spatial numerical operations) - see Johnston & Leek, 2010 Vision Sciences Society). Here we report evidence from studies that tested this hypothesis using data from Parkinson's disease (PD) patients. One known aspect of the underlying pathology of PD is the consequent effects of dopamine depletion in the basal ganglia upon functioning of medial frontal cortex. Thus, PD provides a good model for studying SMA dysfunction and its effects on visuo-spatial processing. The spatial vector transformation hypothesis predicts that PD patients, with impaired SMA function, are likely to exhibit deficits on tasks that require spatial vector transformation. The results showed that, as predicted, PD leads to impairments on transformational but not on non-transformational tasks. These findings support the vector transformation hypothesis and suggest that regions of the SMA are involved in highly abstract visuo-spatial computations that go beyond the preparation and planning of movement. Indeed, these findings suggest that the SMA supports abstract visuo-spatial processes that are potentially recruited in a wide range of visual and spatial tasks.
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