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John Zettel, Tutis Vilis, Jody Culham, Doug Crawford; A comparison of saccade and pointing topography between medial and lateral areas in the human posterior parietal cortex. Journal of Vision 2007;7(9):292. doi: 10.1167/7.9.292.
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Previous functional magnetic resonance imaging (fMRI) studies identified areas in the posterior parietal cortex (PPC) that are active during memory-guided saccades and pointing to visual targets. Both the areas for saccades and pointing are lateralized, showing higher activation for actions directed toward the contralateral hemifield (Medendorp et al., 2003, Journal of Neuroscience; Fernandez-Ruiz et al., in press, Cerebral Cortex). The purpose of this study was to clarify topographical areas of activation in the PPC related to saccading to and pointing to remembered targets as well their activity in an anti-pointing task.
fMRI was used to measure the blood oxygenation level-dependent response while right-handed individuals either made saccades or pointed to remembered visual targets in a blocked design. Subjects also performed an anti-pointing task. Contrasts between right- and left-visual targets revealed topographic areas related to each effector.
The saccade task produced lateralized activation in the medial intraparietal sulcus (IPS; left Talairach coordinates (TC): −23, −60, 43). The pointing task produced topographic activation in a medial region of the PPC (left TC: −12, −82, 32), near the superior parieto-occipital sulcus, as well as the IPS region activated by saccades. These two regions are consistent with previous studies. In addition, a previously unidentified area in the precuneus also demonstrated topography for pointing (left TC: −8, −59, 57).
These results show that a region in the IPS has topographic activity for visually directed saccades and pointing, while a more medial region is topographically activated by pointing alone. Examination of activity in these and other topographically organized areas during an anti-pointing task are expected to provide further insight into their role in visuospatial behavior.
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