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| Lotfi B. Merabet |
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BIDMC 330 Brookline Ave. KS-454, Boston MA, 02215, USA
e-mail: lmerabet@caregroup.harvard.edu |
Poster Presentation: |
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| The role of the occipital cortex in tactile processing: a transcranial magnetic stimulation study. |
| Lotfi Merabet, Jessica Andrews, Janine Ringler, Hugo Theoret, Alvaro Pascual-Leone | |
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| Previous neuroimaging studies have demonstrated that the occipital (visual) cortex is recruited during tactile discrimination tasks in both normally sighted and blind subjects. We have further addressed this issue by investigating the possible role of occipital cortex in tactile tasks requiring fine spatial discrimination. We conducted a tactile discrimination task using arrays of raised dots of different inter-dot spacing. Normally sighted subjects (blindfolded and wearing earplugs) were asked to separately rate (using a scale of 1 to 10) the subjective roughness and distance spacing between dots for a series of 8 patterns presented in random order. When judging roughness, intermediate dot spacing were perceived as the most subjectively rough while closely spaced and widely spaced dot patterns both lead to an impression of little roughness. Plotting subjective roughness against dot spacing yielded an inverted U-shape curve with a maximal value corresponding to a dot spacing of 3 mm. Distance judgment plotted against dot spacing generated a linear line (i.e. greater inter-dot distances were progressively perceived as greater distances). Repetitive TMS (rTMS; 1 Hz, 10 min, 90% of either motor threshold or phosphene threshold) was applied to disrupt either the somatosensory or the occipital cortex in order to determine the relative contribution of these cortical areas in tactile processing. Application of rTMS to the contralateral somatosensory cortex disrupted the amount of overall roughness perceived (i.e. flattening the roughness judgment curve) while distance judgment was not impaired. Conversely, rTMS to the occipital cortex did not affect judgments of roughness but rather disrupted distance perception. Subjects tended to scale increasing inter-dot spacing with less perceived distance compared to the control condition. Our preliminary functional neuroimaging studies are consistent with these results, demonstrating predominant activation of the somatosensory cortex during roughness determination and occipital cortex during distance judgment. Given the differential effect of rTMS on somatosensory and occipital cortex on task performance, these findings suggest that the occipital cortex is engaged in tactile tasks requiring spatial discrimination. |
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