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Abstract:
Supported by NINDS-We previously reported that tactile
discrimination of grating orientation is associated with visual
imagery and activity of an area of parieto-occipital cortex
(Sathian et al., NeuroReport 8: 3877-3881, 1997). The goals of the
present study were to investigate this visual cortical activation
in normal humans with respect to its time course and the effect of
its disruption on tactile discrimination. Gratings were applied to
the immobile index fingerpad using an electromechanical stimulator.
We studied the averaged electrical potentials evoked by this
stimulus. Relative to a condition where the subject was required
merely to count the number of stimuli, discrimination of grating
orientation evoked a potential that was maximal over occipital
cortex with a peak at about 180 msec following grating contact. We
used focal, single-pulse, transcranial magnetic stimulation (TMS),
time-locked to grating application, to interfere with occipital
cortical function while subjects were tactually discriminating
grating orientation. TMS clearly impaired performance when applied
over occipital cortex in the midline or contralateral to the
stimulated hand but not ipsilateral to it. The effect was maximal
at 180 msec after grating contact, consistent with the time course
of the evoked potential. In contrast, TMS did not compromise the
detectability of an electrical stimulus delivered to the fingerpad.
Together with our previous results, these observations indicate a
role for visual cortical activity in normal tactile
discrimination.
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