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Abstract:
Studies with callosotomy ("split-brain") patients have shown
that they perform tasks requiring a choice response more accurately
when the stimulus is presented to the visual hemifield ipsilateral
to the hand making the response. Nevertheless, responses made with
the contralateral hand tend to be fairly accurate. This could
reflect some ability of each hemisphere to control the responses of
the ipsilateral hand. Alternatively, it could reflect information
transfer to the opposite hemisphere, which then emits the response.
We assessed these alternatives in an event-related fMRI study in a
right-handed callosotomy patient. Stimuli were flashed to one
visual hemifield, and the patient was asked to indicate via
button-press whether a critical feature was on the top or bottom of
the stimulus. In one condition, the patient responded with the hand
ipsilateral to the stimulus, so that the hemisphere that processed
the stimulus also controlled the response. In other conditions, she
always responded with the same hand regardless of where the
stimulus appeared. Across all conditions we found motor activation
in the hemisphere contralateral to the hand making the response.
This pattern is consistent with the idea that information is
transferred between the hemispheres prior to the initiation of the
response, which is generated in the hemisphere that controls the
responding hand. There is little evidence to support the
ipsilateral control of manual responses.
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