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Abstract:
Consider a lateralized simple reaction time (SRT) paradigm. A
stimulus presented to the right visual field (RVF) elicits faster
right-hand (uncrossed) responses than left-hand (crossed)
responses. In the uncrossed condition, the stimulus and response
are processed within the left hemisphere. In the crossed condition,
the left hemisphere registers the stimulus while the right
hemisphere makes the response. The crossed-uncrossed difference
(CUD) on both sides measures callosal transfer. Spatial
compatibility (SC) refers to a type of spatial attention in which
left-hand responses to LVF stimuli are faster than left-hand
responses to RVF stimuli, etc. due to spatial proximity. Could SC
rather than anatomy explain the CUD? Previous unimanual experiments
have shown that SC affects choice RT tasks while SRT is purely
anatomical. We put this conclusion to a new test. Methods: We
tested a patient, AA, who had complete cerebral commissurotomy,
with an SC manipulation of the SRT paradigm. Subject responded with
hands in either an uncrossed/natural or a crossed/unnatural
position. Results and Discussion: Data showed a position (SC)
effect on the CUD in the split-brain, but not in the normal-brain.
The split-brain patient showed the standard CUD in the natural
position, but an effect of SC in the unnatural position. This
suggests that the CUD measures anatomical distance rather than
spatial attention in the normal-brain. By contrast, subcallosal
channels are susceptible to spatial attention.
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