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Abstract:
Bimanual choice reaction times (RTs) to indicate the side of
space of a visual stimulus are shorter if that stimulus is on the
same side as the responding hand. We investigated this "Simon
effect" in two callosotomy patients. We also compared simple RTs ,
within-field choice RTs, and between-field choice RTs in these
patients. Simple RTs were assessed by having subjects respond
unimanually to stimulus presentations ipsilateral with the
responding hand. In one choice RT condition, subjects responded to
left visual field (LVF) targets with their left hand (LH) and right
visual field (RVF) targets with their right hand (RH). In a second
condition, this hand-field mapping was reversed. Both conditions
were run with the subject's hands uncrossed and crossed. A Simon
effect should produce longer RTs with crossed hands in the first
case and shorter RTs with crossed hands in the second. Within field
choice RT was assessed in JW by having him respond unimanually to
upper and lower targets. For subject JW, between-field choice RTs
were only slightly longer than simple RTs, and shorter than
within-field choice RTs. For subject VP, between-field choice RTs
were substantially longer than simple RTs. However, robust Simon
effects were found in both patients. Thus, while the relationship
of simple to cross-field choice RTs varies from patient to patient,
a Simon effect occurs irrespective of this relationship. This
implies that each hemisphere in callosotomy patients must represent
events occurring in both sides of visual space.
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