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Abstract:
Since 1912, short (2-5 ms) differences between ipsimanual
(uncrossed) and contramanual (crossed) simple reaction times (RT)
to unilateral visual stimuli (the Poffenberger paradigm) have been
interpreted as reflecting interhemispheric transmission time (IHTT)
across the corpus callosum (e.g. Marzi et al., 1991).
Electrophysiological studies have, however, found longer IHTT
estimates that are uncorrelated with RT (10-20 ms, e.g. Saron and
Davidson, 1989). This study sought the origin of this discrepancy
by examining the spatiotemporal dynamics of visuomotor activation
using high density ERP recordings. Four findings suggest that the
basic assumptions of the Poffenberger paradigm are not correct: 1)
in uncrossed conditions, bilateral frontal, central, and occipital
activations were found pre-movement, violating the exclusive
intrahemispheric processing assumption of the RT subtraction
method; 2) motor cortex activation magnitude was related to
response speed such that a range of RTs were generated by
movement-related activations that differed mainly in magnitude,
bringing into question a strict pathway length interpretation of RT
effects across conditions; 3) multiple movement-related cortical
regions differentially contributed to movement generation as a
function of RT, in accordance with demonstrated corticospinal
efferents from supplementary motor area (SMA), premotor cortex and
caudal cingulate motor areas (e.g. Dum and Strick, 1991), further
complicating interpretation of an equivalence in movement
generation mechanisms across a range of response speeds; and 4)
interhemispheric routes of visuomotor activations differed with
different response speeds, implicating, in some instances, central
routes for faster and posterior routes for slower RTs in crossed
conditions, reflecting different interhemispheric processes for
different RTs.
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