Response competition in the primary motor cortex: corticospinal excitability reflects response replacement during simple decisions.
Journal of Neurophysiology. 2010-07-01; 104(1): 119-127
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It has been suggested that, during decisions about actions, multiple options are
initially specified in parallel and then gradually eliminated in a competition
for overt execution. To further test this hypothesis, we studied the modulation
of human corticospinal excitability during the reaction time of the Eriksen
flanker task. In the task, subjects responded with finger flexion or extension to
a central arrow while ignoring congruent or incongruent flanker arrows.
Single-pulse transcranial magnetic stimulation (TMS) was applied over primary
motor cortex (M1) at one of five different latencies after stimulus onset, and
motor-evoked potentials (MEPs) were measured in the contralateral index finger.
During the control (no flankers) and congruent conditions, MEP size in the
agonist increased gradually over the course of reaction time, indicating an
increase in corticospinal excitability. Conversely, when the same muscle acted as
an antagonist, MEP size decreased, suggesting inhibition. Critically, in the
incongruent condition, MEPs briefly increased in the muscle corresponding to an
initial default response to the flanker arrows and were later replaced by MEPs
corresponding to the correct response to the central arrow. Finally, we found
that the gradually growing MEPs for the three conditions reached a constant
maximum level just before movement initiation. We propose that this dynamic
modulation in corticospinal excitability reflects the competition process,
leading to the selection of one response and the rejection of the other. Our
results suggest that response competition influences activity in primary motor
cortex and that its timing directly influences motor output latency.