| Abstract No.: | B-G2184 |
| Country: | Canada |
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| Title: | CORTICOSPINAL EXCITABILITY REFLECTS RESPONSE COMPETITION DURING A DECISION-MAKING TASK |
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| Authors/Affiliations: | 1 Thomas Michelet*; 1 Hugo Théoret; 1 Paul Cisek;
1 Universite de Montreal, QC, Canada.
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| Content: | It has been suggested that during decision-making, 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 a decision-making task.
Subjects performed the Eriksen-Flanker task while single-pulse transcranial magnetic stimulation (TMS) was applied over primary motor cortex and motor-evoked potentials (MEPs) were measured in the index finger of the contralateral hand. Subjects responded to the direction of a left or right arrow, which appeared in the center of the screen, while ignoring the four adjacent “flanker” arrows. The flankers pointed in the same direction (congruent) or in the opposite direction (incongruent) as the central arrow. A control condition, without flankers, was also presented. Subjects were instructed to flex their right index finger in response to a leftward central arrow or extend in response to a rightward arrow. A single TMS pulse was applied at one of five different latencies after stimulus onset.
As expected, reaction times (RTs) were shorter in the congruent than in the control condition, and longest in the incongruent condition. During the control and congruent conditions, MEP size in the agonist increased gradually over the course of reaction time, indicating an increase in its corticospinal excitability. Conversely, when the same muscle acted as an antagonist, MEP size decreased, suggesting inhibition. Differences between agonist and antagonist MEPs emerged earlier in the congruent than in the control condition. Critically, in the incongruent condition, MEPs first exhibited an increase for the antagonist and a decrease for the agonist, suggesting a “default” response to the distracting flankers. This pattern then reversed and returned to the pattern seen in control and congruent conditions, albeit with a delayed latency. This result is consistent with an interference effect, which produces the longer RTs found in the incongruent condition.
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. In the incongruent condition, a brief initial default response to the flanker arrows is replaced by a later, correct response to the central arrow. Our results suggest that response competition influences activity in primary motor cortex, and that its timing directly influences motor output latency.
Supported by the EJLB Foundation and Fyssen Foundation |
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