Time course of corticospinal excitability in a simple reaction time task


The production of movement in a simple reaction time (RT) task can be separated into two time periods: the foreperiod, which is thought to include preparatory processes, and the RT interval, which includes initiation processes. To better understand these processes, transcranial magnetic stimulation (TMS) has been used to probe the level of corticospinal excitability (CE) at various time points during response preparation and initiation. Previous research has shown that CE decreases as the imperative stimulus (IS) approaches and increases once the IS has been presented; however these two time frames have been examined independently. As CE during the entirety of the process of movement production has not been described, the purpose of this study was to measure when and at what rate changes in CE occur during both the foreperiod and RT interval, relative to a true baseline level. Participants performed a button press movement in the context of a RT task and CE was measured using TMS at 21 time points, encompassing both the 1500 ms fixed foreperiod and the RT interval. Furthermore, a baseline measure was taken prior to the warning stimulus (WS) when the participant was at rest. Results indicated that during the foreperiod CE levels quickly increased from baseline with the presentation of the WS. This was followed by a period of stable CE, and by a subsequent decline in the 500 ms leading up to the IS. This decline was followed by a rapid increase in CE following the go-signal as the movement is initiated. Importantly, even though levels of CE decreased prior to the IS, this does not reflect a decrease in preparatory activation, but rather is thought to reflect selective inhibitory control mechanisms that function to avoid early (premature) response initiation.

Acknowledgments: Supported by NSERC