Corticospinal correlates of urgent motor decisions

Making motor decisions often requires balancing the desire to take time to choose accurately with the urge to act. During a speeded decision, the urge to act increases as time passes but the overall level of urgency also varies depending on the context. Recent work suggests that urgency operates as a gain modulator of task-related activity: when decisions between reaching movements are made under time pressure, activity in motor areas involved in arm movements is amplified. An open question relates to the generalization of this gain modulation in the motor system. Here, we investigated the impact of urgency on corticospinal excitability in different task-related and task-unrelated motor representations in humans, by applying transcranial magnetic stimulation (TMS) over the primary motor cortex. Subjects performed a decision-making task. In each trial, 15 tokens jumped one-by-one every 200 ms from a central circle to one of two lateral target circles; participants had to guess which target circle would ultimately receive the majority of the tokens, and to report their decision on a keyboard with either the left or the right index finger. Importantly, the reward provided for correct choices was proportional to the number of tokens remaining in the central circle at the time of the response. Because this number decreased over time, the urge to act grew accordingly. Further, we manipulated the overall level of urgency in two contexts, by providing a different penalty for incorrect choices in separate blocks. The use of a low penalty encouraged the subjects to make hasty choices, thus ensuring a high urgency (UrgencyHigh context), while a high penalty promoted accurate choices and ensured a low urgency (UrgencyLow). We exploited TMS to elicit motor evoked potentials (MEPs) at different times during the token jumps in each context, in muscles that were either involved in the task (i.e., index finger “task-related” muscles) or not (i.e., thumb, pinky and leg “task-unrelated” muscles). MEP amplitudes obtained from these muscles provided us with a muscle-specific assay of corticospinal excitability at the time of stimulation. MEP amplitudes increased over time in all the investigated muscles (including in the leg) independently of the context, putatively reflecting a global time-dependent gain effect of urgency on corticospinal excitability. Further, amplitudes were higher in the UrgencyHigh than in the UrgencyLow context for task-related muscles, specifically when the MEPs were measured in the hand that was selected for the forthcoming action. The latter finding suggests the presence of a more specific context-dependent effect of urgency on corticospinal excitability