A TMS-induced cortical silent period delays the contralateral limb for bimanual symmetrical movements and the reaction time delay is reduced on startle trials.

Abstract

Bimanual actions are typically initiated and executed in a temporally synchronous manner, likely due to planning bilateral commands as a single motor "program." Applying high-intensity transcranial magnetic stimulation (TMS) to the motor cortex can result in a contralateral cortical silent period that delays reaction time (RT), if timed to coincide with the final motor output stage. The current study examined the impact of a unilateral TMS silent period on the RT and interlimb timing of bilateral wrist extension. In addition, because a loud, startling acoustic stimulus (SAS) can result in the involuntary release of preprogrammed actions via increased reticulospinal activation, it was of interest whether startle-induced speeding of response initiation would moderate the impact of the TMS-induced RT delay. Participants performed blocks of unilateral and bilateral wrist extension in response to an acoustic (82 dB) go-signal. On selected trials, either TMS was applied to the left motor cortex 70 ms before the expected EMG response onset, a SAS (120 dB) replaced the go-signal, or both TMS and SAS were delivered. Results showed that TMS led to a significant RT delay in the right limb during both unimanual and bimanual extension but had no impact on the left limb initiation. In addition, the magnitude of the right limb RT delay was smaller when the response was triggered by a SAS. These results imply that preplanned bimanually synchronous movements are susceptible to lateralized dissociation late into the cortical motor output stage and movements triggered by startle involve increased reticulospinal output.NEW & NOTEWORTHY Bilateral responses are typically planned synchronously and performed symmetrically. Here, we show that delaying the initiation of one limb using transcranial magnetic stimulation (TMS) to produce a cortical silent period does not impact the other limb during bimanual movements. Also, the TMS-induced delay is reduced when a startling acoustic stimulus (SAS) triggers the movement. These results confirm that tightly coupled bilateral responses can be dissociated by contralateral TMS- and SAS-triggered responses involve greater reticulospinal output.