Effects of Task and Persistence on Responses to Transcranial Magnetic Stimulation
Sanjay Yathiraj, William J. Triggs, Michael S. Young, Fabian Rossi
Tuesday April 15 3:00 pm / Exhibit Hall A
To investigate the effects of motor task and task persistence on electromyographic responses to transcranial magnetic stimulation (TMS).
Datta et al. (1989) reported that responses to TMS were larger during index finger abduction than during an electromyographically equivalent power grip. Flament et al. (1993) reported the opposite finding. However, Datta et al. administered TMS while subjects performed these motor tasks intermittently, while Flament et al. administered TMS while subjects sustained isometric contractions in each task, and discarded the initial response to TMS obtained during performance of each task. Thus, neither group tested the possibility that responses to TMS are affected by whether a task has been recently initiated, or is sustained. We hypothesized that responses to TMS would be larger when tasks are repeatedly initiated as opposed to sustained, and that this effect would be greater for index finger abduction than for power grip, consistent with a relatively greater role for the corticospinal system in the control of independent finger movements.
Ten subjects received 20 stimulations with a vertex-centered circular magnetic coil at resting threshold intensity while contracting the right first dorsal interosseous (FDI) muscle at 10% of maximum under each of four conditions: sustained index finger abduction, sustained power grip, intermittent repeated index finger abduction, and intermittent repeated power grip. Motor evoked potential (MEP) amplitude in FDI was averaged for each condition after rectification. Condition order (task and persistence) was counterbalanced across subjects. RESULTS: Repeated measures ANOVA of mean MEP amplitude using task (i.e. finger abduction vs. power grip) and task persistence (i.e. intermittent vs. sustained contraction) as grouping variables showed a significant main effect of task persistence, such that responses to TMS were larger when tasks were performed intermittently and repeatedly.
Task initiation may require higher levels of cortical activity than continued performance of that task. This effect of motor set is greater for index finger abduction than for power grip, consistent with a role for the corticospinal system in the control of independent finger movements.
Datta AK, Harrison LM, Stephens JA. Task-dependent changes in the size of the response to magnetic brain stimulation in human first dorsal interosseous muscle. J Physiol 1989; 418: 13-23.
Flament D, Goldsmith P, Buckley CJ, Lemon RN. Task dependence of responses in first dorsal interosseous muscle to magnetic brain stimulation in man. J Physiol 1993; 464: 361-378.
Sponsored by: National Parkinsons Foundation.