Modulation of motor surround inhibition during motor tasks

Abstract

Surround inhibition (SI) in the motor system is important in individuation of actions, but is sometimes difficult to demonstrate. It has also not been evaluated in real life tasks. In this study, we use real life tasks and a new method where excitability of the surround muscle is assessed with respect to its current activity level rather than when it is at rest. Motor evoked potential (MEP) amplitudes were measured in the abductor digiti minimi (ADM) muscle while participants performed several motor tasks: "writing" on paper, "holding a pen" precisely and, "holding a water bottle" against gravity. These MEPs were compared to ADM MEPs amplitudes measured during a fifth finger abduction (ADM being the center muscle). SI was also measured in the traditional way, by comparing ADM MEPs during an index finger flexion and at rest. For the "writing" and "holding a pen" tasks, but not the "holding bottle" task, the MEP amplitudes were significantly smaller when compared to MEP amplitudes when the ADM was the center muscle with the same level of activation. The ADM MEP amplitudes were not different between rest and during index finger flexion. The new method employed here shows, that motor SI can be measured during tonic movements. The findings also show motor SI during two real-life motor tasks: "writing" and "holding a pen". The lack of modulation of MEP amplitude during "holding bottle" task seems to indicate that SI is action specific rather than muscle specific.

Keywords: Motor cortex; Motor evoked potential; Surround inhibition; Transcranial magnetic stimulation; Writing.