Dissociable roles of preSMA in motor sequence chunking and hand switching - a TMS study

Motor chunking, grouping of individual movements into larger units, is crucial for sequential motor performance. The pre-supplementary motor area (preSMA) is involved in chunking and in other related processes such as task-switching, response-selection, and response inhibition, that are crucial for organizing sequential movements. However, previous studies have not systematically differentiated the pre-SMA's role in motor chunking and hand-switching, thus leaving its relative contribution to each of these processes unclear. The aim of this study is to demonstrate the differential role of preSMA in motor chunking and hand-switching. We designed motor sequences in which different kinds of hand-switches (switching towards the right or left hand or continuing with the right hand) were counterbalanced across between- and within-chunk sequence points. 18 healthy, right-handed participants practiced four short sub-sequences (chunks) of key-presses. In a subsequent task, these chunks had to be concatenated into one long sequence. We applied double-pulse TMS over left preSMA or left M1, at sequence initiation, between chunks, or within chunks. TMS over left preSMA significantly slowed the next response when stimulation was given between chunks, but only if a hand-switch towards the contralateral (right) hand was required. PreSMA stimulation within chunks did not interfere with responses. TMS over left M1 delayed responses with the contralateral hand, both within and between chunks. Both preSMA and M1 stimulation decreased response times at sequence initiation. These results suggest that left preSMA is not necessary for chunking per se, but rather for organizing more complex movements that require chunking and hand-switching simultaneously.

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