Abstract
BackgroundThe interaction between homologous muscle representations in the right and left primary motor cortex was studied using a paired-pulse transcranial magnetic stimulation (TMS) protocol known to evoke interhemispheric inhibition (IHI). The timecourse and magnitude of IHI was studied in fifteen healthy right-handed adults at several interstimulus intervals between the conditioning stimulus and test stimulus (6, 8, 10, 12, 30, 40, 50 ms). IHI was studied in the motor dominant to non-dominant direction and vice versa while the right or left hand was at rest, performing isometric contraction of the first dorsal interosseous (FDI) muscle, and isometric contraction of the FDI muscle in the context of holding a pen.ResultsCompared with rest, IHI was reduced at all ISIs during contraction of either type (with or without the context of pen). IHI was reduced bi-directionally without evidence of hemispheric dominance. Further, contraction of the hand contralateral to the conditioning and test pulse yielded similar reductions in IHI.ConclusionThese data provide evidence for bi-directional reduction of IHI during unimanual contractions. During unimanual, sustained contractions of the hand, the contralateral and ipsilateral motor cortices demonstrate reduced inhibition. The data suggest that unimanual movement decreases inhibition bi-directionally across motor hemispheres and offer one explanation for the observation of ipsilateral M1 activity during hand movements.
Highlights
The interaction between homologous muscle representations in the right and left primary motor cortex was studied using a paired-pulse transcranial magnetic stimulation (TMS) protocol known to evoke interhemispheric inhibition (IHI)
The amplitude of the motor evoked potential (MEP) in the muscle contralateral to the test stimulus (TS) is suppressed at inter-pulse intervals between 6 and 50 ms [5,6,7] suggesting that interhemispheric inhibition (IHI) dominates the interaction during this time frame
Chen et al, (2003)[26] tested IHI during 50% maximum voluntary contraction (MVC) of the first dorsal interosseous (FDI) muscle at 8 and 40 ms interstimulus interval (ISI) and found that inhibition is decreased at short ISI (~10 ms)
Summary
The interaction between homologous muscle representations in the right and left primary motor cortex was studied using a paired-pulse transcranial magnetic stimulation (TMS) protocol known to evoke interhemispheric inhibition (IHI). Interhemispheric interactions between homologous muscle representations may be probed using a paired-pulse TMS technique whereby a conditioning stimulus (CS) is applied to the muscle representation in one hemisphere followed by a test stimulus (page number not for citation purposes). The amplitude of the motor evoked potential (MEP) in the muscle contralateral to the TS is suppressed at inter-pulse intervals between 6 and 50 ms [5,6,7] suggesting that interhemispheric inhibition (IHI) dominates the interaction during this time frame. Transcallosal connections mediate interhemispheric interactions between homologous muscle representations in the primary motor cortices (M1) [8,9]. The functional significance of the two IHI components remains unknown
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