Intended arm use influences interhemispheric correlation of β-oscillations in primate medial motor areas

To investigate the role of interhemispheric β-synchronization in the selection of motor effectors, we trained two monkeys to memorize and perform multiple two-movement sequences that included unimanual repetition and bimanual switching. We recorded local field potentials simultaneously in the bilateral supplementary motor area (SMA) and pre- SMA to examine how the β-power in both hemispheres and the interhemispheric relationship of β-oscillations depend on the prepared sequence of arm use. We found a significant ipsilateral enhancement of β-power for bimanual switching trials in the left hemisphere and an enhancement of β-power in the right SMA while preparing for unimanual repetition. Furthermore, interhemispheric synchrony in the SMA was significantly more enhanced while preparing unimanual repetition than while preparing bimanual switching. This enhancement of synchrony was detected in terms of β-phase but not in terms of modulation of β-power. Furthermore, the assessment of the interhemispheric phase difference revealed that the β -oscillation in the hemisphere contralateral to the instructed arm use significantly advanced its phase relative to that in the ipsilateral hemisphere. There was no arm use-dependent shift in phase difference in the pairwise recordings within each hemisphere. Both neurons with and without arm use-selective activity were phase-locked to the β-oscillation. These results imply that the degree of interhemispheric phase synchronization as well as phase differences and oscillatory power in the β-band may contribute to the selection of arm use depending on the behavioral conditions of sequential arm use. NEW & NOTEWORTHY We addressed interhemispheric relationships of β-oscillations during bimanual coordination. While monkeys prepared to initiate movement of the instructed arm, β-oscillations in the contralateral hemisphere showed a phase advance relative to the other hemisphere. Furthermore, the sequence of arm use influenced β-power and the degree of interhemispheric phase synchronization. Thus the dynamics of interhemispheric phases and power in β-oscillations may contribute to the specification of motor effectors in a given behavioral context.