Further findings document complex interactions of IHI with various intracortical inhibitory circuits in the motor cortex ( Daskalakis et al., 2002 Chen et al., 2003). IHI is additionally modulated by various attributes of the motor output, such as the fineness of movements and contraction strength ( Perez and Cohen, 2008 Morishita et al., 2012 Kuo et al., 2017). Similar to the paired-pulse paradigms, stimulation of the ipsilateral M1 during voluntary movements attenuates the electromyographic (EMG) activity of the moving limb, termed the ipsilateral silent period (iSP) ( Ferbert et al., 1992 Cincotta and Ziemann, 2008 Giovannelli et al., 2009). The interstimulus interval (ISI) between the conditioning and test stimulus affects the strength of IHI ISIs of 3–5 ms can even produce facilitation of the MEPs ( Ferbert et al., 1992 Hanajima et al., 2001). A test stimulus following a conditioning stimulus of the contralateral hemisphere with a 6–15 ms delay results in depressed motor evoked potentials (MEPs) ( Ferbert et al., 1992 Di Lazzaro et al., 1999 Daskalakis et al., 2002). Interhemispheric inhibition has been directly demonstrated with transcranial magnetic stimulation (TMS) using a paired-pulse paradigm. Evidence shows that the motor cortex recruits transcallosal networks that primarily evoke interhemispheric inhibition (IHI) in homologous regions of the contralateral motor cortex ( Ferbert et al., 1992 Cincotta and Ziemann, 2008 Hübers et al., 2008). Although limb movements are predominantly driven by the contralateral motor cortex, entirely unilateral movements elicit correlated activity in the ipsilateral hemisphere ( Tanji et al., 1988 Donchin et al., 1998 Cardoso et al., 2001). However, these studies often have limited timing of stimulation and examine one limb and the contralateral hemisphere in isolation. These results show that changes caused by CS to the functional coupling within and between precentral cortices is contingent on the timing of CS relative to movement.Ĭortical stimulation (CS) delivered during different phases of movement has been shown to affect motor function ( Pascual-Leone et al., 1992 Bütefisch et al., 2004 Thabit et al., 2010 Edwardson et al., 2015). Simultaneous LFP recordings from one animal revealed correlations between changes in interhemispheric alpha band coherence and changes in RT, suggesting that alpha activity may be indicative of interhemispheric communication. All other stimulus conditions as well as random stimulation and periodic stimulation did not have consistently significant effects on either limb.

Stimulation delivered before ipsilateral limb movement decreased ipsilateral RT. In contrast, stimulation delivered after the end of contralateral movement increased contralateral RT but decreased ipsilateral RT. Stimulation delivered before contralateral limb movement onset shortened the contralateral limb RT. During a given session CS was delivered to one hemisphere with respect to movements of either the contralateral or ipsilateral limb. Three macaques were trained in a unimanual reaction time (RT) task and implanted with epidural or intracortical electrodes over bilateral motor cortices. We sought to bridge these two approaches by documenting the consequences of delivering CS to a single motor cortex during different phases of contralateral and ipsilateral limb movement, and simultaneously assessing changes in interactions within and between the hemispheres via local field potential (LFP) recordings. On the other hand, studies exploring interhemispheric interactions typically deliver CS at rest. However, previous studies that have investigated the effects of delivering CS during movement typically focus on a single hemisphere. Cortical stimulation (CS) of the motor cortex can cause excitability changes in both hemispheres, showing potential to be a technique for clinical rehabilitation of motor function.

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