Interhemispheric interactions between M1 and PM during movement preparation

Precise interhemispheric interactions constitute a necessary neural basis for bimanual and complex unimanual coordination, because integration of sensorimotor information of both hemispheres is required. At rest, the effect of interhemispheric interactions between homologous motor cortices (M1) is largely inhibitory (Gerloff et al., J. Physiol. (Lond.) 1998; 510, 249–259). In the process of the generation of a movement, interhemispheric inhibition (IHI) between the M1 ipsilateral to the moving hand (iM1) and the M1 contralateral to the moving hand (cM1) turns into facilitation ('release of IHI') close to movement onset (Murase et al., Ann. Neurol. 2004; 55, 400–409). Anatomical studies suggest that callosal connections between homologous M1 are sparse. In fact, denser connectivity exists between premotor cortex (PM) and M1 of the opposite hemisphere (Rouiller et al., Exp. Brain Res 1994; 102, 227–243). As an electrophysiological correlate of the latter, IHI has also been shown between PM and M1 (Mochizuki et al., J. Physiol (Lond.) 2004; 561, 331–338). In the present study, we set out to evaluate the movement-related dynamic modulation of IHI between the PM ipsilateral to the moving hand (iPM) and cM1. Specifically, we hypothesized that IHI from PM to cM1 is modulated earlier that IHI from iM1 to cM1, following a, in part, serial model of premotor and primary motor cortex activation (e.g., Gerloff et al., J. Clin. Neurophysiol. 1998; 15, 502–513).

IHI between iPM and cM1 as well as iM1 and cM1 was studied in 14 healthy subjects during a simple reaction time task using with an established bi-hemispheric double-pulse TMS paradigm (Murase et al., Ann. Neurol. 2004; 55, 400–409). The conditioning stimulus was either applied over the right iPM (PM-IHI) or the right M1 (M1-IHI) ipsilateral to the moving right hand. The test stimulus was always applied over the left M1.

At the earliest time point immediately after the go-signal comparable IHI was found with iPM and iM1 conditioning stimulation. Over time, IHI was reduced close to movement onset in both conditions (ANOVA, factor TIME, p<0,001). However there was a significant difference between M1-IHI and PM-IHI (ANOVA, p=0,02) with an earlier reduction of IHI following PM stimulation compared with M1 stimulation.

Whereas serial processing steps in the PM and M1 of the hemisphere contralateral to the moving hand are well accepted, the present data extends this view to interhemispheric interactions.