Physiological connectivity of the left ventral premotor cortex to the ipsilateral primary motor cortex explored by TMS

S. Schippling; G. Koch; T. Bäumer; J. Rothwell; A. Münchau

doi:10.1055/s-2006-953003

Aktuelle Neurologie, Table of Contents

Physiological connectivity of the left ventral premotor cortex to the ipsilateral primary motor cortex explored by TMS

S. Schippling ¹, G. Koch ¹, T. Bäumer ¹, J. Rothwell ¹, A. Münchau ¹

¹Hamburg; London, UK

Abstract

Paired transcranial magnetic stimulation (TMS) has been applied as a probe to test functional connectivity within distinct cortical areas of the motor system. Depending on the intensity of a conditioning stimulus applied to different areas of the cortical motor network both facilitation and inhibition may be detected in the primary motor cortex (M1), ipsilaterally or contralaterally to the site of conditioning stimulation. Civardi (2001) and our group (Koch; unpublished data) reported that conditioning stimuli applied to the dorsal premotor cortex (PMd) may induce distinct effects on ipsilateral M1 depending on the intensity of stimulation. Low conditioning intensities provoked inhibition with a maximum at 90% active motor threshold (AMT) which turned into facilitation when higher intensities (120% AMT and 110% RMT, respectively) were applied.

Very recently, Lemon and co-workers have demonstrated robust facilitation of M1 motor outputs to intrinsic hand muscles by conditioning the ipsilateral ventral premotor cortex (PMv/area F5) in the Macaque monkey (Cerri et al., 2003; Shimazu et al., 2004). Here we report data on an area anterolateral to the PMd in humans that elicits strong facilitation on motor outputs of the ipsilateral M1 hand area as tested with single pulse TMS.

We tested five different conditioning stimulus intensities (CSI) (70, 80 and 90% AMT and 90 and 110% RMT, respectively). Interestingly, we found complementary results to those reported on the PMd. Repeated measures ANOVA showed that different effects were obtained depending on the CSI (ANOVA main factor F=13.2; p<0.001), with no specific time course profile. Whereas low intensities evoked facilitation of ipsilateral M1 motor output with a maximum at an interstimulus interval (ISI) of 6 ms and a conditioning stimulus intensity of 80% AMT (140% of the MEP evoked by the test pulse alone; p=0.008), higher intensities provoked inhibition (maximum: ISI 4 ms; CSI 90% RMT; 80% of the baseline MEP; p=0.04). The facilitating effect was even stronger when paired conditioning stimuli were applied (at ISI of 2.6 ms; p=0.013).

In a stereotactically (Brainsight) guided control experiment the area of conditioning stimulation turned out to be slightly anterior to the area of Broca (area 44), in the range of what Rizzolatti and co-workers proposed to be the human ventral premotor cortex and homologue of monkey F5 (Rizzolatti et al., 1998).