Inhibitory and excitatory interhemispheric transfers between motor cortical areas in normal humans and patients with abnormalities of the corpus callosum

Abstract

Transcranial magnetic stimulation of the motor cortex was performed in 10 normal subjects and 10 patients with radiographical abnormalities of the corpus callosum. Seven patients had a complete or partial agenesis or hypoplasia of the corpus callosum, two had a thin corpus callosum due to hydrocephalus or white matter degeneration and one had a circumscript contusion lesion of the corpus callosum. The patients served as a clinical model to investigate transcallosal influences on excitatory and inhibitory effects of motor cortex stimulation and to assess the potential diagnostic use of interhemispheric conduction studies and the contribution of interhemispheric interaction on transcranially elicited contralateral excitatory and inhibitory motor responses. Stimulation over one motor cortex suppressed tonic voluntary electromyographic activity in ipsilateral hand muscles in all subjects with preserved anterior half of the trunk of the corpus callosum. Since this suppression was lacking or had a delayed onset latency in patients with absence or abnormalities of the anterior half of the trunk of the corpus callosum it can be concluded that it is due to a transcallosal inhibition (Ti) of the opposite motor cortex mediated by fibres passing through this part of the corpus callosum. In normal subjects Ti had an mean onset latency of 36.1 +/- 3.5 ms (SD) and a duration of 24.5 +/- 3.9 ms. The calculated mean transcallosal conduction time was 13 ms. The threshold of Ti recorded in muscles ipsilateral to stimulation tended to be higher than the one for eliciting excitatory contralateral motor responses (56 +/- 6% versus 46 +/- 10% maximum stimulator output). Cortical thresholds (at rest) for contralateral excitatory hand motor responses were higher in patients with developmental abnormalities of the corpus callosum than in normals (66 +/- 17% versus 46 +/- 10% maximum stimulator output), which probably reflects also a facilitatory transcallosal interaction of both motor cortices in normals. In contrast, facilitation of cortically elicited motor responses in one hand by strong contraction of the other hand was the same in the patients with agenesis of the corpus callosum and normals, which suggests that this facilitatory spread takes place on a spinal rather than on a cortical level. Central motor latencies and amplitudes of contralateral hand motor responses were the same in patients with developmental abnormalities of the corpus callosum and normals (6.1 +/- 0.7 ms versus 6.3 +/- 0.7 ms and 6.7 +/- 2.4 mV versus 6.6 +/- 2.9 mV) so that callosal transfers do not seem to influence corticospinal conduction properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Transcranial magnetic stimulation in children

“…The iSP was present in both typically developing adolescents and in the adolescents with spastic paraplegia, but was absent in the subjects with spastic diplegia (Heinen et al, 1999). The iSP is mediated via the mid-body of the corpus callosum which carries fibers connecting the motor areas of both hemispheres (Meyer et al, 1995). It would be interesting to determine if the iSP is present but abnormal in children with mild spastic diplegia (providing an accurate reflection of callosal structural deficits in these children (Meyer et al, 1999)) and if they are related to severity of their clinical neuromotor deficits.…”

Noninvasive brain stimulation in stroke rehabilitation

“…65 Transcallosal fibers are known to transmit inhibitory influences between the homologous areas of both hemispheres. 66 These fibers are thought to be glutamatergic and to project onto inhibitory GABAergic interneurons. 67 Patients with stroke have changes in motor cortical excitability 68 -70 and an abnormally high in-terhemispheric inhibition from the contralesional M1 to the ipsilesional M1 with movements of the paretic hand.…”

Stabilization of bimanual coordination due to active interhemispheric inhibition: a dynamical account

“…In particular, links between bilateral motor areas may play an important role in suppressing mirror movements, that is, associated movements in arms or hands not intended to move (Armatas et al 1994;Leinsinger et al 1997;Daffertshofer et al 1999), implying that those connections are effectively inhibitory. The existence of interhemispheric inhibition has indeed been demonstrated by applying transcranial magnetic (conditioning) stimuli over the motor cortex in one hemisphere, which turned out to affect responses to stimuli over the motor cortex in the other hemisphere (Ferbert et al 1992;Meyer et al 1995;Boroojerdi et al 1996;Ikeda et al 2000;Hanajima et al 2001;Meyer-Lindenberg et al 2002). The interhemispheric inhibition in question may be achieved directly via the corpus callosum, although various cortical areas may play a mediating role (for review see, e.g., Chen et al 2003).…”