Interhemispheric facilitation of the hand motor area in humans

Abstract

We investigated interhemispheric interactions between the human hand motor areas using transcranial cortical magnetic and electrical stimulation. A magnetic test stimulus was applied over the motor cortex contralateral to the recorded muscle (test motor cortex), and an electrical or magnetic conditioning stimulus was applied over the ipsilateral hemisphere (conditioning motor cortex). We investigated the effects of the conditioning stimulus on responses to the test stimulus. Two effects were elicited at different interstimulus intervals (ISIs): early facilitation (ISI = 4–5 ms) and late inhibition (ISI ≥ 11 ms). The early facilitation was evoked by a magnetic or anodal electrical conditioning stimulus over the motor point in the conditioning hemisphere, which suggests that the conditioning stimulus for early facilitation directly activates corticospinal neurones. The ISIs for early facilitation taken together with the time required for activation of corticospinal neurones by I3‐waves in the test hemisphere are compatible with the interhemispheric conduction time through the corpus callosum. Early facilitation was observed in responses to I3‐waves, but not in responses to D‐waves nor to I1‐waves. Based on these results, we conclude that early facilitation is mediated through the corpus callosum. If the magnetic conditioning stimulus induced posteriorly directed currents, or if an anodal electrical conditioning stimulus was applied over a point 2 cm anterior to the motor point, then we observed late inhibition with no early facilitation. Late inhibition was evoked in responses to both I1‐ and I3‐waves, but was not evoked in responses to D‐waves. The stronger the conditioning stimulus was, the greater was the amount of inhibition. These results are compatible with surround inhibition at the motor cortex.

Modulation of motor cortex excitability after upper limb immobilization

“…Hyperexcitability of the right cortex, projecting to the immobilized arm, might have been partially mirrored to the contralateral cortex, as a result of interhemispheric transfer of plasticity between corresponding cortical fields (Calford and Tweedale, 1990). Moreover, the functional inactivity of the right cortex might have resulted in a reduction of the inhibitory influences on the contralateral active areas (Hanajima et al, 2001). An alternative explanation is that the slight hyperexcitability of the projections to the free arm may depend on 'use-dependent' plasticity, because patients were increasingly dependent on the free right arm in everyday life.…”

Human Motor Corpus Callosum: Topography, Somatotopy, and Link between Microstructure and Function

“…The resting motor threshold (RMT) was determined for the conditioning pulse to the nearest 1% of maximum stimulator output and was defined as the minimum stimulus intensity that resulted in liminal MEPs Ͼ50 V in at least 5 of 10 trials. IHI increases with the intensity of the conditioning stimulus (Ferbert et al, 1992;Hanajima et al, 2001). Accordingly, the intensity of the conditioning stimulus was varied from 100 to 150% RMT in 10% steps (i.e., six different intensities) to obtain a wide range of IHI magnitudes from threshold to maximum.…”

“…The intensity of the test pulse was adjusted to produce an unconditioned MEP of on average 1 mV in peak-to-peak amplitude. The interstimulus interval between conditioning and test pulse was set to 12 ms, because previous studies showed reliable IHI at this interval (Ferbert et al, 1992;Hanajima et al, 2001). Eight trials per condition were run in randomized order, and conditional averages of the single-trial MEP amplitudes were calculated.…”

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).…”