Cortical stimulation over the motor cortex of one hemisphere (conditioning motor cortex) had two effects on responses to stimulation over the motor cortex of the other hemisphere (test motor cortex). (1) Responses to the test stimulus could be facilitated at short ISIs (early facilitation). This only occurred if (a) the test stimulus evoked I3-waves and (b) medially directed currents were used for the conditioning stimulus (induced by magnetic or electrical stimulation). Test responses produced by I1-waves or D-waves were never facilitated. No early facilitation was observed if posteriorly directed induced currents were used for the conditioning stimulus, or if anodal electrical stimulation was applied over a point 2 cm anterior to the motor point of the conditioning hemisphere. Facilitation was most prominent if the conditioning stimulus was over the hand motor area. (2) Responses to both I1- and I3-waves were suppressed by a conditioning stimulus at ISIs of 11-15 ms (late inhibition), but those to D-waves were not. This suppression was evoked by magnetic conditioning stimuli with medially or posteriorly directed currents in the conditioning motor cortex or by anodal electrical conditioning stimuli applied over a point 2 cm anterior to the motor point. There was no inhibition if the conditioning stimulus was an anodal electrical shock over the conditioning motor cortex at an intensity of +2 %. The amount of inhibition depended upon the absolute intensity of the magnetic conditioning stimuli irrespective of their current directions.
The lack of facilitation of I1-waves, even at ISIs of 7 or 8 ms, in the present study seems at first sight to be inconsistent with our previous report that facilitation of I1-waves could occur at these ISIs (Ugawa et al. 1993). However, this is probably due to the different size of the control responses in the two experiments. In our previous report, facilitation at these ISIs was only seen when the test responses were smaller than 0.1 mV (0.05 mV). There was no facilitation of responses larger than 0.2 mV (Ugawa et al. 1993). In the present experiments, we studied effects on responses of around 0.3 mV, and hence did not find any facilitation. The intensity of the conditioning stimulus (+5 %) was about the same in the two sets of experiments.
One explanation for these results is that the facilitatory effect on I1-waves is very weak and difficult to observe unless the I1-wave is small. Another possibility is that the early facilitation of I1-waves can be masked by late inhibition. However, responses to I1-waves were not enlarged at ISIs of 7 or 8 ms even when the intensity of the conditioning stimulus (+5 %) was adjusted to evoke no late inhibition. Thus inhibitory masking of facilitation seems unlikely even though we cannot completely exclude that possibility. Another possibility is that I1-waves are saturated when test responses have an amplitude > 0.2 mV. However, previous experiments with single motor unit and surface EMG recordings have shown that this is not the case, and that larger I1-waves can be evoked at higher stimulus intensities (Hanajima et al. 1998). Therefore saturation of I1-waves cannot explain the lack of facilitation. Our conclusion is that early facilitation of I1-waves occurs, but is too weak to have an influence on responses larger than 0.2 mV. In summary, we consider that early facilitation occurs moderately in I3-waves, weakly in I1-waves and does not occur in D-waves. We hereafter discuss the above two effects separately.
The fact that the best site for the conditioning stimulus for early facilitation was over the hand motor area of the conditioning hemisphere indicates that the facilitation is not due to current spread of the conditioning stimulus to the opposite hemisphere. The fact that test responses evoked by D-waves were not facilitated suggests that the effect occurs within the test motor cortex, which is consistent with a previous report (Salerno & Georgesco, 1996). The ISIs for the facilitation (4-5 ms) are compatible with the transcallosal effect. Cracco et al. (1989) reported that motor cortical stimulation evoked a potential over the contralateral motor cortex with an onset latency of 8.8-12.2 ms. In patients with cortical myoclonus (Shibasaki et al. 1978; Wilkins et al. 1984; Brown et al. 1991), differences in the onset latency between jerks of homologous muscles on the two sides of the body were about 10 ms. A posteriorly directed test stimulus over the test motor cortex is thought to activate corticospinal neurones about 4.5 ms after the test stimulus is given (I3-waves). Therefore, the facilitation at ISIs of 4 or 5 ms means that facilitation occurs in corticospinal neurones of the test motor cortex about 10 ms after the conditioning stimulus (4-5 ms plus 4.5 ms). A slight facilitation of small I1-waves at ISIs of 7-8 ms is also consistent with a transcallosal effect because ISIs of 7 or 8 ms added to the latency difference between I1- and D-waves (1.5 ms) makes about 10 ms. The best position for the conditioning stimulus for facilitation is also compatible with the interhemispheric facilitatory connection between homotopic areas of the motor cortices in the two hemispheres in animals (Asanuma & Okamoto, 1959, 1962; Naitou, 1970). Based on these arguments, we suppose that early facilitation of the test motor cortex is produced by transcallosal inputs evoked by cortical stimulation of the conditioning motor cortex.
Which structure was activated by the conditioning stimulus? Facilitation was only evoked by medially directed currents in the conditioning motor cortex, whether these were induced by magnetic or electrical stimulation. Posteriorly directed induced currents never produced facilitation. Previous work (Werhahn et al. 1994) has shown that medially directed currents preferentially activate the pyramidal tract neurones directly and posteriorly directed currents tend to activate them via interneurones in the motor cortex. This was confirmed in the present study, since the latency of responses in the left FDI was compatible with D-waves if the conditioning stimulus used medially directed currents. Low intensity posteriorly directed currents evoked responses that had a latency compatible with I3-wave activation. Thus the type of stimulation that produced the best early facilitation was the same as that which activated the corticospinal neurones directly. Indeed, the intensity required to produce the two effects was also very similar. It is therefore possible that collateral fibres of corticospinal neurones activated by the conditioning stimulus facilitate the contralateral test motor cortex through the corpus callosum. However, anatomical evidence for the existence of such collaterals is under debate (Künzle, 1975; Castman-Berrevoets et al. 1980; Matsunami & Hamada, 1984; Ezrokhi et al. 1985). Another possibility is that the conditioning stimulus activates small or medium-sized pyramidal neurones in layer III or V projecting to the corpus callosum (Jacobson & Trojanowski, 1974; Wise, 1975; Wise & Jones, 1976; Beck & Kaas, 1994). This seems less likely than the former possibility because smaller pyramidal neurones should have a higher threshold than the corticospinal neurones.
Why was early facilitation absent if posteriorly directed currents were used as the conditioning stimulus? After all, the conditioning stimulus still activates the corticospinal tract, and therefore should activate the postulated collaterals to the opposite hemisphere. The explanation is probably that the latency of any facilitation evoked by posteriorly directed conditioning stimuli will overlap with the late inhibition. Thus, facilitation would begin at I3-wave latency (4-5 ms) plus 4-5 ms (the ISI for early facilitation elicited by a medially directed conditioning stimulus), i.e. 8-10 ms in total, which is the same as the onset latency of late inhibition. In addition, the absolute intensity of a posteriorly directed conditioning stimulus was 10-15 % higher than an anteriorly directed one and would have elicited strong late inhibition, which could easily mask any small facilitation.
It is unlikely that the facilitatory and inhibitory effects on the excitability of the test hemisphere are the result of direct projections to large pyramidal cells in layer V. Such transcallosal connections have not been described in animal experiments (Chang, 1953; Jacobson & Marcus, 1970; Jones et al. 1979). Instead it seems more probable that callosal fibres activated in the present experiments affect cortical interneurones in the test motor cortex. The experiments of Schnitzler et al. (1996) in man are also compatible with this.
Previous studies in humans emphasized the inhibitory interaction between the hand motor areas in the two hemispheres (Ferbert et al. 1992; Meyer et al. 1995, 1998; Boroojerdi et al. 1996; Gerloff et al. 1998). Ferbert et al. (1992) showed inconsistent early facilitation in some subjects, and we later showed that this facilitation was consistently evoked only when small test responses (< 0.1 mV) were conditioned by a weak stimulus (Ugawa et al. 1993). In animals, stimulation of the motor cortex in one hemisphere also evokes a minimal facilitation in the motor cortex of the contralateral hemisphere followed by inhibition at late intervals (Chang, 1953; Asanuma & Okamoto, 1959, 1962; Matsunami & Hamada, 1984). However, even in animal experiments, this facilitation was not always present. When high intensity conditioning stimuli were applied, the facilitation was masked by suppression (Chang, 1953; Asanuma & Okamoto, 1959, 1962). Asanuma & Okuda (1962) showed that the hand motor area had a facilitatory connection with the homotopic area in the contralateral motor cortex, which was surrounded by a larger area of more powerful inhibition (surround inhibition). Because this surround effect is so prominent, interhemispheric facilitation may be difficult to observe in humans using transcranial stimulation. However, in the present paper, we identified the conditions under which it can be consistently studied. There are two methodological reasons for our success. One is that we used test responses that result from I3-wave volleys to the spinal cord. In most previous papers, test responses were elicited by anteriorly directed currents in the brain, which preferentially evoke I1-waves. I1-waves are facilitated much less. The other reason is that we used medially directed induced currents as a conditioning stimulus. In many previous reports, anteriorly directed induced currents were used as a conditioning stimulus. One possible explanation for our result that I3-waves were more enhanced by the early facilitation than I1-waves is that I3-wave generation involves more synapses in the motor cortex than I1-wave generation, and synapses are very susceptible to excitability changes. Another possibility is that different sets of cortical interneurones are responsible for I1- and I3-waves, and those for I3-waves are susceptible to transcallosal inputs.
Inhibition was provoked by the medially and posteriorly directed conditioning stimuli at ISIs later than 11 ms. These intervals are consistent with the ISIs for the interhemispheric inhibition reported by Ferbert et al. (1992). They proposed that the inhibition occurs at the cortical level because no inhibition was evoked in responses to electrical stimuli. Direct recordings of descending volleys (Di Lazzaro et al. 1999) also confirmed that the inhibition occurs at the cortex. Studies of patients with a lesion in the corpus callosum (Meyer et al. 1995, 1998; Boorojerdi et al. 1996) confirmed that this inhibition is transcallosal. Our results show that early facilitation was not associated with late inhibition when the conditioning stimulus was a magnetic stimulus of +5 % intensity or an electrical stimulus of +2 % intensity. This suggests that the inhibition is not a rebound phenomenon following the early facilitation. We consider that the late inhibition reported here is the same effect previously reported by Ferbert et al. (1992). We did not study this inhibition in detail for this reason.
At the motor point of the conditioning hemisphere, the threshold of the conditioning stimulus for late inhibition was higher than that for early facilitation. The amount of inhibition was positively correlated with the intensity of a conditioning stimulus, irrespective of its current direction. These results suggest that activation of large areas surrounding the motor point of the conditioning motor cortex or many callosal fibres is required for late inhibition. Many callosal fibres are activated by a strong conditioning stimulus and such large callosal inputs presumably evoke powerful surround inhibition in the test motor cortex. The fact that a conditioning electrical stimulus applied over a point 2 cm anterior to the motor point of the conditioning motor cortex evoked late inhibition without the early facilitation also supports the idea that the concept of surround inhibition is relevant to the mechanisms of late inhibition.
When late inhibition was elicited by electrical stimulation over a point 2 cm anterior to the motor point, it occurred at an ISI of 12 ms (Fig. 4B), which was a few milliseconds later than the onset of the late inhibition evoked by the medially or posteriorly directed magnetic conditioning stimulus over the motor point (Figs 2A and 4A). This may be due to the fact that activation of neurones surrounding the motor point at a point 2 cm anterior to it needs a few milliseconds more to affect the corticospinal neurones in either the conditioning or test motor cortex than that just over the motor point. The fact that even the shortest onset of late inhibition was longer than that of facilitation is also compatible with surround inhibition.
We have demonstrated a transcallosal facilitatory connection between the homotopic hand motor areas in the two hemispheres at short intervals in humans. At later intervals this was replaced by inhibition. This pattern of facilitation followed by inhibition is consistent with the effect of callosal stimulation on the motor cortex in animal experiments (Chang, 1953; Asanuma & Okamoto, 1959, 1962).