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Transcranial magnetic stimulation (TMS) of the motor cortex of one hemisphere (conditioning stimulus, CS) inhibits EMG responses evoked in distal hand muscles by a magnetic stimulus given at appropriate interval later over the opposite hemisphere (test stimulus, TS). The common interpretation attributes this effect to an inhibition produced at cortical level via a transcallosal route. The variability of cortical excitability as measured by the interhemispheric paired-pulse (PP) technique has been assessed in healthy subjects in order to compare sub- and supra-threshold intensity of CS (80% versus 120% of individual motor threshold, MT). Within- and between-subject variability relating, respectively, to interhemispheric and gender differences were also assessed. Results point to an efficacy of a magnetic CS on one hemisphere in inhibiting EMG responses of the abductor digiti minimi (ADM) stimulated by a TS delivered over the opposite hemisphere in a range of intervals centered at 12ms. These reductions were produced by the 120% suprathreshold CS, while the 80% subthreshold CS did not affect EMG responses. Females showed a higher transcallosal inhibition than males, suggesting gender differences in interhemispheric connectivity that concern the anterior half of the trunk of the corpus callosum.

Transcranial magnetic stimulation (TMS) of the motor cortex of one hemisphere (conditioning stimulus (CS)) inhibits EMG responses evoked in distal hand muscles by a later magnetic stimulus given at an appropriate interval, over the opposite hemisphere (test stimulus (TS)). This effect is commonly attributed to an inhibition produced at cortical level via a transcallosal route. The present study assessed the reproducibility of the transcallosal inhibition effects in different sessions in healthy subjects. Within- and between-subject variability, relating to interhemispheric differences was also evaluated. A magnetic CS on one hemisphere effectively inhibited EMG responses of the abductor digiti minimi stimulated by a TS delivered over the opposite hemisphere in a range of intervals centered at 12 ms. Even though group effects were reproduced in separate sessions, the high between- and within-subject variability yielded low test-retest correlations. This differentiation forces the definition of reproducibility (or repeatability), as the replication of the same mean curves of EMG reduction, and of reliability, as the between- or within-subject correlations between values of specific EMG measures. © 2003 Elsevier Science Ireland Ltd and the Japan Neuroscience Society. All rights reserved.