On the calculation of transcallosal conduction time using transcranial magnetic stimulation.

Transcallosal conduction time (TCT), based on the results of transcranial magnetic stimulation studies, is currently calculated as a function of the ipsilateral silent period (iSP) and of the motor evoked potential (MEP) obtained from a target muscle (TCTcurrent = iSP latency - MEP latency). We argue that this measure overestimates TCT and may lead to a bias in statistical group comparisons. We propose an alternative measure, TCTproposed, which we defined as TCTproposed = iSP latency - cSP latency, where cSP is the contralateral silent period. We report our results on the comparison of the two measures in twenty healthy individuals and provide a theoretical basis for TCTproposed.