Electrophysiological correlates of short-latency afferent inhibition: a combined EEG and TMS study

Cutaneous stimulation produces short-latency afferent inhibition (SAI) of motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS). Since the demonstration of SAI is primarily based on the attenuation of MEPs, its cortical origin is not yet fully understood. In the present study we combined TMS with concurrent electroencephalography (EEG) in order to obtain direct cortical correlates of SAI. TMS-evoked EEG responses and MEPs were analysed with and without preceding electrical stimulation of the index finger cutaneous afferents in ten healthy volunteers. We show that the attenuation of MEPs by cutaneous stimulation has its counterpart in the attenuation of the N100 EEG response. Moreover, the attenuation of the cortical N100 component correlated positively with the strength of SAI, indicating that the transient changes in cortical excitability can be reflected in the amplitude dynamics of MEPs. We hypothesize that the hyperpolarization of the pyramidal cells due to SAI lowers the capacity of TMS to induce the inhibitory current needed to elicit N100, thus leading to its attenuation. We suggest that the observed interaction of two inhibitory processes, SAI and N100, provides further evidence for the cortical origin of SAI.