Movement-related changes in cortical excitability: a steady-state SEP approach

This study explored the use of steady-state somatosensory evoked potentials (ssSEPs) as a continuous probe on the excitability of the somatosensory cortex during the foreperiod and the response time of a cued choice reaction time task. ssSEPs were elicited by electrical median nerve stimulation at the left and right wrist, using a stimulation frequency of 22.2 Hz. Scalp-recorded ssSEPs were analysed by means of dipole source analysis to achieve optimal separation of left and right hemisphere ssSEPs. The time course of ssSEP modulation at the source level was extracted by means of a wavelet transform. In addition to the extraction of ssSEPs, the analysis included the derivation of lateralized attention and movement-related potentials, i.e. the attention-directing anterior negativity (ADAN) and the lateralized readiness potential (LRP). The results revealed a time course of ssSEP modulation remarkably similar to the time course of ADAN and LRP. The time course was characterized by a reduction of ssSEP amplitude at latencies just following the peak latency of the ADAN (approximately 400 ms) and the peak latency of the LRP (approximately 1200 ms). This reduction was greater for contralateral than for ipsilateral movements. The study demonstrates that ssSEP methodology represents a feasible approach to the measurement of movement-related changes in cortical excitability, which may be used to resolve ambiguities in the interpretation of lateralized event-related brain potentials.