LAUSR

Objectives Fluctuations in cortical excitability is a candidate mechanism involved in the trial-to-trial variation of motor evoked potentials (MEPs) to transcranial magnetic stimulation. We explore if oscillations in the infraslow (0.01–0.1 Hz), delta, theta, alpha and beta frequency bands are possible modulators of cortical excitability. Methods In ten healthy subjects 200 single pulses were given at the resting motor threshold. Induced motor responses were either regarded as present (MEP amplitude > 50  μ V) or absent. Temporal clustering of MEP presence/absence was tested ( α  = 0.05) using 1000 random generated surrogates with a similar presence/absence distribution. Secondly, the phases at stimulation were extracted for each frequency band using the Hilbert transform and divided into 18 equal bins between – π and π . For each phase bin, we determined the absolute value of the difference in percentage MEP presence and MEP absence. We compared the sum over all bins to the sums obtained in surrogates. If the sum was larger than the 75th percentile in surrogates, moderate coupling between phase and MEP presence/absence was assumed. For a sum larger than the 95th percentile, coupling was assumed to be strong. Results In all ten subjects we found significant temporal clustering of MEP presence/absence. In 30% of the subjects we found moderate, and in 10% strong, coupling between the phase of the infraslow frequency and MEP presence/absence. For the delta/theta/alpha/beta frequency bands coupling was moderate in 40%/20%/20%/30% of the subjects and strong in 20%/50%/30%/10% of the subjects, respectively. Discussion Temporal clustering of MEP presence/absence suggests oscillatory modulation of cortical excitability instead of randomly generated fluctuations. Our results do not provide strong evidence for one specific oscillatory frequency. Conclusions Oscillations in the infraslow, delta, theta, alpha and beta frequency band are all possible modulators of cortical excitability. Significance Gaining insight into the oscillations modulating cortical excitability might provide possibilities to reduce the large trial-to-trial variation, as stimulation can be targeted at a specific oscillatory phase.