Acute changes in motor cortical excitability during slow oscillatory and constant 2

26 Transcranial oscillatory current stimulation has recently emerged as a non-invasive 27 technique that can interact with ongoing endogenous rhythms of the human brain. 28 Yet, there is still little knowledge on how time-varied exogenous currents acutely 29 modulate cortical excitability. In ten healthy individuals we used online single-pulse 30 transcranial magnetic stimulation (TMS) to search for systematic shifts in 31 corticospinal excitability during anodal sleep-like 0.8 Hz slow oscillatory tDCS (so32 tDCS). In separate sessions, we repeatedly applied 30s trials (2 blocks á 20min) of 33 either anodal so-tDCS or constant tDCS (c-tDCS) to the primary motor hand area 34 during quiet wakefulness. Simultaneously and time-locked to different phase angles 35 of the slow oscillation, motor evoked potentials (MEP) as an index of corticospinal 36 excitability were obtained in the contralateral hand muscles 10, 20, and 30s after the 37 onset of tDCS. MEPs were also measured offline before, between and after both 38 stimulation blocks to detect any lasting excitability shifts. Both tDCS modes increased 39 MEP amplitudes during stimulation with an attenuation of the facilitatory effect 40 towards the end of a 30s tDCS trial. No phase-locking of corticospinal excitability to 41 the exogenous oscillation was observed during so-tDCS. Offline TMS revealed that 42 both c-tDCS and so-tDCS resulted in a lasting excitability increase. The individual 43 magnitude of MEP facilitation during the first tDCS-trials predicted the lasting MEP 44 facilitation found after tDCS. We conclude that sleep slow oscillation-like excitability 45 changes cannot be actively imposed on the awake cortex with so-tDCS, but phase46 independent online as well as offline facilitation can reliably be induced. 47 48