OP 10. Phasic modulation of somatosensory perception by means of transcranial alternating current stimulation

Abstract

Ongoing oscillations are associated with brain functions such as somatosensory perception. For example, the amplitude of the sensorimotor mu rhythm can be linked to the perception of near-detection-threshold somatosensory stimuli ( Linkenkaer-Hansen et al., 2004 ). Furthermore the phase of neuronal oscillations affects the perception of near-threshold stimuli ( Busch et al., 2009 ). Transcranial alternating current stimulation (tACS) may offer the possibility to modulate oscillatory activity. Recently it was shown that tACS increased the amplitude of visual alpha oscillations ( Zaehle et al., 2010 ) and had a phase dependent influence on auditory perception ( Neuling et al., 2012 ). We examined the effect of tACS applied at participants’ individual mu frequency on threshold levels of somatosensory perception. We hypothesized that (a) tACS modulates somatosensory perception thresholds as compared to sham and (b) perception thresholds vary as a function of the phase of tACS. In a randomized, single-blinded, crossover design, 17 participants (mean age: 27; female: 10) underwent a combined EEG/tACS experiment in two separate sessions (real or sham tACS). In the beginning, subject’s individual mu frequency was derived from the event-related desynchronization over the left somatosensory cortex (S1) induced by electric pulses to the right index finger ( Fig. 1 b). Subsequently, somatosensory detection thresholds were determined in a block of 16 min using an adaptive staircase procedure of weak electric stimuli that were presented with electrodes at the right index finger. During the second third of the task 5 min of tACS was applied at the individual mu frequency in a bilateral montage over both primary somatosensory cortices (S1). For sham, 30 s of 1 mA random noise stimulation was applied ( Fig. 1 a). Behavioral performance was assessed with respect to (i) an average effect of tACS as compared to sham and (ii) a modulation dependent on the tACS phase. No differences in the average somatosensory perception thresholds were observed between real and sham stimulation. However, during tACS, somatosensory detection thresholds changed as a function of the phase of tACS. Thresholds were differing maximally for stimuli presented at opposite phases in both maxima of the tACS signal curve( Fig. 2 ). We conclude that tACS applied at the individual mu frequency over S1 is capable of modulating perception of near-threshold somatosensory stimuli in a phase-dependent manner. Our findings suggest that functionally relevant intrinsic oscillations may be modulated using non-invasive brain stimulation.