Abstract
Introduction
Major functional modes of the human brain rely on neuronal oscillatory activity. Ongoing oscillatory activity can be associated with several brain functions such as sensory perception. For example, the amplitude of the mu rhythm, the predominant sensorimotor background oscillation, could be linked to perceiving near-detection-threshold somatosensory stimuli (Linkenkaer-Hansen et al., 2004). Transcranial alternating current stimulation (tACS) may offer the possibility to modulate such oscillatory activity, as shown recently for visual alpha oscillations (Zaehle et al., 2010). Here, we systematically examined the effect of different stimulation frequencies on the modulation of somatosensory mu rhythm. Specifically we hypothesized, that application of tACS, using each subject’s individual mu frequency (μind), has stronger modulatory effects than tACS with neighbouring frequencies (μind ± 1 Hz) or using a fixed frequency in a different range (20 Hz).
Methods
A total of 18 healthy volunteers took part in a combined EEG/tACS experiment. In a pre-test, each subject’s individual mu frequency was derived from the event-related desynchronization (for a review: Pfurtscheller and da Silva, 1999) over the left somatosensory cortex (S1) induced by electric pulses to the right index finger. Subsequently, tACS was delivered in a bilateral montage over both S1 during 4 blocks of 5 min length each, while the EEG was recorded simultaneously. The order of the blocks was counterbalanced across subjects and differed in stimulation frequency only: μind, μind +1Hz, μind -1Hz and 20Hz. Furthermore one sham stimulation block was applied. For analysis, we compared the mu amplitude during 120 sec before to 120 sec after each block using an ANOVARM with the factor (STIMULATION).
Results and conclusions
Application of tACS using the individual mu frequency resulted in an increase of the mu-rhythm amplitude (F = 3.475, p < .05), while the other stimulation conditions did not induce changes in this. We conclude, that tACS is capable of modulating ongoing somatosensory oscillations only when the exact individual frequency of the respective rhythm is applied.
References
Linkenkaer-Hansen, K., Nikulin, V. V., Palva, S., Ilmoniemi, R. J., & Palva, J. M., (2004), ‘Prestimulus Oscillations Enhance Psychophysical Performance in Humans’, J. Neurosci., vol. 24, no. 45, pp. 10186-10190.
Zaehle, T., Rach, S., & Herrmann, C. S., (2010), ‘Transcranial Alternating Current Stimulation Enhances Individual Alpha Activity in Human EEG’, PLoS ONE, vol. 5, no. 11, e13766.
Pfurtscheller, G., & Lopes da Silva, F. H. (1999), ‘Event-related EEG/MEG synchronization and desynchronization: basic principles’, Clinical Neurophysiology, vol. 110, no. 11, pp. 1842-1857.
