Title: Eeg Mean Frequency Changes in Healthy Subjects during Prefrontal Transcranial 1 Direct Current Stimulation (tdcs) 2 3 Dept of Neurological Sciences

30 31 In this pilot study we evaluated electroencephalographic (EEG) mean frequency changes induced 32 by prefrontal transcranial direct current stimulation (tDCS) and investigated whether they 33 depended on tDCS electrode montage. Eight healthy volunteers underwent tDCS for 15 minutes 34 during EEG recording. They completed six tDCS sessions, one week apart, testing left and right 35 direct current (DC) dipole directions with six different montages: four unipolar montages (one 36 electrode on a prefrontal area, the other on the opposite wrist) and two bipolar montages (both 37 electrodes on prefrontal areas), and a single sham session. EEG power spectra were assessed 38 from four one-minute EEG epochs, before, during and after tDCS. During tDCS the outcome 39 variable, brain rate (fb), changed significantly, and the changes persisted for minutes after tDCS 40 ended. With the DC dipole directed to the left (anode on the left prefrontal area or wrist) fb 41 increased, with the DC dipole directed to the right (anode on the right prefrontal area or wrist) fb 42 decreased, suggesting asymmetric prefrontal cortex functional organization in the normal human 43 brain. Anodal and cathodal effects were opposite but equally large. Gender left these effect 44 unchanged. 45 46