Spectral Signatures of the Effects of Caffeine and Occipitally Applied Transcranial Magnetic Stimulation in a Task-Free Experimental Setup

Spectral activity of the brain strongly depends on the subject's arousal state. To study the dynamics of state-dependent activity in response to occipitally applied brain stimulation free from task-related confounds, we used a combination of transcranial magnetic stimulation (TMS), electroencephalography (EEG), and caffeine administration in a double-blind experimental setup with eight male subjects. Caffeine increased pre–TMS baseline gamma-band power compared with placebo control in both low-gamma (30–50 Hz) and high-gamma (50–80 Hz) bands. Surprisingly, TMS led to a decreased relative power of the poststimulation low-gamma-band activity under caffeine as compared with TMS in a placebo condition. In addition, caffeine administration was associated with a reduction of TMS-evoked alpha power of about 400 ms after TMS. When we analyzed the TMS-related raw power without baseline normalization, the gamma-band activity in both frequency bands was stronger for the caffeine conditions. These results show that c...