Nonlinear analysis of EEG after repetitive transcranial magnetic stimulation

The purpose of this study was to investigate whether repetitive transcranial magnetic stimulation (rTMS) can change nonlinear dynamic properties of the cerebral cortex. Two rTMS trains (10 Hz, 3 seconds, 100% of motor threshold) were administered to the left frontal area in healthy subjects. EEG signals were collected at 14 electrode sites before and after stimulation, and were filtered digitally into delta, theta, alpha, beta, and gamma bands. Basing on an improved algorithm introduced in the authors' recent study, dimension estimates were calculated on these signals as well as on the corresponding surrogate data. Sham treatment was designed into this study. The data showed that EEG signals obviously exhibited lower dimension estimates than the surrogate data, whereas the theta and alpha rhythms presented the lowest values among the frequency components. rTMS increased the dimension estimates of EEG signals during the first 2 minutes. Similar findings were also obtained on the delta and beta components. This study revealed that EEG signals in the normal state can be described by a nonlinear dynamic process. This process can be affected temporarily by rTMS. Neuronal networks revealed by EEG signals show more complicated properties after stimulation.