Changes in EEG power spectra during biofeedback of slow cortical potentials in epilepsy

The goal of the study was to explore parallel changes in EEG spectral frequ encies during biofeedback of slow, cortical potentials (SCPs) in epilepsy p atients. Thirty-four patients with intractable focal epilepsy participated in 35 sessions of SCP self-regulation training. The spectral analysis was c arried out for the EEG recorded at the same electrode site (Cz) that was us ed for SCP feedback, The most prominent effect was the increase in the thet a 2 power (6.0-7.9 Hz) and the relative power decrement in all other freque ncy bands (particularly delta 1, alpha 2, and beta 2) in transfer trials (i .e., where patients controlled their SCPs without continuous feedback) comp ared with feedback trials. In the second half of the training course (i.e., sessions 21-35) larger power values in the delta, theta, and alpha bands w ere found when patients were required to produce positive versus negative S CP shifts. Both across-subject and across-session (within-subject) correlat ions between spectral EEG parameters, on the one hand, and SCP data, on the other hand, were low and inconsistent, contrary to high and stable correla tions between different spectral variables. This fact as well as the lack o f considerable task-dependent effects during the first part of training, in dicates that learned SCP shifts did not directly lead to the specific dynam ics of the EEG power spectra. Rather these dynamics were related to nonspec ific changes in patients' brain state.