Frequency characteristics of evoked and oscillatory electroencephalic activity in a human memory scanning task.

Research on memory scanning tasks indicates that oscillatory activity increases with load during retention, whereas evoked power (EP, comprising the P3) decreases during retrieval. We investigate the question, whether both phenomena are primarily related to theta oscillations. However, we found that during retention alpha oscillations increased with load and that exactly this frequency exhibits increased phase locking - measured by a specially developed phase locking index (PLI) - during retrieval. The decreased P3 amplitude was related to decreased delta EP and PLI. The P3 coincides with the last of three evoked alpha peaks. Thus, alpha may be important for the timing of the scanning and the evaluation of the read out process that most likely is manifested by the P3.