Distinct patterns of brain oscillations underlie two basic parameters of human maze learning

We examine how oscillations in the intracranial electroencephalogram (iEEG) relate to human maze learning. Theta-band activity (4-12 Hz in rodents; 4- 8 Hz in humans) plays a significant role in memory function in rodents and in humans. Recording intracranially in humans, we have reported task-relate d, theta-band rhythmic activity in the raw trace during virtual maze learni ng and during a nonspatial working memory task. Here we analyze oscillation s during virtual maze learning across a much broader range of frequencies a nd analyze their relationship to two task variables relevant to learning. W e describe a new algorithm for detecting oscillatory episodes that takes ad vantage of the high signal-to-noise ratio and high temporal resolution of t he iEEG. Accounting for the background power spectrum of the iEEG, the algo rithm allows us to directly compare levels of oscillatory activity across f requencies within the 2- to 45-Hz band. We report that while episodes of os cillatory activity are found at various frequencies, most of the rhythmic a ctivity during virtual maze learning occurs within the theta band. Theta os cillations are more prevalent when the task is made more difficult (manipul ation of maze length). However, these oscillations do not tend to covary si gnificantly with decision time, a good index of encoding and retrieval oper ations. In contrast, lower- and higher-frequency oscillations do covary wit h this variable. These results suggest that while human cortically recorded theta might play a role in encoding, the overall levels of theta oscillati ons tell us little about the immediate demands on encoding or retrieval. Fi nally, different patterns of oscillations may reflect distinct underlying a spects of memory function.