Non-invasive detection of ongoing neuronal population activity in normal human hippocampus

A 122-channel magnetoencephalographic array was used to monitor ongoing neuronal population activity in six normal human subjects during the performance of a mental calculation and passive viewing of a picture. Signal-space projection was utilized to obtain waveforms and spectra for activity in anterior hippocampus and sensorimotor cortex. Hippocampal waveforms were complex. Spectral components below 12 Hz in hippocampus included task-dependent peaks superimposed on a broadband background of increasing amplitude at decreasing frequencies. MEG imaging of both waveforms and spectra for ongoing activity within hippocampus makes possible the investigation of the role of rhythmic and non-rhythmic hippocampal activity in normal human cognition.