Theta activity, virtual navigation and the human hippocampus.

Although the authors do not emphasize the point, there appears to be more theta from the temporal lobe electrodes than from the others. Because surface electrodes were used, the present study could not identify the generator for this activity, much less localize it to the hippocampal formation. Furthermore the authors did not comment on the relationship between theta activity in different areas so it is difficult to tell whether one is dealing with one or more synchronizing mechanisms. It is also not clear from the paper how specific the theta activation was to the navigational task. Ideally these results need to be corroborated by investigations using depth electrodes and a wider range of behavioral tasks. There has also been recent interest in the related field of frequency analysis of EEG recorded from scalp electrodes. For example, increases in the power present in the EEG at theta frequencies have been shown to be related to the successful encoding of new information (see Ref. 4 for a review). However, these studies have not specifically addressed navigation, nor do they typically demonstrate the peak in the power spectrum at theta frequencies or the long continuous records trains of theta activity shown by Kahana et al. (see Fig. 1). Localizing the source of theta in scalp-recorded EEG is even more problematic than from subdural electrodes and, in one experiment where it was attempted, implicated the anterior cingulate rather than the hippocampus (in a task showing increased theta with increased working memory load5). In addition, spectral peaks at theta frequencies have been found in experiments using magnetoencephalography, and have been interpreted as consistent with a generator near the hippocampus6, although this technique also suffers from problems of accurate source localization. Nevertheless, these findings, and particularly the subdural recordings of Kahana et al., clearly show that theta activity exists in the human brain and tempt one to speculate that it might be related to the hippocampal system. O ’ K e e f e a n d B u r g e s s – H u m a n t h e t a a c t i v i t y Update Comment