Hippocampal theta or rhythmic slow activity (RSA) occurring during exploratory behaviors
and rapid-eye-movement (REM) sleep is a characteristic and well-identifiable oscillatory rhythm in
animals. In contrast, controversy surrounds the existence and electrophysiological correlates of this
activity in humans. Some argue that the human hippocampal theta occurs in short and phasic bursts.
On the contrary, our earlier studies provide evidence that REM-dependent mesiotemporal RSA is
continuous like in animals but instead of the theta it falls in the delta frequency range. Here we used a
virtual navigation task in 24 epilepsy patients implanted with foramen ovale electrodes. EEG was
analyzed for 1-Hz wide frequency bins up to 10 Hz according to four conditions: resting, non-learning
route-following, acquisition and recall. We found progressively increasing spectral power in frequency
bins up the 4 Hz across these conditions. No spectral power increase relative to resting was revealed
within the traditional theta band and above in any of the navigation conditions. Thus the affected
frequency bins were below the theta band and were similar to those characterizing REM sleep in our
previous studies providing further indication that it is delta rather than theta that should be regarded
as a human analogue of the animal RSA
