 Theta oscillations have classically been described in rats, but evidence is emerging that they also occur in humans.In 1993, O'Keefe and Recce discovered a relationship between the theta wave and the firing patterns of individual place cells.Although the occasional action potentials of cells when rats were outside of the place fields occurred in phase with (at the peaks of) the theta waves, the bursts of more rapid spikes elicited when the rats reached the place fields were out of synchrony with the oscillation.