Our analysis showed that the shifts from high gamma at the onset of the fixation to low gamma at the offset of the fixation occurred in both tasks; however, these trends were more pronounced in the memory-guided task. The differential intensity in the modes of information processing in CA1 likely reflects the necessary usage of memory in the memory-guided alternation task; this information needed to be suppressed in the visually guided random cue task.

Gamma and theta oscillations had been observed previously in several regions in the brain. In a previous study, theta activity had been seen in free moving rats [3]. Here, we show gamma and theta association during a quiet immobile period in CA1. Hippocampal areas CA1 and CA3 contribute to the retrieval of events in memory-dependent tasks [4]. Learning and cognition, on the other hand, appear to depend on the connectivity between CA1 and entorhinal cortex [5]. These different roles in the hippocampus reflect the different manipulation of received information. A recent study has reported that different waves of gamma were correlated with input from different areas in the hippocampus, with fast gamma originating from entorhinal cortex and slow gamma from CA3; these gamma oscillations occurred during different phases of the theta rhythm in CA1 [6].

Here we find that theta and gamma coupling occurs during the fixation period while rats are completely immobile and waiting for the next event to make their choice. We noted that the activation of the low gamma band during the memory-guided alternation task in CA1 was more pronounced than in the visual discrimination task, providing further empirical support for the role of hippocampus in sequence learning during memory-guided tasks.

Entorhinal cortex receives input from different reward-related areas in the brain like amygdala, and conveys the information to hippocampus. These different manipulations enable the extraction of contextual information and the translation of this information to codes necessary for decision-making. Previous studies have shown memory-guided sequence-dependent activity in hippocampus [7] and reported that the theta phase in CA3 was modulated by the low gamma band when rats learned to make associations in the spatial context [8]. In addition, type-two theta tended to increase during the immobile period [9].