Theta Oscillations and the Comparator Function of the Hippocampus

Abstract

Detecting environmental changes/novelty is of basic importance for adaptive behavior. By comparing the current context with the previous one, living organisms can make predictions and adjust their actions. The mechanisms and structures of the brain involved in the function of comparison have not yet been sufficiently elucidated. The available studies emphasize the special contribution of the hippocampus to the process of comparison; it is indicated that the identification of novelty is carried out by hippocampal neurons through the mechanisms of match/mismatch, or misalignment. Here, we provide information about existing hypotheses of how these mechanisms occur, which other brain structures are involved in detecting inconsistencies, how they are related to the hippocampus, and what processes contribute to this. In particular, it is assumed that it is not novelty in itself, but only novelty that contrasts with previously acquired experience that initiates the process of misalignment. The arguments that the theta rhythm plays a crucial role in the functioning of the hippocampus as a comparator are analyzed. Theta oscillations caused by the appearance of a new signal or a change in a situation mediate the mechanism of temporal coordination of structures involved in the comparison function. In comparison, the theta rhythm functions as an active filter: it participates in the selection and transmission of a new signal to the registration system in the hippocampus. An increase in theta oscillations and their coherence in the brain structures processing new information serves as a signal of misalignment, facilitating a change in behavior strategy. In addition to theta rhythm, gamma oscillations are also involved in comparison: during the generation of theta rhythm in the prefrontal cortex, the temporary coincidence of gamma oscillations in other areas of the brain with a certain phase of the theta cycle can perform the function of comparison during the memorization process. A deep understanding of the mechanisms of comparator function and its disorders can help in the treatment of pathologies such as schizophrenia, Alzheimer’s disease, and temporal lobe epilepsy.