Theta rhythm across the species: Bridging inconsistencies with a multiple memory systems approach

The discovery of movement-modulated 7-8 Hz theta rhythm in rodents provided one of the earliest indications of synchronicity of neuronal firing in the hippocampus. Subsequent research expanded on this finding on theta rhythm and its role in other domains such as spatial navigation and memory. Nevertheless, discrepancies among animal models posed the question of how well the animal literature represents human mechanisms. Technological advances have since facilitated research taking place directly in humans, typically neurosurgical patients implanted with intracranial electrodes. Human studies have observed lower frequency oscillations in participants engaged in virtual spatial navigation studies, compared to the 7-8 Hz oscillations seen in rodents. Recently, research in humans engaged in real-world task demonstrating 7-9 Hz oscillations infer that the previously observed lower frequency theta may have been constrained by the absence of movement-related processing that occurs in physical activities. Together, these findings suggest that humans may indeed be more similar to rodent models than previously reported when completing tasks more analogous to those used in rodent experiments. This review summarizes early work on theta rhythm across species and outlines certain remaining discrepancies as well as a novel proposed hypothesis of the behavioral correlates of theta rhythm, taking a multiple memory systems perspective. (PsycInfo Database Record (c) 2021 APA, all rights reserved).