Modulation of Theta and Gamma Oscillations during a Place and Response Task

The hippocampus contributes to the formation of long term memory. However, exactly how the hippocampal neurons contribute to memory formation has yet to be fully elucidated. There is evidence that rhythmical oscillations such as theta (4-12 Hz) and gamma (25-140 Hz) are involved in mnemonic processes. Neural oscillations transiently synchronize distributed neurons processing similar information and align periods of inhibition thereby allowing precise coordination of neuronal input from behaviorally relevant stimuli. ^ Theta oscillations in the hippocampus are postulated to support various cognitive processes such as working memory, decision making, and spatial navigation in humans and rodents. Theta is thought to provide a clocking system for neuronal, support synaptic plasticity, and facilitate the encoding new information. Additionally, lesions that impair theta oscillations subsequently disrupt spatial learning. ^ Gamma oscillations support perception, sensory binding, attention, working memory, language processing, and synaptic plasticity. Gamma oscillations are also involved in non-mnemonic processes such as movement initiation and reward processing. Recent studies suggest that gamma oscillations can be segregated into low (25-55 Hz) and high (65-140 Hz) gamma frequencies, each with their own internal generators and behavioral functions. ^ There are anatomical, electrophysiological, and functional dissociations along the longitudinal (dorsal-ventral) axis of the hippocampus. Few studies have simultaneously recorded theta and gamma oscillations from dorsal and ventral hippocampus; though dissociations have been noted in theta power and in how running speed modulates theta power. ^ In the current study rats were trained to continuously switch between a hippocampal-dependent place or striatal-dependent motor-response strategy. The level of difficulty and hippocampal-dependence was varied while theta and gamma were recorded in the dorsal and ventral hippocampus during different stages of decision making and learning. ^ The most striking and consistent finding was that theta and gamma oscillations were modulated during the cognitive component of the task. Theta power increased in the dorsal, but decreased in the ventral hippocampus. Low gamma power decreased in both the dorsal and ventral hippocampus. High gamma power selectively increased in the dorsal hippocampus. Understanding what these different neural oscillations do during different cognitive processes will help elucidate how they contribute to the formation of memory.