Coupling of Theta Oscillations between Anterior and Posterior Midline Cortex and with the Hippocampus in Freely Behaving Rats

Theta oscillations in the hippocampus support cognitive processing. Theta-range rhythmicity has also been reported in frontal and posterior cortical areas - where it tends to show consistent phase-relations with hippocampal rhythmicity. Theta-range rhythmicity may, then, be important for cortico-cortical and/or cortico-hippocampal interactions. Here, we surveyed the rat frontal and posterior midline cortices for theta-related oscillations and examined their relationships with hippocampal activity in freely moving rats. Variation in electroencephalography across 4 general classes of spontaneous behavior demonstrated different profiles of theta-like activities through the rat midline cortices. Analysis of cortico-cortical and cortico-hippocampal coherences showed distinct, behavior-dependent, couplings of theta and delta oscillations. Increased theta coherence between structures was most obvious during nonautomatic behaviors and least during immobility or grooming. Extensive coupling of theta oscillations throughout the rat midline cortices and hippocampus occurred during rearing, and exploratory behavior. Such increases in coherence could reflect binding of cortico-hippocampal pathways into temporary functional units by behavioral demands. Extensive coupling of frontal delta, which lacked coherence with posterior areas (including the hippocampus), suggests that different frequencies of rhythmicity may act to bind groups of structures into different functional circuits on different occasions.