Abstracts

Rationale: Theta rhythm of the rat is a 4-10 Hz oscillatory electrical activity found in the hippocampus and related structures. Theta rhythm is suggested to participate in sensorimotor integration, spatial navigation, and memory. In this study, functional magnetic resonance imaging (fMRI) is used as a novel approach to study the spatial extent of the brain associated with the theta state. Methods: Simultaneous EEG-fMRI data were acquired in a 9.4T scanner, combining the superior spatial resolution of the blood-oxygenation-level dependent (BOLD) signal with the high temporal resolution of EEG. Rats were anesthetized with ketamine-xylazine. EEG was recorded using silver-chloride wires placed subdermally in the scalp, and theta oscillations and vibrissal movements were induced using electrical stimulation of the pontis oralis nucleus (PnO; 1 s train of 100-Hz pulses of 0.2 ms duration), once every 60 s. The stimulus electrode, a Teflon-coated platinum wire of 100 μm diameter, was chronically implanted in the PnO and induced susceptibility artifacts only in the immediate vicinity (< 1 mm) of the wire. We employed a two-shot echo planar imaging (EPI) acquisition (TR/TE = 500/15 ms, FOV = 25.6 X 25.6 mm, matrix = 64 X 64, inplane resolution = 400X 400 μm2). fMRI data was analyzed using both block and event related designs. Results: The results showed robust changes in limbic structures and uncharacteristic BOLD responses following theta rhythm generation. Independent component analysis (ICA) was used to examine spatiotemporal changes in response to the stimulation, without assumption of a vasculature response template. Specific spatiotemporal components corresponding to bilateral activation of both hippocampi, septal nuclei and other subcortical areas were correlated with the theta rhythm generation in a network dependent manner. Conclusions: The results show for the first time that a positive BOLD response in the hippocampus and related structures during a theta rhythm. Connectivity between brain structures using the BOLD response will be studied during theta and non-theta states.