Theta behaviors in the hippocampus fall in several categories, e.g. movement related (Type 1), immobility related (Type 2), and REM sleep theta, with distinct layer specific current source distributions, different pharmacological features, and characteristic frequencies. The mechanism of the generation of theta at different frequencies is poorly understood but its strong association with different aspects of behavior (sensory-motor integration, sleep, etc.) indicates that subcortical structures play an essential role. A number of subcortical nuclei exhibit theta rhythmic neuronal activity synchronized with hippocampal oscillations and our goal is to identify the functional relationship among these structures and rhythmic activity in the hippocampus. The commonly used symmetrical methods of signal analysis have limited utilities in this regard as they could not provide information on causal influences. Thus the goal of the present R21 is to test the effectiveness of a newly proposed method called Granger causality on two subcortical structures generating theta in the hippocampus: medial septum (MS) and supramammillary nucleus (SUM). First (Specific Aim 1), we will validate the method in the context of the hippocampal network in two experiments;one in which the causal direction is known (using stimulation of input pathways and recording field potentials in their target layers of the hippocampus) and the other in which the causal direction comes from prior knowledge of the information flow along the classic hippocampal """"""""three-synaptic circuit"""""""". Then, in Specific Aim 2, we will address the causal influence and directional driving of hippocampal theta by subcortical theta generators (SUM and MS) and test whether Granger causality can be a tool in hypothesis-driven investigation of this system. We hypothesize, in particular, that SUM involvement is limited to specific states, and thus expect to find significant directional drive from SUM to hippocampus associated with fast theta during waking exploration and phasic episodes of REM sleep but not during slow theta of waking immobility, and tonic REM sleep. On the other hand, we hypothesize that MS, providing a final common pathway for ascending drives from different sources, will show significant directional drive to the hippocampus in all theta states.
The mechanism of the generation of theta at different frequencies is poorly understood but its strong association with different aspects of behavior indicates that subcortical structures play an essential role. The goal of the present R21 is to test the effectiveness of a newly proposed method called Granger causality on two subcortical structures: medial septum (MS) and supramammillary nucleus (SUM).
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