Growing evidence suggests that synchronized oscillatory activity in distributed local networks of neurons may lead to functional integration in the brain. Neural synchronization is typically revealed by a consistent phase delay between two signals generated in two separate neuronal sources. Here we show that local field potentials recorded from the frontal and visual cortical areas of mice during motor quiescent (passive state) and while actively exploring environment (active state) become synchronized without phase delay in the theta band. We then investigated whether theta oscillations in the hippocampus could be responsible for anterior-posterior isochronal synchronization. To achieve this goal, the two behavioral states (passive and active) were modeled with a recurrent network involving the hippocampus, acting as a relay element, and the two cortical areas. Results provided by the model are in agreement with experimental evidence. Synchrony without time lag between frontal and visual cortices was found to occur simultaneously with prominent theta oscillations in the hippocampal formation during both passive and active behavioral states. We hypothesize that this zero-lag long-range synchronization mediated by hippocampal relay may play a crucial role in integrating top-down and bottom-up control mechanisms during spatial navigation and memory formation.
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