Conjunctive and complementary CA1 hippocampal cell populations relate sensory events to immobility and locomotion

This study investigated the dynamics of recruitment of cells in the CA1 region of the hippocampus in response to sensory stimuli presented during immobility, movement, and their transitions. Two-photon calcium imaging of somal activity in CA1 neuron populations was done in head fixed mice. Sensory stimuli, either a light flash or an air stream, were delivered to the mice when at rest, when moving spontaneously, and while they were induced to run a fixed distance on the conveyor belt. Overall, 99% of 2083 identified cells (from 5 mice) were active across one or more of 20 sensorimotor events. A larger proportion of cells were active during locomotion. Nevertheless, for any given sensorimotor event, only about 17% of cells were active. When considering pairs of sensorimotor event types, the active cell population consisted of conjunctive (C [isin] A and B) cells, active across both events, and complementary (C [isin] A not B or C [isin] B not A) cells that were active only during individual events. Whereas conjunctive cells characterised stable representations of repeated sensorimotor events, complementary cells characterised recruitment of new cells for encoding novel sensorimotor events. The moment-to-moment recruitment of conjunctive and complementary cells across changing sensorimotor events signifies the involvement of the hippocampus in functional networks integrating sensory information with ongoing movement. This role of the hippocampus is well suited for movement guidance that secondarily might include spatial behavior, episodic learning and memory, context representation, and scene construction.