POSTDICTIVE CODING OF POSITION BY CA1 NEURONS

Hippocampal place cells fire when an animal is at a particular part of the environment. Hippocampal neurons also fire in relation to movement speed or direction. It has been suggested that the hippocampus forms a cognitive map for navigating the environment. Navigation requires knowledge of both past and future positions, but it remains unclear whether hippocampal activity predicts kinematics, memorizes the recent past, or both.
We record the activity of well isolated CA1 in mice that explore an 80 cm arena for several hours at a time, without being restricted or rewarded. We quantify coding of position and velocity using firing rate maps and mutual information across multiple time lags. This approach allows direct evaluation of predictive, instantaneous or postdictive (retrospective) encoding of position and velocity.
We find that the spiking of almost all units carries information about movement kinematics. For pyramidal cells (PYRs), information about position is higher than about velocity. Information peaks at positive time lags ranging 100-200 ms, indicating postdictive coding of position. In contrast, PV-like interneurons (INTs) exhibit higher information for velocity compared with position, with near zero optimal time lags.
These findings suggest cell-type dependent temporal parcellation of CA1 activity, with PYRs encoding position postdictively and INTs representing ongoing movement. Thus, instead of trajectory planning, CA1 PYRs may contribute to error correction of ongoing trajectories, allowing to compare a desired position with the one just passed through. The results are consistent with the view that the cognitive map may be used as a memory device.