Time or distance: predictive coding of Hippocampal cells

The discovery of place cells within the hippocampus has pointed to the importance of the hippocampus for navigation. The more recent discovery of hippocampal time cells has broadened the perspective of encoding in the hippocampus. An alternative hypothesis to the existence of time cells is based on the notion that hippocampal cells deduce location by integrating travelled distance ("path integration"). According to this alternate hypothesis, time cells, which fire at particular times when animals are running on a treadmill without changing location, actually encode accumulated distance on the treadmill. To examine this hypothesis, Kraus et al.1 performed treadmill experiments in which animals either ran for a fixed time or a fixed distance with varying velocities. Two distinct coding modes of hippocampal principal cells were found. Some cells encoded travelled distance and others elapsed time, thus refuting the notion that all hippocampal cells were performing path integration. Using the data from these experiments, we asked whether the two populations depended on the type of task the rats were engaged in. We show that the type of experiment determined the cells encoding, such that in fixed-distance experiments distance-encoding cells dominated, while on fixed-time experiments time-encoding cells dominated. These results suggest that the cells encoding contains a predictive element, dependent on the important variables of the experiment.