Mapping of a non-spatial dimension by the hippocampal/entorhinal circuit

During spatial navigation, neural activity in the hippocampus and the medial entorhinal cortex (MEC) is correlated to navigational variables like location(1,2), head direction(3), speed(4), and proximity to boundaries(5). These activity patterns are thought to provide a map-like representation of physical space. However, the hippocampal/entorhinal circuit is involved not only in spatial navigation, but in a variety of memory-guided behaviors(6). The relationship between this general function and the specialized spatial activity patterns is unclear. A conceptual framework reconciling these views is that spatial representation is just one example of a more general mechanism for encoding continuous, task-relevant variables(7–10). We tested this idea by recording hippocampal and entorhinal neurons in a task that required rats to use a joystick to manipulate sound along a continuous frequency axis. We found neural representation of the entire behavioral task, including activity that formed discrete firing fields at particular sound frequencies. Neurons involved in this representation overlapped with the known spatial cell types in the circuit like place cells and grid cells. These results suggest that common circuit mechanisms in the hippocampal/entorhinal system are used for representations of diverse behavioral tasks, possibly supporting cognitive processes beyond spatial navigation.