Traces of Experience in the Lateral Entorhinal Cortex

A growing body of evidence suggests that memories are stored in the hippocampus by integrating spatial information from specialized cell types in the medial entorhinal cortex (MEC) with nonspatial information from cells in the lateral entorhinal cortex (LEC) [1-5]. LEC neurons show little spatial modulation when rats run in empty open-field environments [6, 7] but fire in the vicinity of discrete objects [7], suggesting that they provide information about the specific content of the spatial environment. It is unclear, however, whether firing at objects is elicited purely by stimulus properties, in a sensory-like manner, or whether any higher-order property, such as the history of experience, is also relevant. To address this question, we recorded from LEC neurons in an open field where objects were present on a subset of the trials. Whereas some neurons fired at the objects, other cells developed specific firing at places where objects had been located on previous trials, providing a readout of past experience in the environment. The latter cells generally did not respond to the object when it was present, suggesting that object cells and object-trace cells are independent cell classes. These findings identify LEC as a component of the hippocampal-cortical circuit for object-place memory.