Object × position coding in the entorhinal cortex of flying bats

Successful navigation requires knowledge of the specific locations of different objects. However, currently only an object-invariant signal was discovered, marking all objects at all locations (carried by object-vector cells in the superficial layers of the medial entorhinal cortex (MEC)). It is thus unknown where and if this general object signal is crossed with a signal for a specific location (as is carried by place cells in the hippocampus), and whether these two variables are encoded conjunctively. We hypothesized that if indeed the brain represents object × position in a conjunctive manner, such encoding will be found at the “end” of the MEC-hippocampal-MEC loop – the deep layers of MEC – where all-object information from MEC converges with location-specific information from hippocampal place-cells and is sent to the neocortex. Here we recorded from MEC of flying bats as they foraged for food in a large flight-room where 6–11 identical rest-objects were placed at various heights and locations. We found that a substantial fraction of cells in the deep layers of MEC (but not superficial layers) fired at the vicinity of specific rest-objects at specific locations. These cells fired near the rest-object when the bat flew from or to the object, but not when it flew through the same location without object-engagement – thus encoding object × position. Our results suggest a broader prevalence than currently thought for conjunctive-coding of navigational variables – including the encoding of objects, which are crucially important for navigation.