2018-05-30 at 14:00
Influence of external information on retrosplenial head direction cells and entorhinal grid cells
When an animal freely moves in an open arena, some entorhinal cortex neurons, called grid cells, show a spatial activity forming a regular triangular pattern across the environment. This activity is thought to form a euclidian map-like representation of the rat’s location and orientation. Head direction (HD) cells fire when the animal’s head faces in a particular direction in space. HD cells have been found in several cortical and sub-cortical brain regions and express a global directional signal. Previous studies suggested head direction cells and grid cells to be mostly independent from external information but governed by movement-related information. Here, I will present two experiments showing that, on the contrary, head direction cells and grid cells are strongly influenced by external information. In the first study, I recorded retrosplenial HD cells while rats freely moved in two visually symmetrical compartments polarized by a visual cue card and connected by a central door. I found a sub-population of HD neurons which fired in a given direction in one compartment and rotated (“flipped”) their directional activity by 180° in the other compartment, unlike standard HD cell which maintained a single directional firing across compartments. The flipping HD cells were restricted to a portion of RSC and were not found in any other recorded area. The finding that some RSC HD neurons can operate independently of the main HD signal overturns the previous assumption of a global, unitary head direction signal; that this is controlled by environmental cues suggests a specific role for RSC in landmark-based directional encoding of local sub-spaces. In a second recent study, entorhinal grid cells were recorded while rats performed a place navigation task. Animals learnt to reach an unmark area, stay it for two seconds to release a food pellets randomly in the arena. Results show that the grid cell pattern is locally deformed in the goal zone: the closer to the goal zone, the greater the field scaling. This local deformation questions current properties of grid cells and suggests a specific influence of the goal area on the hexagonal grid cell pattern.