Place cell: Encyclopedia II - Place cell - Place fields

Place cell - Place fields

Place cells show increased frequency of firing (up to 100Hz at times, for CA1 cells) when an animal is in a specific area, which is referred to as the cell's place field. On initial exposure to a new environment, place fields become established rather quickly. The place fields of cells located close to each other in the brain will tend to be evenly distributed over the environment and tend to be stable over repeated exposures to the same environment. In a different environment, however, a cell may have a completely different place field or no place field at all. In any particular environment, roughly 40-50% of the hippocampal place cells will be active.

In an environment with few or no directional cues (for instance, a circular environment surrounded by black curtains), place fields will tend to have a fixed radial position, but the entire set of place fields may rotate around the maze unpredictably. If a polarizing cue is introduced (commonly a large white rectangle of paper), place fields will tend to have fixed positions relative to the cue. If the cue is moved while the animal can see it, place fields will tend to remain unaffected; however, if the animal is briefly removed from the environment then the cue is moved and the animal returned, the place fields will rotate so as to maintain their position relative to the cue card. Although visual cues seem to be the primary determinant of place cell firing, it is worth noting that firing persists in the dark, suggesting that proprioception or other senses contribute as well.

In an environment in which a rat is constrained to walk alone a linear track, place fields will often have a directional component in addition to a place component. A place cell that fires at a particular location while the rat walks in one direction along the track will not necessarily fire as the rat visits that location from the other direction. If the rat frequently turns around at the same point, however, place fields there will often be independent of direction.

The size of place fields and their signal to noise ratio varies depending on the region of brain in consideration. In the hippocampus, place fields are smallest and sharpest at the dorsal pole, becoming larger toward the ventral pole. CA3 place fields are slightly larger and noisier than CA1 place fields. Place fields in entorhinal cortex are larger and noisier still.

Adapted from the Wikipedia article " Place fields ", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki