When the Cognitive Map Theory of the hippocampus was originally postulated, it was suggested that spatial representation was the job solely of the hippocampus. However, it has since been discovered that other structures, such as the entorhinal cortex, are integral for establishing spatial representations in the brain. Specifically, this chapter focuses on the medial entorhinal cortex, which, compared to the lateral entorhinal cortex, seems to be preferentially involved in spatial representation rather than object representation.
The entorhinal cortex is home to a particular cell type called grid cells. Grid cells are a lot like place cells in that they fire when the animal traverses a particular location space. Unlike place cells, they seem to fire in multiple locations of a given area, generating a hexagonal, grid-like pattern. Grid cells are highly selective in the dorsal entorhinal cortex as compared to the ventral portion. These cells seem to be modulated by external visual cues, as evidenced by experiments which show that a rotation of the environment rotates entorhinal firing patterns in a similar fashion.
In addition to grid cells and head direction cells, the entorhinal cortex is home to other spatially tuned cells such as head direction x grid conjunction cells, and border cells. Border cells increase their firing when the organism is near a physical boundary in the environment. These same cells will fire near borders when the shape of the environment is changed. Not present in this chapter is whether or not border cells will fire near a non-border object that may be more massive (or not) than the animal. Do they fire near doors that the experimental animals can pass through?
Lastly, what is the relationship between grid cells and place cells? Some hypotheses suggest that grid cells are integral in the establishment of hippocampal place fields. Inhibition of the medial entorhinal cortex while recording from CA1 place cells, rather than abolishing place fields, modulates their size and stability.
Being that these cells were very recently discovered (first report in 2004), many questions remain:
- How are these cells generated?
- What exactly are they computing?
- What is the relationship between entorhinal activity and memory?