  About a decade ago grid cells were discovered in the medial entorhinal cortex
of rat. Their peculiar firing patterns, which correlate with periodic locations
in the environment, led to early hypothesis that grid cells may provide some
form of metric for space. Subsequent research has since uncovered a wealth of
new insights into the characteristics of grid cells and their neural
neighborhood, the parahippocampal-hippocampal region, calling for a revision
and refinement of earlier grid cell models. This survey paper aims to provide a
comprehensive summary of grid cell research published in the past decade. It
focuses on the functional characteristics of grid cells such as the influence
of external cues or the alignment to environmental geometry, but also provides
a basic overview of the underlying neural substrate.

Neurons and Cognition