Trends Neurosci. 2015 Dec;38(12):763-75. doi: 10.1016/j.tins.2015.10.004. Epub 2015 Nov 24.

Sanders H, Rennó-Costa C, Idiart M, Lisman J.

Much has been learned about the hippocampal/entorhinal system, but an overview of how its parts work in an integrated way is lacking. One question regards the function of entorhinal grid cells. We propose here that their fundamental function is to provide a coordinate system for producing mind-travel in the hippocampus, a process that accesses associations with upcoming positions. We further propose that mind-travel occurs during the second half of each theta cycle. By contrast, the first half of each theta cycle is devoted to computing current position using sensory information from the lateral entorhinal cortex (LEC) and path integration information from the medial entorhinal cortex (MEC). This model explains why MEC lesions can abolish hippocampal phase precession but not place fields.

http://www.ncbi.nlm.nih.gov/pubmed/26616686