Navigational Cell Systems Located in Human Brains

I was just reading this paper (http://www.nature.com/neuro/journal/vaop/ncurrent/full/nn.34...), and I noticed that they say only 5% of cells in the parahippocampal gyrus show grid-like activity p < 0.05, but in Fig. 2 and the supplemental information they still plot the grid responses from the parahippocampal gyrus, which are presumably noise, lumped together with the responses from the entorhinal cortex, cingulate, and hippocampus, at least some of which are presumably real. This doesn't invalidate the rest of the paper, which suggests there are really grid cells in the latter three regions, but it's frustrating. But I probably care more whether there are grid cells in the parahippocampal gyrus than most people.

What the NYT article omits is that a triangular grid [1] encoded by the grid cells is used to track movement, according to Wikipedia's entry for Grid cell. [2]

Further questions: How many grid cells are there? What is the largest area that can be fully represented? How are grid cells reused as the subject travels?

[1] https://twitter.com/th222/status/364156774309314560

[2] http://en.wikipedia.org/wiki/Grid_cell