  Grid cells recorded in the parahippocampal formation of freely moving rodents
provide a strikingly periodic representation of self-location whose underlying
mechanism has been the subject of intense interest. Our previous work(1) showed
that grid cells represent the most stable subset of a larger continuum of
spatially periodic cells (SPCs) which deviate from the hexagonal symmetry
observed in grid cells. Recently Navratilova et al(2) suggested that our
findings reflected poor isolation of the spikes from multiple grid cells,
rather than the existence of actual non-grid SPCs. Here we refute this
suggestion by showing that: (i) most SPCs cannot be formed from hexagonal
grids; (ii) all standard cluster isolation measures are similar between
recorded grid cells and non-grid SPCs, and are comparable to those reported in
other laboratories; (iii) the spikes from different fields of band-like SPCs do
not differ. Thus the theoretical implications of the presence of cells with
spatially periodic firing patterns that diverge from perfect hexagonality need
to be taken seriously, rather than explained away on the basis of hopeful but
unjustified assumptions.
