In their seminal paper Hafting, Torkel, et al. "Microstructure of a spatial map in the entorhinal cortex." Nature 436.7052 (2005): 801-806. the Mosers have discovered the grid cells.
To show the existence of these cells, they have counted spikes as a function of location, plotted (figure 1.b) a rate map and finally the most known figures of autocorrelogram which beautifully visualize the grid response pattern of these cells.
Auto-correlation is measured between a signal and itself (e.g. when one of the signals is shifted in time). My question is, what is the signal from which the autoelectrogram is calculated and what are the different states of that signal, between which the correlation is calculated (explanation is given at the paper's methods section, but it still didn't help me much).