Figure 8.

Firing rates of individual interneurons correlate with the gamma frequency of hippocampal LFP. Data from two interneurons, recorded simultaneously on the same tetrode as the rat ran on a y-shaped track. A, C, E, Interneuron 1, which had a mean firing rate of 33 Hz. B, D, F, Interneuron 2, which had a mean firing rate of 69 Hz. A, B, Both interneurons increase their firing rate as a function of increasing speed. Insets, The narrow extracellular spike shapes are indicative of putative fast-spiking interneurons. C, D, As the normalized (Z-scored) rate of each interneuron increases, there is a shift to higher LFP frequencies. The interneuron and LFP were recorded simultaneously on different hippocampal electrodes. Note that this shift is independent of the absolute firing rates of the two interneurons, and instead depends on the firing rate of each interneuron relative to its mean rate. E, F, Lower LFP frequencies are negatively correlated with each interneuron's Z-scored firing rate, whereas higher frequencies are positively correlated. Thus the normalized rate of interneuron firing is predictive of the shift in LFP frequencies across the gamma-band.