Figure 9.

The gamma oscillation, and its slowing with age, was observed in the firing patterns of single neurons and pairs of single neurons. A, Autocorrelation firing rates in a cluster of fast-spiking neurons. All fast-spiking neuron autocorrelations of neurons recorded from aged (blue) and young adult (red) rats are averaged, and the dashed lines show these same traces after smoothing. Autocorrelation peaks were significantly later in aged rat neurons than those of young adults; overall firing rates (height of autocorrelations) were not significantly different. B, Cross-correlations between simultaneously recorded putative excitatory and inhibitory neurons are normalized by converting to z scores and then averaged across all aged and young adult pairs. Putative monosynaptic connections were observed in both age groups, reflected by the strong peak of excitatory neuron firing in the milliseconds before firing of inhibitory neurons. In aged rat cell pairs, there was a significantly higher incidence of excitatory neuron firing 4–6 ms before inhibitory neuron firing, consistent with the observation of larger inhibitory–excitatory offsets in the spike–LFP relationships. Inset shows that gamma oscillations were apparent in the excitatory–inhibitory cross-correlations of both aged and young adult rats.