We computed for each cell its average stimulus-evoked response, Alectinib mw which we defined as the average over the mean firing rates to each of the 125 stimuli within either the familiar or novel set (Figures 4A–4D). Paralleling previous reports that have grouped neurons into two distinct classes based on extracellular spike waveform (Diester and Nieder, 2008 and Mitchell

et al., 2007), we first note that putative inhibitory units had much larger stimulus-driven activity than putative excitatory units. This can be appreciated by comparing the axes in Figure 4A (putative excitatory) and Figure 4B (putative inhibitory) and by comparing the blue (putative excitatory) and red (putative inhibitory) points in Figures 4C and 4D. To quantify this difference, we compared the average stimulus-evoked firing rates of putative excitatory cells to those of putative inhibitory cells within each unique combination of stimulus set (familiar/novel) and time epoch (early/late). All comparisons were highly significant (mean ± SEM Hz for putative excitatory versus putative inhibitory: familiar early, 8.62 ± 0.70 versus 35.12 ± 3.24; familiar late, 5.90 ± 0.60 versus 22.96 ± 3.54; novel early, 9.20 ± 0.92 versus 44.26 ± 4.21; novel late, 7.79 ± 0.91 versus 44.00 ± 4.01; p < 0.001 for every comparison, uncorrected, two-sample t tests).

Because it has been shown that current injections ATM inhibitor can drive fast-spiking inhibitory units to very high firing rates (McCormick et al., 1985), the higher average responses of narrow-spiking units further support the labeling of this cell class as putative inhibitory. We observed a similar difference in firing rates when we looked at spontaneous activity, which we took as the last 500 ms of the fixation epoch (putative excitatory, 5.20 ± 0.68 Hz; putative inhibitory, 15.01 ± 2.87 Hz; and p = 0.004, two-sample t test). Notably, we found that in both cell classes the novel set elicited higher average responses than the familiar set (Figures 4A–4D). Like the maximum response effect in

putative inhibitory units, these experience-dependent differences in average firing rate emerged, in both cell classes, after the initial visual transient (Figures 4A and 4B). In particular, in the early epoch (Figure 4C), the population-averaged difference for the putative excitatory cells was small and not significant (familiar − novel, mean ± SEM, −0.59 ± 0.42 Hz; p = 0.17, paired t test), and whereas the difference was larger and significant in the putative inhibitory subset (familiar − novel, −9.14 ± 2.85 Hz; p = 0.006), it was only observed in one monkey (compare Figures S3C and S3D). It was in the late epoch (Figure 4D) that population-averaged differences in average firing rate for both classes of cells became significantly different from zero (familiar − novel; putative excitatory, −1.90 ± 0.67 Hz, p = 0.006; putative inhibitory, −21.04 ± 4.01 Hz, p < 0.