09). Consistent with these observations, we also observed that experience led to decreases in the proportion of stimuli eliciting a significant elevation in firing rate and to increases in the proportion of stimuli eliciting a significant reduction in firing rate (Figure S4). Furthermore, although both cell classes showed reduced average responses to familiar stimuli, this R428 ic50 decrease was much larger in putative inhibitory than excitatory cells (early epoch, p = 0.001; late epoch, p < 0.001; two-sample t tests; early epoch effect not significant in the same monkey whose

effects tended to arise later), which can be seen by comparing the red and blue arrows in the histograms of Figures 4C and 4D. To convey information, neurons modulate their firing rates. The greater and/or more reliable this modulation, the more informative the neuron’s firing rate becomes about the presence (or absence) of some stimulus. Because we have shown that visual experience not only led to an increase in maximum response (in putative excitatory cells) but also to a decrease in average response, we have already implicated visual experience in sharper stimulus selectivity. Here, we make this idea explicit. To capture increases in selectivity with a single metric, we computed the value of (lifetime) sparseness (Olshausen and Field, 2004, Rolls and Tovee, 1995,

Vinje DNA Damage inhibitor and Gallant, 2000 and Zoccolan et al., 2007) (see Experimental Procedures). Sparseness quantifies how much of a single neuron’s total firing rate, across a stimulus set, is concentrated within a few stimuli. A neuron with high sparseness will be quiet

most of the time, but there will be a few stimuli that elicit robust firing rates. By definition, this is a selective neuron. An unselective neuron, one with low sparseness, will respond with an elevated firing rate to many stimuli. We calculated the sparseness of cells’ responses across the familiar and novel stimulus sets, first with a sliding window (Figures 5A and 5B) and then in the previously defined early and late epochs (Figures 5C and 5D). As with the average response analyses, one of the more conspicuous features of the data was that putative inhibitory units had much lower sparseness than putative excitatory Cell Penetrating Peptide units for every combination of stimulus set and epoch (mean ± SEM putative excitatory versus putative inhibitory; familiar early, 0.53 ± 0.03 versus 0.16 ± 0.02; familiar late, 0.65 ± 0.03 versus 0.32 ± 0.04; novel early, 0.42 ± 0.02 versus 0.17 ± 0.02; novel late, 0.57 ± 0.02 versus 0.24 ± 0.02; p < 0.001 for every comparison, uncorrected, two-sample t tests). The broad tuning of putative inhibitory units is consistent with recent functional data (Kerlin et al., 2010, Liu et al., 2009 and Sohya et al., 2007) as well as neuroanatomical data showing that these units can receive highly convergent and heterogeneous input from the surrounding excitatory population (Bock et al., 2011).