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Abstract

The morphological and electrophysiological properties of parvalbumin-expressing inhibitory interneurons (PV+ neurons) suggest their role as synchronizers and normalizers of the local cortical microcircuit. PV+ cells are thought to average the local activity and dynamically regulate its overall level. In apparent agreement with this model, previous studies have shown stable patterns of correlations of the spiking activity of the PV+ neurons among themselves and with the local excitatory cells. However, we have previously shown that, in sufficiently dense recordings, estimates of the partial pairwise correlations of the spiking activity can yield a more insightful picture of interactions in the circuit, or its functional connectivity. Using high-speed 3D two-photon imaging of calcium signals and genetically encoded fluorescent markers of PV+ neurons, we recorded the activity of the majority of neurons in 200 um x 200 um x 100 um volumes in layers 2/3 and 4 of mouse visual cortex during visual stimulation. If PV+ neurons simply pooled the activity of the local circuit, their activity would be predicted from the local circuit and the partial correlations among the PV+ neurons would all but vanish. Surprisingly, we found that the partial pairwise correlations among the PV+ cells were exceptionally high. In fact, the partial pairwise correlations enhanced the differentiation of PV+ neurons from other cell types. The average partial pairwise correlation between PV+/PV+ pairs was 4.9 times higher than between PV-/PV- pairs whereas the average noise correlations differed by the factor of 1.5. This effect was insensitive to the choice of the temporal scales of correlation analysis. Although other explanations cannot yet be excluded, the present finding may suggest that the correlations among the PV+ neurons are shaped predominantly by structured input from outside the local circuit such as, for example, by input from layer 5.