Constructs used to produce AAV included pGP-AAV-syn-GCaMP-WPRE and the Cre recombinase-activated construct pGP-AAV-syn-flex-GCaMP-WPRE. Virus was injected slowly (30 nL in 5 minutes) at a depth of 250 μm into the primary visual cortex (two sites, 2.5 and 2.9 mm lateral from the lambda suture). For population imaging and electrophysiology (Fig 2-3), AAV2/1-syn-GCaMP-WPRE virus (titer: ∼1011 -1012 genomes/mL) was injected into the visual cortex of C57BL/6J mice (1.5-2 months old)6. For dendritic imaging (Fig 4, 5 and 6a-f), sparse labeling was achieved by injecting a mixture of diluted AAV2/1-syn-Cre particles (titer: ∼1012 genomes/mL, diluted 8000-20,000 fold in PBS) and high titer, Cre-dependent GCaMP6s virus (∼8×1011 genomes/mL). This produces strong GCaMP6 expression in a small subset of neurons (∼3-5 cells in a 250 μm × 250 μm × 250 μm volume), defined by Cre expression56. Both pyramidal (Fig. 4-5) and GABAergic (Fig. 6) neurons were labeled using this approach, but they could be distinguished based on the presence or absence of dendritic spines. Post hoc immunolabeling further identified the imaged cells. For specific labeling of parvalbumin interneurons (Fig. 6g and Supplementary Fig.
