Fig. 2.

Population tuning curves for excitatory and inhibitory neurons in V1 are contrast-invariant. (A) Schematic of the setup for extracellular silicon probe recordings in V1. (B) Histology of example V1 recording site. Blue indicates DAPI, and white outline indicates V1, labeled according to ref. 26. Bregma, –2.7 mm. (Scale bar, 1 mm.) (C) Classification of single units based on extracellular wave shape into putative excitatory (broad-spiking, E; orange) and inhibitory (narrow-spiking, I; teal) neurons. (Left) Normalized extracellular wave shapes. (Right) Clustering based on wave shape parameters. Large dots indicate neurons recorded in sessions relevant to the current study, i.e., sessions containing the flashed grating stimulus, $(N=204 \text{E}+38 \text{I})$; small dots indicate V1 neurons recorded in other sessions, used to improve N for clustering. (D) Flashed gratings stimulus paradigm consisting of three stimulus intervals containing a random sequence of gratings (white bars), with interleaved blank periods (gray bars). In some sessions, the blank periods were used for photoactivation of ChR2 expressed in V1 PV+ inhibitory interneurons. (E) Responses of two V1 example neurons to combinations of orientation and contrast (Left), first SVD component (Middle), and SVD residual (Right). Spatial patterns in the residual, such as those that are evident for the lower example neuron, reveal violations of contrast invariance. (F) Violations of contrast invariance were assessed by the power of the SVD residual (> 5%) and significance of spatial autocorrelation ($g_z>1.96$). Light dots indicate contrast-dependent neurons (14/144 E, 0/30 I neurons). Solid dots indicate contrast-invariant neurons, considered for further analysis (130 E, 30 I neurons). Purple indicates example neurons from E. (G) Two-dimensional tuning fit consisting of a product of a hyperbolic ratio function and a wrapped Gaussian (27) for an example V1 neuron. (H) Distribution of fit quality across neurons. Dashed line indicates fit quality threshold (0.4). Solid bars indicate neurons considered for further analysis (125/130 E, 30/30 I neurons). (I) Normalized orientation tuning component for E (Left) and I (Right) neurons. (K) Cumulative distribution of orientation selectivity (OSI; 28, 29). (Inset) Density histogram of orientation selectivity. The x axis is the same as the cumulative distribution; y scale bar represents two neurons per bin of OSI. (J and L) Same as I and K but for normalized contrast response component and cumulative distribution of contrast sensitivity (contrast at which the contrast response function reaches half height). (M and N) Pooled population responses from V1 (mean ± SEM). In C, F, and H–N, orange indicates putative excitatory neurons, and teal indicates putative inhibitory neurons.