Figure 1. Near-linear scaling with excitatory optogenetic stimulation in mouse V1. A, Schematic of experimental stimulus protocol. If scaling is linear, the same input pulse produces the same response when baseline (spontaneous) rate is changed. B, Baseline rates are raised using ChR2 in excitatory (E) neurons (Cre-dependent virus in Emx1-Cre mouse line). C, Population histograms showing responses to combined ChR2 and visual (90% contrast) stimuli. In the top row, columns show three groups of neurons, divided based on size of ChR2 baseline firing rate changes: left, smallest ChR2 effects (n = 94; 36 single, 58 multiunits); middle, intermediate ChR2 effects (n = 101; 31 single, 70 multiunits); right, largest ChR2 effects (n = 94; 28 single, 66 multiunits). Brown, responses to visual stimulus with no optogenetic stimulus; cyan, responses to visual stimulus when baseline rates are changed by sustained optogenetic stimulus. The bottom row shows the same data as the top row, with spontaneous firing rates subtracted. Visual responses differ somewhat between columns because each column is a different group of neurons, but within each group there is little response change as spontaneous rate varies. D, Linear scaling is seen across a wide contrast range. Top row, responses without baseline subtraction; bottom row, baseline subtracted. Error bars are SEM of pooled unit responses. E, Linear scaling is seen on average, across neurons with a variety of ChR2-induced baseline rate changes, with some weak sublinearity at the highest rate changes and highest contrasts. Y axes, difference in visual responses (relative to baseline) with and without ChR2 stimulation; dashed line at zero shows a perfectly linear response. Red, LOWESS regression; shaded region is a bootstrapped 95% confidence interval. Two outlier points in 90% contrast plot are omitted for visual clarity although they are included in the regression; the two outliers are shown in Fig. 2A.