So in mice where we did not record from PV cells we used this range of light intensity, i.e., light intensity was set to 0.05–0.1 mW/mm2, and increased until change in the activity Pyr cells was observed. The population response of the visual cortex to visual stimuli was monitored using local field potential recordings during this process. Light intensities never exceeded 1 mW/mm2. When recording from PV cells while photo stimulating Arch or ChR2 (Figure 2)

cortical illumination started before the visual stimulus selleck compound (to monitor the effect on spontaneous activity) and ended before the end of the visual stimulus (to determine the kinetics of recovery to visually evoked firing rates). Spontaneous spike rate was calculated as the average firing rate during a 0.5 s period before the presentation of the stimulus. The visual response to a given stimulus was the average

rate over the stimulus duration or over the period when both cortical illumination and visual stimulus occurred (1–2 s). Orientation selectivity index (OSI) was calculated as 1 − circular variance (Ringach et al., 1997). Responses to the 12 grating directions were fit with orientation tuning curves i.e., a sum-of-Gaussians (Figure 1, Figure 3 and Figure 4). The Gaussians are forced to peak 180 degrees apart, and to have the same tuning sharpness (σ) but can have unequal height (Apref and Anull, to account for direction selectivity), and a constant baseline B. The tuning sharpness was measured as 17-DMAG (Alvespimycin) HCl σ (2 ln(2))1/2, check details i.e., the half-width at half height (HWHH). Direction selectivity index (DSI) was calculated as (Rpref – Rnull) / (Rpref + Rnull), where Rpref is the response at the preferred direction and Rnull is the response 180 degrees away from the preferred direction.

Contrast-response curves were fit with the hyperbolic ratio equation ( Albrecht and Hamilton, 1982): R(C) = Rmax cn / (C50n + cn) + Roffset, where c is contrast, C50 is the semisaturation contrast, and n is a fitting exponent that describes the shape of the curve, Rmax determines the gain, and Roffset is the baseline response. To obtain the threshold-linear fit, we first computed a summary of Pyr cell responses in layer 2/3. The tuning curves of all cells were aligned to the same preferred orientation, a nominal value of 0 degrees and the maximal response was scaled to a nominal value of 100% (Figure 4A). We then plotted the median Pyr cell response measured during the suppression or activation of PV cells against the median response measured in the control condition (Figure 4B). The bootstrapped distribution of median responses was used to calculate errors bars in OSI, DSI, and HWHH. Please see Supplemental Experimental Procedures for more details. The membrane potential tuning, or net depolarization, as a function of orientation, θ, was modeled as: ΔV(θ)=gLRL+gE(θ)RE+gI(θ)RIgL+gE(θ)+gI(θ)−Vr gx=gmin+(gmin−gmax)e−θ22σ2.