Characterizing Light Stimulus Intensity

We measured the spectral intensity profile (in μW·cm⁻²·nm⁻¹) of our light stimuli with a calibrated USB2000+ spectrophotometer (Ocean Optics). We transformed the stimulus intensity into equivalents of photoisomerizations per rod and second, assuming dark-adapted rods^{42 (link)}. Briefly, the spectrum was converted to photons·cm⁻²·s⁻¹·nm⁻¹, convolved with the normalized spectrum of rod sensitivity^{5 (link)}, and multiplied with the effective collection area of rods (0.5 μm²)^{55 (link)}. The results for a stimulus intensity of ‘30’ range from 1 R*·s⁻¹ per rod (ND8) to 10⁴ R*·s⁻¹ per rod (ND4), see Fig. 1b. These calculations, and recordings from mice lacking functional rods and functional cones (not shown), suggest that ND8 and ND7 correspond to scotopic conditions, ND6 weakly activates cones, ND5 is fully mesopic, and ND4 is photopic. Note that our characterization of ND7 as scotopic may partly be owed to our use of rather low-contrast stimuli. We cannot exclude that stimuli with stronger contrast might activate cones already at ND7 (see e.g. refs. 5 (link),56 ).