Pupil diameters associated with large and reliable visually evoked V1 responses are significantly larger than those associated with optimal visual detection performance. A, Histograms (counts = number of animals) of pupil diameter bins during stillness associated with, left to right, largest visually evoked firing rate, spike reliability, and visual detection d′ (combined from the two visual detection tasks). Largest evoked firing rate and spike reliability recorded per animal were significantly more concentrated in bins of larger pupil diameter than the largest d′ recorded per animal (p-values from Fisher's exact test; after Bonferroni correction, significant p-values at the 0.05 level must be lower than 0.017). For largest d′ recorded per animal, values were most strongly concentrated in pupil diameter bins ranging from 40 to 60% constricted from the maximal pupil diameter. B, As in A, but for still and locomotion periods combined. For pupil diameter bins, bin widths were chosen such that an equal amount of data fell into each bin.