Two different stimuli, labeled s1 and s2, were presented to a salamander retina. This is schematically shown here as grey patches on light and dark backgrounds, but Tkačik et al. used sequences of flickering light. They triggered two different sets of neural responses σ to repeats of the same stimuli. Out of these repeats, a distance Dret between s1 and s2 was computed as the difference between P(σ|s1) and P(σ|s2). The bottom right panel shows the matrix of distance for different pairs of stimuli indexed by t[s]. The upper diagonal shows the Euclidian distance between the stimuli, the lower diagonal the Dret distance estimated from the retinal acitivity.