Izmailov's model of color space[REF][REF][REF][REF] represents both aperture and pigment colors as points on a four-dimensional sphere, such that the Euclidean distance (chord length rather than arc length) between two colors is nearly proportional to the estimates of their dissimilarity.
Another line of research led Chingis Izmailov to a spherical model of facial expressions of emotions [REF].