||P.1 bottom paragraph: "One would like to achieve a similarly satisfying understanding of human smell [as that of color perception]. Human olfaction begins with the binding of odor molecules to olfactory receptors, of which there exist ∼400 types (primary source). It is believed that these receptor types all differ in their relative sensitivity to various odorants. Two odors can only be distinguished if they cause different patterns of activity among these types. Thus the neural space of odors at the very input to the olfactory system would
seem to have 400 dimensions, many more than encountered in the color system. But what is the perceptual space for odors? The exact analog of the early color mixing experiments has not been
done, but from a quantitative analysis of perceptual similarities it has been argued that the space of odors is dominated by just one or two dimensions (Secundo et al., 2014), much fewer than the 400 dimensions at the level of sensory receptors."