An evolutionary scaling law for the primate visual system and its basis incortical function

A hallmark of mammalian brain evolution is the disproportionate increase in neocortical size as compared with subcortical structures(1). Because prima ry visual cortex (V1) is the most thoroughly understood cortical region, th e visual system provides an excellent model in which to investigate the evo lutionary expansion of neocortex. I have compared the numbers of neurons in the visual thalamus (lateral geniculate nucleus; LGN) and area V1 across p rimate species. Here I find that the number of V1 neurons increases as the 3/2 power of the number of LGN neurons. As a consequence of this scaling la w, the human, for example, uses four times as many V1 neurons per LGN neuro n (356) to process visual information as does a tarsier (87). I argue that the 3/2 power relationship is a natural consequence of the organization of V1, together with the requirement that spatial resolution in V1 should para llel the maximum resolution provided by the LGN. The additional observation that thalamus/ neocortex follows the same evolutionary scaling law as LGN/ V1 may suggest that neocortex generally conforms to the same organizational principle as V1.