Brain organization, not size alone, as key to high-level vision: Evidence from marmoset monkeys

Bigger brains are thought to support richer abilities, including perceptual abilities. But bigger brains are typically organized differently (e.g., with more cortical areas). Thus, the extent to which a neural system’s size versus organization underlies complex abilities remains unclear. The marmoset monkey is evolutionarily peculiar: it has a small brain, yet many cortical areas. We used this natural experiment to test organization as source of high-level visual abilities independent of size, via large-scale psychophysics comparing marmosets to different species on identical tasks. Marmosets far out—performed rats—a marmoset-sized rodent—on a simple visual recognition task. On another visual task, which is difficult for both humans and machines, marmosets achieved high performance. Strikingly, their image-by-image behavior revealed that they did so in a manner highly similar to humans—marmosets were nearly as human-like as were macaques. These results suggest a key role for brain organization—not simply size—in the evolution of sophisticated abilities.