All New World monkeys differ slightly from Old World monkeys in many aspects, but the most prominent of which is the nose. This is the feature used most commonly to distinguish between the two groups. The scientific name for New world monkey, Platyrrhini, means "flat nosed", therefore the noses are flatter, with side facing nostrils, compared to the narrow noses of the Old World monkey. Most New world monkeys have long, often prehensile tails. Unlike the other New World monkeys, their tail is not used for climbing, but as a kind of "balancing pole" and also as a tool. Their movements in the branches are extremely speedy. Many are small, arboreal and nocturnal.

(Source: Wikipedia )

Although ocular dominance columns are present in all macaques and humans tested so far, their expression in some species is inconsistent. For example, in the squirrel monkey, ocular dominance columns are present in some animals and not in others. Even more surprisingly, in some animals they are present in only part of the cortex.