Intuitively, intellectual skills and perceptual-motor skills seem very different because perceptual-motor skills appear more primitive. Ontogenetically, perceptual-motor skills develop before intellectual skills, or at least before most intellectual skills are manifested. Phylogenetically, creatures “high on the evolutionary ladder” are more obviously capable of intellectual skills than are creatures “lower down.” Perceptual-motor skills also seem more closely tied to specific forms of expression. Being a chess player does not mean one can only play with pieces of a certain size, that one can only move pieces with one's right hand, and so on. By contrast, being a violinist means one can play an instrument whose size occupies a fairly narrow range and that one must play with a rather rigid assignment of functions to effectors (bowing with the right hand, and fingering with the left). The seeming narrowness of this perceptual-motor skill expression, contrasted with the seeming openness of intellectual skill expression, seems to follow from intellectual skills having symbolic outcomes and perceptual-motor skills having non-symbolic outcomes. Symbolic outcomes need not be realized in specific ways and can rely on abstract rules. Non-symbolic outcomes, by contrast, need more specific forms of realization and seem to depend on restricted associations between stimuli and responses . Another difference between intellectual and perceptual-motor skills is that the two kinds of skill seem to be represented in different parts of the brain. For example, structures homologous to the optic tectum, a nucleus located on the dorsal surface of the midbrain, have a common function in all vertebrates―coordinating visual, auditory, and somatosensory information relevant to the control of orienting movements of the eyes, ears, and head. Similarities in structure and function between these and other brain areas associated with perceptual-motor behavior suggest that mechanisms for control of perceptual-motor skills are both highly specialized and conserved across species. In contrast, what distinguishes the human brain from the brains of other species―even closely related ones -- is the differential growth of brain regions most strongly associated with intellectual skills, such as the association areas of the cerebral cortex. The contention that these areas serve intellectual functions is supported by a large body of clinical and experimental literature. Together, these diverse sources of information suggest that perceptual-motor and intellectual skills depend on distinct brain circuits .