The contribution of the reach and the grasp to shaping brain and behaviour

Skilled hand use is a striking feature of human behaviour, but in contrast to the view that it recently evolved in the primate lineage, the present paper argues that skilled hand movements have an early phylogenetic origin dating back to the synergies displayed by the pectoral fins of bony fishes. Insights into the function and evolution of hand use stem from the dual visuomotor channel theory, which proposes that prehension is a composite of 2 movements, the reach and the grasp. The reach is in part an egocentric act directed toward the extrinsic (location) features of objects while the grasp is in part an allocentric act directed toward the intrinsic (shape and size) features of objects. Phylogenetic, developmental, and behavioural evidence suggest that the reach and the grasp evolved separately under somatosensory control and were subsequently coordinated with visual control in the primate lineage. Accordingly, parallel pathways from visual cortex came to influence separate reach and grasp systems in parietofrontal cortex, and new descending pathways to the spinal cord came to assist in visually guided reaching. Neural processes related to the "where" of the reach and the "what" of the grasp have had a formative role in shaping cognition more generally.