4.02 - The Evolution of Sensory and Motor Systems in Primates

Abstract

The course of the evolution of the sensorimotor system from early primates to modern humans can be reconstructed from comparative studies of how sensorimotor systems are organized in extant primates and their nonprimate relatives. Early mammals appear to have had no separate motor cortex. Somatosensory areas included a primary area, S1, adjoining somatosensory belts rostral (SR) and caudal (SC) to S1 in the positions of area 3a (SR) and area 1 (SC) of primates, the second somatosensory area (S2) and the parietal ventral area (PV). S1 and S2 were activated in parallel by inputs from the ventroposterior nucleus of the thalamus, which in turn was activated by the dorsal column – medial lemniscus pathway. Somatosensory areas, especially SR, S1, and S2, participated in motor as well as sensory functions. Placental mammals added a primary motor area (M1) and probably a premotor or secondary motor area (M2). In early primates, the hand representation in somatosensory and motor areas had an enlarged representation, the SR and SC regions acquired most of the features that distinguish somatosensory areas 3a and 1 of primates, and motor cortex included ventral and dorsal premotor areas, a supplementary motor area, and rostral and caudal cingulate motor areas. The sensorimotor thalamus was more differentiated, with a distinct ventroposterior inferior nucleus projecting especially to S2, but to other somatosensory areas as well, and a ventroposterior superior nucleus relaying information from muscle spindle receptors to area 3a. A posterior parietal region was involved in relaying somatosensory and visual information to motor areas of frontal cortex. Early anthropoids (monkeys) were characterized by more serial processing in somatosensory cortex so that S2 depended on S1 (area 3b) for activation. Area 1 contained a distinct representation of cutaneous receptors, while a mixed representation of deep (muscle spindle) and cutaneous receptors emerged in area 2, just caudal to area 1. Posterior parietal cortex had expanded and differentiated to reflect an increased role in sensorimotor functions. Subdivisions of premotor cortex emerged in Old World simians. In the hominid line of bipedal ancestors leading to humans, brains increased in absolute size and in size relative to body size, resulting in scaling adjustments in sensorimotor systems. Different specializations emerged in the right and left cerebral hemispheres, while posterior parietal and frontal sensorimotor regions enlarged, and probably added cortical areas.