THE FRONTOPARIETAL CORTEX OF THE PROSIMIAN GALAGO - PATTERNS OF CYTOCHROME-OXIDASE ACTIVITY AND MOTOR MAPS

We mapped the motor areas of the prosimian Galago crassicaudatus using intracortical electrical microstimulation and morphological and histo chemical (cytochrome oxidase) techniques. Stimulation data showed that on the brain convexity there is an area (area Frontalis posterior, F post.) from which movements could be evoked at low threshold (< 10 mu A). This area is somatotopically organized, with the leg represented m edially, the arm centrally and the face and mouth laterally. Proximal and distal movements are not segregated. Most of the evoked movements, even at threshold, consist of movements involving two or more joints. F post. is characterized by a three-band cytochrome oxidase activity pattern. It has an agranular structure, but it lacks pyramidal cells t hat are larger than those observed in other areas. In front of F post. there is an area histochemically similar to it, Frontalis intermedial is (F int.). This area consists of two cytoarchitectonic divisions: an agranular division (F int. pars caudalis) and a disgranular division (F int. pars rostralis). The excitability threshold of F int. is relat ively high (10 to 30 mu A). Eye, ear and neck movements are elicited f rom its lateral part, whereas trunk movements associated with limb mov ements are elicited from its medial part. Caudal to F post., there is another region from which movements can be evoked with currents betwee n 10 to 30 mu A This region has the same medio-lateral somatotopic arr angement of F post. Typically, single joint movements are elicited fro m it. Proximal and distal movements are not segregated. In spite of it s homogeneity in terms of motor response, the posterior excitable regi on is formed by two anatomically separate areas: anterior somatic area (S ant.) and posterior somatic area (S post.). S ant. has a typical k oniocortex structure, whereas S post. resembles the parakoniocortex as defined by Sanides (J. Hirnforsch., 9 (1967) 225-252). Histochemicall y both areas are made up of four longitudinal stripes differing for en zymatic activity. The three superfical stripes tend to merge together and are sharply separated from a deeply located, light stripe. This st ripe is homogeneous in S ant., whilst its central part shows an increa se in activity in S post. The possible homologies between the motor an d somatic areas of the galago and monkey as well as their role in move ment control are discussed.