Converging evidence from microstimulation, architecture, and connections for multiple motor areas in the frontal and cingulate cortex of prosimian primates

Abstract

In the present study, somatotopic organization, architectonic features, and patterns of connections were used to define motor areas in the frontal and cingulate cortex of the prosimian primate Galago garnetti. Sites throughout portions of the motor cortex were electrically stimulated with microelectrodes at the approximate depth of layer V. In some of the same animals, injections in primary motor cortex (M1), and in the spinal cord, revealed patterns of connections with physiologically identified motor areas. Results were related to cortical architecture in brain sections processed for Nissl, myelin, cytochrome oxidase, acetylcholinesterase, or neurofilaments. Evidence was obtained for a number of fields previously identified in simian primates, including M1, dorsal premotor field with caudal (PMDc) and rostral (PMDr) divisions, ventral premotor area (PMV), supplementary motor area (SMA), presupplementary motor area (pre-SMA), frontal eye field (FEF), and cingulate motor areas, CMAr and CMAc located rostrally and caudally, respectively. In addition, we distinguished area 6Ds of Preuss and Goldman-Rakic (1991a) between PMV and PMDc, and a more posterior cingulate sensorimotor area (CSMA) with motor connections that may correspond to the supplementary sensory area of monkeys. Areas M1, SMA, PMDc, PMV, CMAr, CMAc, and CSMA projected to the spinal cord, while all of these areas and 6Ds projected to M1. Although area M1 had the lowest stimulation thresholds for evoked movements, movements were also evoked from the other motor areas, as well as from somatosensory areas 3a and 3b. These results indicate that prosimian galagos have a complex of motor areas that closely resembles that in monkeys and suggest that at least 10 motor fields emerged early in primate evolution.