MOTOR EFFECTS OF STIMULATION OF CEREBRAL CORTEX OF SQUIRREL MONKEY (SAIMIRI SCIUREUS)

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Abstract
The cerebral cortex was stimulated with 3 second 60 cycle AC pulse trains through a unipolar stainless steel electrode. The indifferent electrode was attached to a metal ring sutured to the scalp surrounding the exposed cortex. At least 2 minutes separated successive stimulations. The medial, dorsolateral, parietal opercular and insular cortices were stimulated in a total of 17 squirrel monkeys under moderately deep pentobarbital anesthesia. Thresholds for movement were found for every point stimulated and these movements were simple and devoid of after-discharge. The current required for threshold stimulation varied from 0.2 mA to 2.8 mA. The data from individual animals were superimposed on standard brain diagrams and average composite charts were prepared. Movements which were elicited from alternate 2-mm squares were represented on figurines. On the basis of topographical organization, threshold, and cytoarchitectual criteria, 4 cortical areas were delineated which yielded body movements upon stimulation. A complete body representation was found for 3 of these areas. The precentral motor area largely overlies agranular cortex. Stimulus thresholds were lowest in this area. The postcentral area overlies cortex with a well developed fourth layer, and has higher thresholds than the precentral area. The topographical pattern in this area is a mirror image of that of the pre-central area and coincides approximately in rostrocaudal and medio-lateral organization with the postcentral tactile pattern in this animal. The supplementary motor area lies rostral and medial to the precentral motor area and has a higher threshold than that area. Representation for the face was not found in this area. The "second" motor area lies on the superior bank of the sylvian fissure and extends onto the caudal portion of the insula. It coincides in location and pattern of organization with the "second" somatic sensory area of this animal. Except for the precentral face area, ipsilateral responses were not observed in any of these areas when stimulated with threshold stimulus current. The details of organization of these motor patterns are described and they parallel those found in studies of other primates. The locations of the pre- and post-central areas and of the "second" motor area are similar to those found by other workers, but the supplementary area extends onto the dorsal surface in this animal, whereas it is confined largely to the medial surface in man, chimpanzee and macaque. The degree of correspondence of somatic and motor patterns is discussed and causes of intra-animal variability in results are considered.