The Effects of Precentral Lesions on the Behavior of Monkeys ¶

Bucy, in the introduction to the recent monograph on the precentral motor cortex, makes the following statement: "The editor regards the precentral motor cortex as the principal efferent or effector cerebral cortical mechanism by which the brain expresses its activity through the skeletal musculature."' Implicit in this statement is the suggestion that all learned behavior finds expression through efferents leaving the precentral region; that afferents relay signals to the "primary projection" cortices whence internuncials lead to the "association areas" which, in turn, play upon the precentral cortical areas. To be validated, this viewpoint must be supported by the following evidence: anatomical, showing a preponderance of separation between afferent and efferent tracts in the cerebrum; electrophysiological, showing a minimum of overlap between those areas responding (by "evoked potentials") to peripheral stimulation, and those areas which, when electrically excited, give rise to peripheral responses; behavioral, showing that learned behavior is impaired by injury to the more-or-less "final" common path. This paper deals with some experiments in the last category. The