Digit-muscle responses evoked from multiple intracortical foci in monkey precentral motor cortex

1. The precentral motor cortex, including the anterior bank of the central sulcus of monkey (Macaca fuscata), was systemically penetrated with microelectrodes to determine the spatial organization of the microexcitable cortical elements that can produce responses in digit muscles. 2. At 200-microns intervals on each electrode track, low-current intracortical microstimuli were delivered and the muscle responses evoked from four digit muscles were recorded. The responses, obtained with 5, 8, 15, and 25 microA, were quantified and plotted on a map displaying an unfolded view of the precentral gyrus. 3. For all four muscles studied [first interosseus, thenar, extensor digitorum communis (EDC) and flexor digitorum profundus (FDP)], the effective stimulus points evoking muscle responses at a current of 5 microA were scattered over wide areas. The low-threshold foci, largely buried in the anterior bank of the central sulcus but partly extending to a region rostral to the sulcus, were found in multiple spots separated by a few millimeters. 4. Stimulation of individual sites at a current of 5 microA often evoked responses in several different muscles. Antagonist muscles were frequently coactivated. 5. A three-dimensional display of the distribution of response magnitude evoked from the precentral cortex indicates several peaks for each digit muscle. The peaks were either sharply demarcated from surrounding areas of minimal responses or gradually shifted into regions of low-grade responses. 6. Taken together, the data suggest that the digit area of motor cortex does not have a simple organization in which each muscle is represented by a single focus. Rather, each muscle has multiple foci that have varying degrees of efficacy in producing responses and with variable overlap onto foci of other muscles.