Representations of Fine Digit Movements in Posterior and Anterior Parietal Cortex Revealed Using Long-Train Intracortical Microstimulation in Macaque Monkeys

The current investigation in macaque monkeys utilized long-train intracortical microstimulation to determine the extent of cortex from which movements could be evoked. Not only were movements evoked from motor areas (PMC and M1), but they were also evoked from posterior parietal (5, 7a, 7b) and anterior parietal areas (3b, 1, 2). Large representations of digit movements involving only the index finger (D2) and thumb (D1), were elicited from areas 1, 2, 7b, and M1. Other movements evoked from these regions were similar to ethologically relevant movements that have been described in other primates. These include combined forelimb and mouth movements and full hand grasps. However, many other movements were much more complex and could not be categorized into any of the previously described ethological categories. Movements involving specific digits, which mimic precision grips, are unique to macaques and have not been described in New World or prosimian primates. We propose that these multiple and expanded motor representations of the digits co-evolved with the emergence of the opposable thumb and alterations in grip type in some anthropoid lineages.