The effect of fasciculus cuneatus lesions on finger positioning and long-latency reflexes in monkeys.

Previous studies have reported abnormalities in fine hand and finger movements following interruption of the fasciculus cuneatus (FC) in primates. We report here that many of these deficits could be caused by an inability to actively regulate the position of the finger. Three macaques were trained to maintain the index finger in one position against constant or changing loads. Periodically, torque pulses were used to elicit reflexes in finger muscles. Following unilateral FC lesions, the monkeys failed to adjust finger position during the trials, and the normal M2 long-latency response was absent in the finger muscles. Performance on the task was impaired only in monkeys with complete lesions that included the deep ventral portion of the FC. These results suggest that afferent fibers in the FC regulate finger position, and do so partly through reflexive mechanisms. When the FC is interrupted, the inability to control finger position disturbs fine motor activities.