Alterations of natural hand movements after interruption of fasciculus cuneatus in the macaque.

As part of a series of investigations on the control of fine finger movements in the macaque, spontaneous use of the hand in grooming, scratching, and manipulation was observed before and after interruption of fasciculus cuneatus (FC). Videotaped observations were made of four stumptail macaques (Macaca arctoides) living outdoors in social groups. The monkeys were followed for 1 to 3 years postoperatively. For the first 2 weeks following surgery, all monkeys neglected the affected hand and did not use it for support, locomotion, climbing, scratching, foraging, or grooming. Recovery of gross arm and hand movements occurred over a 1- to 3-month period. All the monkeys eventually used the hand for support, climbing, and object manipulation, but fine control of the fingers did not recover. Also, there was an apparent hypotonia of the fingers, imparting a "floppy" appearance to the hand. The animals coped with the loss of fine control by decreasing the frequency of some behaviors, eliminating others, and developing alternative strategies. Exploratory movements that were utilized for investigating the anogenital area or foraging for small food items were eliminated by FC interruption. There were obvious deficits in grip formation and grasp of small food objects (see Glendinning et al., this issue), but effects on similar movements during grooming only became obvious after repeated inspection of videotaped records. Self-scratching and sweeps of the hand in grooming were preserved but altered in form and frequency. The component movements in these behaviors were relatively uncoordinated, and the fingers were splayed (abducted). Often the hand was formed in a rigid posture throughout the sweeping motion, and the fingers did not stroke the skin individually. Frame-by-frame analysis of videotapes revealed that the morphology of the precision grip during grooming, in movements termed "plucks," was permanently altered. Preoperatively, the monkeys kept the index finger and thumb closely apposed and routinely made contact on the distal surfaces of the digits, as has been described for precision grip in humans. Postoperatively, this relationship was altered. The index finger frequently missed the thumb tip and made contact on the proximal part of the phalanx, or missed the thumb altogether. Thus, the dorsal column input is important for proprioceptive guidance of movements that achieve "tactile foveation," when objects or surfaces are actively contacted by the receptive areas of keenest sensitivity (on the fingertips).