The flexibility of the human motor system is remarkable. Even when parts of the system are damaged, the output often remains optimal or near-optimal. The neuromotor system is designed to keep the output optimal by shifting between input sources. This capability is termed the principle of continuous adaptation. This article describes an experiment in which patients suffering from a hereditary motor and sensory neuropathy, type la (Charcot Marie Tooth disease, type la), had to perform fine motor movements. We examined whether they were able to regulate these movements in spite of the fact that the somatosensory input and motor output was substantially impaired as a result of the chronic, slowly progressing neuropathy. It was predicted that these patients were able to perform fine movements as long as the movements were well known and over-learned. Furthermore, it was predicted that these patients would compensate for the loss of somatosensory information by becoming more dependent on vision. A second prediction was that the quality of the motor performance would break down when these patients had to perform a novel motor pattern. The performance of the patients (n = 10) was contrasted with the performance of 20 healthy subjects. The results indicated that the patients, indeed, were able to perform the over-learned movements and that their performance deteriorated significantly when they had to perform a novel motor pattern. No indication, however, could be found for visual compensation.
T. Mulder is with the Institute of Human Movement Sciences, University of Groningen, P.O. Box 196, 9700 AD Groningen, The Netherlands. R. den Otter is with the Sint Maartenskliniek-Research, Nijmegen, The Netherlands. B. van Engelen is with the Neuromuscular Centre, Nijmegen, Institute of Neurology. University Medical Centre Sint Radboud. Nijmegen, The Netherlands.