TIMING OF ONSET OF AFFERENT RESPONSES AND OF USE OF KINESTHETIC INFORMATION FOR CONTROL OF MOVEMENT IN NORMAL AND CEREBELLAR-IMPAIRED SUBJECTS

A coordinated triggering task requiring use of kinesthetic information was employed to assess the timing of use of kinesthetic information i n normal subjects and patients with cerebellar dysfunction. Passive mo vements of varying velocity were imposed in the flexor direction about the metacarpophalangeal joint of the right index finger, Subjects att empted to depress a switch with their left thumb when the index finger moved past a specified angle that was learned during a training sessi on. The velocities ranged from 10 degrees/s to 88 degrees/s in 2 degre es/s increments. After 200 trials, subjects were then instructed inste ad to react as quickly as possible (reaction-time task) to the onset o f movement for an additional 200 trials. For the same movements, the t iming of onset of responses of muscle spindle afferents and cutaneous mechanoreceptors was determined by recording the responses of these af ferents using microneurography. For slow velocities, patients were abl e to perform similarly to normals but at faster velocities patients tr iggered too late compared with normals. Patients required more time to use kinesthetic information than did normal subjects. An estimate of kinesthetic processing was not longer in patients. The chief explanati on for the prolonged time required to use kinesthetic information in p atients was that their reaction times were prolonged by 93 ms. In addi tion, the movement time was also prolonged, but this accounted for onl y 23 ms. Impaired motor performance in tasks requiring the use of kine sthetic information in cerebellar patients can be explained largely by their prolonged reaction times. Muscle spindle afferents responded on average much sooner than cutaneous mechanoreceptors. Because of the l imited time available to perform the kinesthetic triggering task, the role for cutaneous mechanoreceptors to provide signals for on-line coo rdination of movement appears limited compared with muscle spindle aff erents.