Perception of non-voluntary brief contractions in normal subjects and in a deafferented patient.

The accuracy of force perception by human subjects in the absence of voluntary motor command was evaluated by exploring how they perceived isometric twitches of wrist extensor muscles produced by external stimulation. Twelve normal subjects and a well-known patient lacking large-diameter afferent fibres (GL) performed estimation, production and reproduction tasks. Magnetic stimulation of the radial nerve and, in normal subjects, transcranial magnetic stimulation (TMS) of the motor cortex were used to produce weak brief non-voluntary twitches. In estimation tasks, the subjects had to ascribe verbal marks on a 1-5 scale to the forces of stimulation-induced twitches. Loose covariations of marks and forces were observed, while directions of force variations between successive twitches were relatively well detected. GL did less well than normal subjects in detecting directions of force variations. In production tasks, subjects had to produce twitches matching verbal command marks in a 1-5 range, with or without visual feedback. Performances of normal subjects and GL resembled those of estimation tasks and were not improved by visual feedback. In reproduction tasks, subjects had to duplicate stimulation-induced test twitches: first without visual feedback, second with and third again without. Large errors were observed but all subjects did better with visual feedback. In the third step, improvement with respect to the first one was significantly more marked with TMS than with peripheral stimulation. GL improved her performance in the third step, possibly because she could use information provided by group III and group IV afferents still present in her nerves. Altogether, for normal subjects (1) the performances in estimation tasks are consistent with the known behaviour of Golgi tendon organs as observed in animal experiments, and (2) results observed in reproduction tasks suggest that cortical stimulation might elicit, in addition to corticospinal activation of motoneurones, collateral discharges that could be stored as a memory of motor command.