As muscle spindles are involved in the sensation of position and movement of the body, we tested their involvement in the pathophysiology of idiopathic focal dystonia. Twenty patients with torticollis, nine with writer's cramp, two with blepharospasm and 16 healthy control subjects participated. In the first task, the blindfolded subject matched the position of the passively moved forearm with the opposite forearm when the elbow joint was slowly flexed. In a second matching task, passive movement was replaced by stimulation of one biceps tendon with a 50-Hz vibratory stimulus (a selective stimulus for muscle spindle Ia afferents). In normal individuals, this stimulus produces flexion of the vibrated arm around the elbow joint. Movement in both arms was recorded electronically. In experiments without vibratory stimuli, dystonic subjects showed normal movement of the tracking arm during attempts to match the position of the passively moved arm and no difference between the arms in the initial and final steady state positions. In experiments using vibratory stimuli, vibration of biceps tendons in normal subjects elicited flexion of the stimulated arm at the elbow and a matching movement of the opposite arm. In patients with dystonia, there was a similar flexion response to the vibratory stimulus in the stimulated arm but movement of the tracking arm was reduced. Taken together, these experiments suggest that there is abnormal perception of motion, but not position, in dystonic subjects. Dystonic subjects showed bilateral abnormalities of perception of the tonic vibration reflex which were remote from the clinically affected site. These findings are discussed in relationship to the role of muscle spindle Ia afferents in focal dystonia.

Although the pathophysiological basis of idiopathic focal dystonia (IFD) remains unclear, we recently reported abnormal perception of the tonic vibration reflex (TVR) in the biceps brachii in IFD. In this study we examined whether the abnormality affects muscles other than the biceps brachii. A 100-Hz vibration stimulating predominantly the muscle spindle afferent was transcutaneously applied to one muscle tendon of the triceps brachii, the wrist extensor and flexor muscles in 29 subjects with IFD (18 with torticollis, 9 with writer's cramp, 2 with blepharospasm) and 15 control subjects. The blindfolded subjects were instructed to copy any perceived movement with the opposite tracking arm. The elbow or wrist angle changes were quantified electronically. The TVR and subjects' perception of the TVR were evaluated by angle movements of the vibrated joint and of the tracking joint, respectively. Perception of the TVR was significantly reduced in dystonic subjects at both elbow and wrist joints, while magnitude of the TVR did not differ between the two groups. Abnormal central perception of the TVR is a feature of IFD, suggesting a widespread involvement of abnormal muscle spindle afferent processing in IFD.

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Idiopathic torsion dystonia is characterized by persistent abnormalities of posture. We tested the hypothesis that abnormal sensorimotor processing is involved pathophysioiogically by looking for asymmetry of sensory processing in patients with asymmetric symptoms. Sixteen patients with torticollis (ten with head turning to the right and six to the left), seven with simple writer's cramp and 19 healthy control subjects were tested. The tasks involved matching one of five lengths of stick or weights presented to one hand with sticks and weights chosen by the other hand. All experiments were performed with the subject blindfolded. Patients with torticollis tended to underestimate weights presented to the hand away from which the head tended to turn. Control subjects showed no significant tendency to overestimate or underestimate lengths or weights with either hand, and dystonic patients showed no tendency to overestimate or underestimate lengths. Those with writer's cramp underestimated weights when the stimulus was presented to the affected hand. An asymmetry can thus be detected in muscles remote from the site of dystonia, indicating a generalized abnormality of sensorimotor processing.

Responses produced by the parallel fibre stimulation in the cerebellar cortex were studied in detail and reported recently (ANDERSEN, ECCLES and VOORHOEVE, 1964, ECCLES, LLINAS and SASAKI, 1966c, ECCLES, SASAKI and STRATA, 1966). Weak pulse current through a concentric electrode placed on the surface of the cerebellar cortex excites parallel fibres under the electrode and sets up a narrow band or "beam" of the excited parallel fibres in a shape like a half cylinder (see FIG.1). Impulses in the parallel fibres elicit EPSPs in dendrites of Purkinje cells and of cerebellar interneurones such as basket , stellate and Golgi cells (ECCLES, LLINAS and SASAKI, 1966b, c, ECCLES, SASAKI and STRATA, 1966). The basket and stellate cells in turn produce IPSPs in

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