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Objective To study short interval intracortical inhibition (SICI) in a rare patient with segmental dystonia of the left upper limb due to a vascular lesion in the contralateral putamen without corticospinal tract involvement. Methods Paired-pulse transcranial magnetic stimulation (TMS) was applied to both hemispheres in a conditioning-test paradigm. Six interstimulus intervals (ISIs) and 4 conditioning stimulation intensities were investigated in two separate sessions. Results Motor evoked potentials upon single-pulse TMS were within the normal range, whereas paired-pulse TMS revealed major changes in cortical excitability, proving that SICI was bilaterally absent. Conclusions The bilateral impairment of SICI cannot be considered the cause of dystonic contractions, but just a predisposing factor. Significance The absence of SICI might be regarded as a condition able to promote maladaptive plastic changes triggered by focal lesions in the putamen.

Objective : To investigate whether the same mechanisms underlie muscle relaxation in proximal and distal arm muscles of normal subjects. Methods : Fourteen healthy subjects were studied using a simple visual reaction time paradigm. Relaxation reaction time (R-RT) and contraction reaction time (C-RT) were compared across different tasks involving distal (first dorsal interosseus, FDI, flexor carpi radialis, FCR) and proximal (biceps brachii, BB, triceps brachii, TR) arm muscles. Changes of FCR H-reflex before and during voluntary relaxation were investigated in two subjects. Results : No significant difference was observed between R-RT and C-RT in the distal muscles. The R-RT was significantly shorter than C-RT in both the BB and TR muscles. The relaxation latency (R-RT) was significantly correlated to the subjects' age in all the muscles except the FDI. No inhibition of the FCR H-reflex could be observed in the 20 ms preceding muscle relaxation. Conclusions : Our findings suggest that neural mechanisms contribute differently to the relaxation of muscles with a different functional role. Voluntary relaxation in distal arm muscles is mainly related to the reduction of motor cortical output, while in proximal muscles a spinal disfacilitation is also present and possibly sustained by the modulation of presynaptic inhibition.

Objectives : To verify the usefulness of early recording of motor evoked potentials (MEPs) in predicting motor outcome after stroke and to investigate the neural mechanisms underlying functional recovery following stroke. Methods : We performed a comparative analysis of the behaviour of motor responses evoked by transcranial magnetic stimulation (TMS) of the ipsilateral and contralateral motor cortex in the affected and unaffected thenar muscles of 21 consecutive patients with acute stroke. Results : According to the behaviour of MEPs in the affected muscles, patients could be divided into 3 groups: (a) 10 subjects with absent responses to TMS of both the damaged and undamaged hemisphere, whose motor recovery was poor and related to the size of MEPs on the normal side; (b) 5 subjects with larger MEPs upon TMS of the ipsilateral (undamaged) than of the contralateral (damaged) cortex, whose good recovery possibly resulted from the emergence of ipsilateral pathways; (c) 6 subjects with larger MEPs in the affected than in the unaffected muscles, whose good recovery was possibly subserved by alternative circuits taking over cortical deafferentation. Conclusions : Early MEP recording in acute stroke provides useful information on the clinical prognosis and the different mechanisms of motor recovery.

Objectives: To assess whether cortico-cortical inhibition (CCI) induced by paired-pulse transcranial magnetic stimulation (TMS) is influenced by ‘preferential' or ‘non-preferential' activation of the motor cortex. Methods: Paired-pulse TMS (conditioning-test paradigm with interstimulus intervals of 2–5 ms) with a round coil centered over the vertex was performed in 10 normal subjects using opposite current flow directions. The amount of CCI in the opponens pollicis and first dorsal interosseus muscles was determined. Results: When a clockwise current was induced in the brain (side A of the coil uppermost) a ‘preferential' activation of the left hemisphere (right hand muscles) was observed, but the suppression of the test response by the conditioning stimulus (i.e. the CCI) was significantly greater in the left hand muscles. The situation was reversed when an anticlockwise current (side B of the coil uppermost) was induced in the brain. These effects occurred independently of the interstimulus interval, or of the absolute conditioning stimulus strength. Conclusions: CCI is more effective in the ‘non-preferentially' stimulated hemisphere, and the neural elements generating the indirect I3 wave are more sensitive to intracortical inhibition than those generating the I1 wave.