Interhemispheric Inhibition Predicts the Magnitude of Mirror Movements in Healthy Subjects

It has been proposed that interhemispheric inhibition mediated through the corpus callosum is important to prevent mirror movements (MM) during intended unimanual finger movements. To test this, we correlated interhemispheric inhibition (IHI) with the magnitude of MM in 12 healthy volunteers (age, 21–40 years). MM were quantified according to an established protocol (Mayston et al. Ann Neurol 1999; 45: 583). Subjects were instructed to perform 50 ballistic index finger abductions with one hand (task hand) at a self-paced rate of about 0.2Hz whilst maintaining an isometric contraction with the first dorsal interosseus (FDI) muscle of the other hand (mirror hand). EMG in the mirror FDI was rectified, time-locked to EMG burst onset of the task FDI and averaged. MM were quantified as increase of EMG activity relative to pre-burst EMG. Two measurements were obtained. In one, the task hand was the right hand, in the other, it was the left hand. The order was random and counterbalanced. IHI was measured using transcranial magnetic stimulation (TMS, two figure-of-eight-shaped coils) according to a previous protocol (Ferbert et al. J Physiol 1992; 453: 525). A conditioning pulse over the motor cortex (M1) of one hemisphere (conditioning hemisphere) preceded the test pulse over the other M1 by 12 ms. The intensity of the conditioning stimulus varied from resting motor threshold (RMT) to 150% RMT in 10% steps (IHI100-IHI150). The intensity of the test pulse was adjusted to produce a test motor evoked potential (MEP) of 1 mV. IHI was expressed by the ratio of the mean conditioned over unconditioned MEP. IHI was tested from the left to right M1 and vice versa in random and counterbalanced order. IHI was correlated with the MM ipsilateral to the conditioning hemisphere. MM amounted to 17.1±9.3%. IHI increased roughly linearly from IHI100 (89.7±23.7%) to IHI 150 (34.8±22.5%). IHI correlated inversely with MM. The correlation was best with IHI140 (p=0.0031, R²=0.33). Findings show for the first time that, in healthy subjects, the magnitude of mirror movements can be predicted by the strength of interhemispheric interaction. Quantification of mirror movements may be used as an easy means to study the integrity of interhemispheric interaction in neurological disease, such as patients with multiple sclerosis.