Ipsilateral responses in the biceps brachii muscle after focal transcranial magnetic stimulation of the motor cortex

Focal TMS of the motor cortex evokes corticospinally mediated responses in contralateral muscles and EMG effects in ipsilateral upper limb muscles. Th e aim of the study was to characterize ipsilateral responses m the biceps b rachii muscle (BB) and to compare with ipsilateral EMG effects in the first dorsal interosseus muscle (FDI). In 20 normal subjects and one patient wit h agenesis of the corpus callosum the motor cortices of both hemispheres we re stimulated. EMG activity was recorded bilaterally with surface electrode s from FDI and BE. Stimulation was performed during strong tonic muscle con traction, stimulus intensity was set to 1.5 fold of threshold at rest (n = 10, group A) or to a higher intensity of 80 % of maximum stimulator output, i.e. about 1.8 times the response threshold (n = 10, group B). In all subj ects corticospinally mediated contralateral excitatory responses occurred i n FDI and BE (FDI: latency 21.7 +/- 1.5 ms, amplitude 6.5 +/- 1.7 mV; BE: l atency 11.5 +/- 1.2 ms, amplitude 8.7 +/- 4.4 mV; n = 40). All healthy adul ts had an ipsilateral inhibition of tonic EMG activity in the FDI (latency 37.4 +/- 2.1 ms; duration 24.6 +/- 5.7 ms; n = 40), which was not present i n the patient. In the BE ipsilateral excitatory responses occurred in 33/40 muscles of normal subjects and in the patient. These responses had a highe r threshold, longer onset latency and smaller amplitude compared to contral ateral excitatory responses and were more dependent on facilitation by toni c contraction. According to their latency, ipsilateral BE responses could b e attributed to two different types: short latency responses (group A: late ncy 17.8 +/- 2.1 ms, amplitude 1.3 +/- 1.2 mV; group B: latency 18.1 +/- 1. 9 ms, amplitude 1.2 +/- 0.8 mV) and long latency responses (group B: latenc y 45.8 +/- 4.6 ms, amplitude 0.5 +/- 0.3 mV). The results show that in norm al subjects TMS can elicit ipsilateral excitatory responses in preactivated BB, which are probably mediated by ipsilaterally projecting oligosynaptic and/or slowly conducting pathways.