Ipsilateral and contralateral cortical control of the external oblique muscles revealed by TMS

Transcranial magnetic stimulation (TMS) is used widely to explore human motor function. This study was conducted to investigate the differences between contralateral and ipsilateral cortical control on trunk muscles for eliciting contralateral and ipsilateral motor evoked potentials (MEPs) in the external oblique muscles.

Twenty healthy participants were examined: 10 younger and 10 older adults. They underwent TMS to locate contralateral and ipsilateral hot spots. For the identified hot spots in the younger group, the coil was positioned in eight orientations, each separated by 45 degrees, to measure active motor threshold (AMT) and onset latencies.

Contralateral MEPs were obtained from all participants, whereas ipsilateral MEPs were detected in 14 out of 20 participants. The ipsilateral hot spots were located more laterally from Cz (p < 0.05). Coil orientation that induced an antero-medially directed current in the brain produced the shortest onset latency and lowest AMT for both stimulations. However contralateral MEPs required lower AMT and exhibited shorter onset latency than ipsilateral MEPs (p < 0.05).

Ipsilateral motor pathways of the external oblique muscles have physiological features unlike those of contralateral motor pathways.

These findings suggest differential cortical control between contralateral and ipsilateral pathways in trunk muscles and provide bases for additional investigations into ipsilateral motor control.