Effect of motor tasks on the cortical topography of the human masseter muscle.

Non-invasive magnetic stimulation of the brain was earlier used to reveal the corticomotor representation of the human masseter muscle but it is unclear how motor tasks affect this map. An experimental approach incorporating transcranial magnetic stimulation of verified locations on the scalp, surface electromyography, and controlled muscle facilitation was used to disclose the corticomotor output map of the masseter during three isometric tooth-contact tasks, viz., tooth clenching in the intercuspal position, biting on the left molar teeth, and biting on the incisors. Map area was significantly different for all tasks, and map height and volume were also different for biting on the incisor teeth (p<0.05). There was evidence of task-related modulation of corticobulbar activity that appeared to be mainly of corticomotoneuronal origin, although the role of differential, task-associated peripheral afferent input from orofacial receptors could not be discounted.