ORIGIN OF THE FACIAL LONG-LATENCY RESPONSES ELICITED BY MAGNETIC STIMULATION

With magnetic stimulation (MS) it is possible to elicit bilateral long latency facial motor responses (LLRs). Due to a relatively wide magne tic field, the site of neural activation may take place in many differ ent structures. The purpose of this study was to determine the site of origin of facial LLRs. The motor long latency responses were recorded bilaterally on the naso-labial folds (NLFs) with reference electrodes on the nose, and on some subjects also with reference electrodes on t he chin. The stimulating coil was placed in the right parietal area. L LRs obtained with MS were compared to LLRs elicited electrically at th e right stylomastoid foramen, supraorbital foramen, as well as cutaneo us sensory area V1 of the trigeminal nerve. In addition, right sided h igh intensity electrical stimuli, paired magnetic stimulation and elec trical stimulation with interstimulus intervals ranging from 0 to 80 m sec were also applied for comparison. LLRs recorded with reference to the nose were always elicitable with MS as well as with the other stim ulation procedures. The responses elicited with MS did not differ from those elicited electrically at various extracranial stimulation sites . With paired stimuli the second LLRs were inhibited by the preceding stimulation, whether given magnetically or electrically. In subjects w ith elicitable LLRs with chin references, the responses were always bi lateral. Based on the similar characteristics with extracranial electr ical stimuli, bilateral distribution of the responses, and inhibition of the second response with paired stimuli, it is concluded that the n eural origin of LLRs to MS is in the extracranial trigeminal or facial nerve branches.