Modulation of facial sensitivity by navigated rTMS in healthy subjects

Repetitive transcranial magnetic stimulation (rTMS) has had partly incongruous effects on cutaneous sensibility, and there are no systematic studies on the effects of rTMS on facial sensory function. We assessed modulation of thermal sensitivity of facial skin in healthy subjects by navigated rTMS (10 Hz), enabling accurate localization of predefined cortical targets: right primary motor cortex (M1) of facial muscles, primary somatosensory cortex (S1) representing the cheek, dorsolateral prefrontal cortex (DLPFC), and secondary somatosensory cortex (S2); the control site was occipital cortex (OCC). Applying signal detection theory, we investigated whether the rTMS-induced changes in heat-pain threshold (HPT) relate to an alteration in the subject's discriminative capacity (sensory factor) or response criterion (non-sensory factor). HPT increased after stimulation of S2, but also 45 min after stimulation of DLPFC and OCC. S2 stimulation produced the most effective and long-lasting heat hypoalgesia that was associated with a decrease in discriminative capacity and an increase in response criterion. Cold-pain threshold was elevated after S2 stimulation only in men. Stimulation of M1 decreased capacity to discriminate painful heat without influencing HPT; there was large interindividual variation in rTMS effects in the M1/S1 areas. Detection threshold for innocuous warming rose similarly after rTMS of M1, S1, DLPFC, S2 and OCC, whereas sensibility to innocuous cooling transiently improved after rTMS of S1. The results indicate that rTMS applied anatomically accurately to S2 may produce analgesia in the face via multiple mechanisms, partly depending on gender, and involving decreased discriminative capacity and increased response criterion.