10. Sensorimotor interaction and motor learning in facial muscles

Abstract

Background and aim : Integration of sensory information with motor output is thought to be important in motor learning. In limb muscles, this is studied using the short afferent inhibition (SAI) paradigm, to assess sensorimotor interaction, and paired associative stimulation (PAS), to evaluate LTP-like plasticity. As far as we know, SAI and PAS paradigms have never been used in the territory of the cranial nerves. The present study was aimed at testing in normal subjects whether sensorimotor interaction and LTP-like plasticity can be observed in facial muscles as well as in limb muscles. Methods : Motor evoked potentials (MEPs) were evoked in the depressor angulis oris (DAO) muscle of 7 subjects. MEPs were recorded from the contralateral DAO at rest and during 10% maximal voluntary contraction (active condition). SAI was tested in 5 subjects, by pairing electrical stimulation (ES, intensity 3 times the perceptual threshold) of the facial nerve, with magnetic stimulation (TMS, 120% of motor threshold intensity) of the facial motor cortex. Intervals between ES and TMS were 5, 10, 15, 20, 25 and 30 ms. The LTP-like plasticity protocol (200 pairs of ES and TMS, 20 ms ISI, at 0.25 Hz) was tested in X subjects by evoking twenty MEPs in both resting and active conditions, before and at 0, 20 and 30 min after paired stimulation. Results : Facial nerve stimulation in the SAI paradigm had no significant affect on MEP amplitude, either in the active or in the relaxed DAO muscle. By contrast MEP amplitude at rest showed a trend of facilitation (p < 0,072) after the PAS protocol administration. When tested at rest this effect was observed at baseline and after 10 min. On the contrary MEP amplitude recorded during activity was significantly enhanced at 10 (p < 0,026) and 30 (p < 0,014) minutes after PAS. There was a significant difference between the time course in resting and active conditions (p < 0,019). Conclusions : These data show that there is no short latency afferent inhibition in the facial motor cortex, yet despite this, there is evidence for maintained LTP-like plasticity. Further studies are needed to understand how and why PAS works in facial muscles despite the absence of SAI. A larger sample is also required to confirm the PAS effect on resting DAO MEPs. A second point to be clarified is why the PAS time course at rest is different from that operating during active condition.