Application of focused ultrasound for research on pain

Exteroceptive silent periods (ESPs) of masseter muscle activity evoked by electrical stimulation of the mental nerve were studied over a large range of prepain intensities and at pain threshold in 44 normal subjects. Seven levels of stimulus intensity, based on individual sensory and pain thresholds, were applied and the relationship between ESPs, stimulus intensity and perception, as manifested by the subjective verbal response, was investigated. The analysis revealed that the occurrence of ESPs was not related to the stimulus intensity at the pain threshold. There were individually different patterns of progressive response to increasing current intensities within the pre-pain range in many cases. On the other hand, almost half of all the subjects investigated showed no or only occasional ESPs. In view of this variability the concept of ESPs being a nociceptive behavioural response has to be questioned.