Cortical interactions and integration of nociceptive and non-nociceptive somatosensory inputs in humans

There is experimental evidence indicating that, in humans, avoiding the concurrent activation of non-nociceptive A beta-fibers is a necessary condition for slower A delta-fiber nociceptive input to elicit reproducible event-related brain potentials (ERPs). Similarly, numerous studies have shown that for unmyelinated C-fiber nociceptive input to elicit ERPs, the concurrent activation of A delta-fibers must be avoided. As studies have shown that expectancy of the stimulus greatly conditions the magnitude of these evoked responses, it was hypothesized that the absence of cortical responses related to A delta- or C-fiber somatosensory input that is shortly preceded by A beta- or A delta-fiber somatosensory input could be explained by the fact that the first-arriving afferents render later-arriving afferents highly expected. To test this hypothesis, ERPs were recorded in response to (1) an electrical stimulus selectively activating A beta-fibers of the nervus radialis superficialis, (2) a CO2 laser stimulus selectively activating A delta-nociceptors located in the sensory territory of that same nerve, or (3) the combined activation of nociceptive and non-nociceptive fibers using both stimulation methods applied either concurrently, or with a short inter-stimulus interval. This paradigm aimed at preventing first-arriving inputs from increasing the expectancy of late-arriving inputs. Results showed that under these particular conditions, A delta-fiber ERPs were not abolished by preceding A beta-fiber input. However, their amplitude was significantly reduced. Furthermore, the amplitude of A beta-fiber vertex potentials was similarly reduced by shortly-preceding A delta-fiber input. As expectancy of the stimulus could not account for this reduction, a new hypothesis was proposed, based on processes related to the perceptual fusion of multisensory inputs.