Tempo-spatial discrimination to non-noxious stimuli is better than for noxious stimuli

Abstract

Abstract Aims The exteroceptive sensory system is responsible for sensing external stimuli in relation to time and space. The aim of the study was to investigate the tempo-spatial properties of the exteroceptive system using laser heat and mechanical touch stimulation. Methods 13 healthy subjects were stimulated in the volar forearm. Each subject was stimulated using two paradigms, a continuous stimulation along a continuous line on the skin, and a simultaneous 2-point stimulation. The line stimulations were delivered in both a distal and proximal direction with lengths of 25, 50, 75, and 100 mm. The 2-point stimulations were delivered with a point-to-point distance ranging from 10 to 100 mm, in steps of 10 mm. Both paradigms were delivered using painful heat (laser) stimulation and mechanical (touch) stimulation. Following each stimulation, subjects had to report the intensity (0–10 NRS, 3 being pain threshold) and either direction (line stimuli) or number of perceived points (2-point stimuli). Results All four line lengths and both directions were reported correctly for all mechanical line stimulation (an accuracy of 100%). For laser line stimulation the directional discrimination threshold was 68.5 mm and 70.2 mm for distal and proximal directed stimuli, respectively. The 2-point discrimination threshold for heat stimulation (67.9 mm) was higher than for the mechanical stimulation (34.5 mm). NRS was significant higher for laser stimulations than for the mechanical stimulation for both line and 2-point stimulations. NRS increased both with line length and distance between the two points (ANOVA, p < 0.001). The average NRS was 3.15 and 0.91 for laser line and mechanical line stimulations, respectively. For 2-point stimulation the average NRS was 3.72 and 1.06 for laser and mechanical stimulations, respectively. Conclusions The findings indicate that the tempo-spatial acuity of the exteroceptive system is lower for noxious stimuli than for innoxious stimuli. This is possible due to the larger receptive fields of nociceptive neurons and/or less lateral inhibition.