Tempo-spatial discrimination is lower for noxious stimuli than for innocuous stimuli

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The exteroceptive sensory system is responsible for sensing external stimuli in relation to time and space. The aim of this study was to investigate the tempo-spatial properties of the exteroceptive system using painful laser heat and nonpainful mechanical touch stimulation. Thirteen healthy subjects were stimulated on the volar forearm using 2 paradigms: a continuous stimulation along a line on the skin and a 2-point stimulation. The line stimulations were delivered in both the distal and proximal direction with lengths of 25, 50, 75, and 100 mm. The 2-point stimulations were assessed by simultaneous stimuli at a point-to-point distance ranging from 10 to 100 mm, in steps of 10 mm. The subjects reported the intensity (0-10 numeric rating scale, 3: pain threshold) and either direction (line stimuli) or number of perceived points (2-point stimuli). All mechanical line stimulations were reported correctly, ie, a directional discrimination threshold of less than 25 mm. For painful laser line stimulation, the directional discrimination threshold was 68.5 and 70.2 mm for distally and proximally directed stimuli, respectively. The 2-point discrimination threshold for painful laser stimulation (67.9 mm) was higher than for the mechanical stimulation (34.5 mm). Numeric rating scale increased both with line length and distance between the 2 points (linear mixed model, P < 0.001). The findings indicate that the tempo-spatial acuity of the exteroceptive system is lower for noxious stimuli than for innocuous stimuli. This is possible due to the larger receptive fields of nociceptive neurons and/or less lateral inhibition.

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