Abstract
The directional discrimination is lower for painful laser heat compared to non-painful mechanical stimulation. The aim of the current study was to investigate how the directional discrimination of radiant heat stimulation depends on stimulation intensity and displacement velocity. Fifteen healthy subjects were stimulated in the right volar forearm with a CO2 laser at intensities that were expected to be either painful (46°C) or non-painful (39°C). The laser beam was continuously displaced distal-proximally along the arm during the stimulation. After the stimulation, subjects indicated the perceived direction and intensity (NRS: 0: perception 3: pain 10: maximum pain). Stimulations were delivered with five lengths (20, 40, 60, 80 and 100mm) and three velocities (10, 30 and 100mm/s). To estimate the directional discrimination threshold (DDT) the data were fitted to a sigmoidal curve. For the lower intensity (39°C) the DDT was 81.8mm for the slowest velocity, and above 100mm for the two faster velocities. For the higher intensity (46°C) the DDT was 58.8 and 69.6mm for the slowest velocity and middle velocity, respectively, and above 100mm for the fastest velocity. The perceived intensity increased with stimulation length, stimulation intensity and decreasing velocity (LMM, p<.001). This study shows how the DDT for thermal stimuli is shorter for higher intensity and lower displacement velocities. Additionally, it was shown that for the velocity where directional discrimination is optimal for mechanical stimuli it is not possible to discriminate a thermal stimulus. This study showed that the directional discrimination of painful laser stimuli is better than that for non-painful laser stimuli. These findings supplements our current knowledge regarding the tempo-spatial discrimination in the nociceptive system, where evidence from previous discrimination studies differs somewhat regarding difference between painful and non-painful discrimination. This, therefore, indicates that there is lacking knowledge regarding the discrimination within the nociceptive system.
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