Directional discrimination is better for noxious laser stimuli than for innocuous laser stimuli

Abstract

Background

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.

Methods

Fifteen healthy subjects were stimulated in the right volar forearm with a CO₂ laser at intensities that were expected to be either painful (46 ⁰C) or non‐painful (39 ⁰C). 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, 100 mm) and three velocities (10, 30 and 100 mm/s). To estimate the directional discrimination threshold (DDT) the data was fitted to a sigmoidal curve.

Results

For the lower intensity (39 ⁰C) the DDT was 81.8 mm for the slowest velocity, and above 100 mm for the two faster velocities. For the higher intensity (46 ⁰C) the DDT was 58.8 and 69.6 mm for the slowest velocity and middle velocity, respectively, and above 100 mm for the fastest velocity. The perceived intensity increased with stimulation length, stimulation intensity and decreasing velocity (LMM, p < .001).

Conclusions

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.

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