Directional discrimination in the nociceptive system is enhanced for non-continuous lines

Directional discrimination in the nociceptive system is a measure of how temporospatial information is integrated. When investigating these mechanisms, the stimulus is typically continuously moved across the skin. However, mechanisms such as lateral inhibition may affect the discrimination if non-continuous stimuli are applied. Thus, the aim of this study was to investigate if continuous and non-continuous line stimuli are discriminated differently.

21 healthy participants were stimulated in the right forearm using linearly moving laser stimulation. The directional discrimination was estimated for three different stimulation paradigms, a continuous moving line, and two non-continuous lines (discrete). The discrete stimuli (points) were separated by either 10 or 20 mm. A computational model was used to investigate how these stimulation paradigms affected the temperature profiles at receptor level.

The directional discrimination threshold (DDT) was higher for continuous lines (58.2 mm [95 % CI]) compared to non-continuous lines (DDT@10 mm: 41.6 mm [95 % CI], and DDT@20 mm: 29.8 mm [95 % CI]), indicating better discrimination for the discrete stimuli. The perceived intensity was significantly higher for the continuous lines (ANOVA, p<0.001). The computational model indicated that the receptor temperature is higher for continuous stimuli. But with greater temperature difference i.e. contrast along the line for non-continuous stimuli.

In this study it was found that directional discrimination is enhanced for non-continuous lines compared to continuous. The computational model showed that continuous stimulus caused higher receptor temperature, indicating stronger activation, which appears to explain the increase perceived stimulation intensity. This may be caused by higher spatial contrast for discrete stimuli, and is comparable to what has been found in the visual and tactile systems.