Motor Responses to Noxious Stimuli Shape Pain Perception in Chronic Pain Patients

Abstract

Pain serves vital protective functions, which crucially depend on appropriate motor responses to noxious stimuli. Such responses not only depend on but can themselves shape the perception of pain. In chronic pain, perception is often decoupled from noxious stimuli and motor responses are no longer protective, which suggests that the relationships between noxious stimuli, pain perception, and behavior might be changed. We here performed a simple experiment to quantitatively assess the relationships between noxious stimuli, perception and behavior in 22 chronic pain patients and 22 age-matched healthy human participants. Brief noxious and tactile stimuli were applied to the participants' hands and participants performed speeded motor responses and provided perceptual ratings of the stimuli. Multi-level moderated mediation analyses assessed the relationships between stimulus intensity, perceptual ratings and reaction times for both stimulus types. The results revealed a significantly stronger involvement of motor responses in the translation of noxious stimuli into perception than in the translation of tactile stimuli into perception. This significant influence of motor responses on pain perception was found for both chronic pain patients and healthy participants. Thus, stimulus-perception-behavior relationships appear to be at least partially preserved in chronic pain patients and motor-related as well as behavioral interventions might harness these functional relationships to modulate pain perception.

Keywords: behavior; chronic pain; motor; pain; perception.

Figure 1.

Paradigm. Pain and touch stimuli of varying intensity were applied to the right hand of healthy participants and chronic pain patients. Pain stimuli were brief cutaneous laser stimuli which selectively activate nociceptive afferents without activating tactile afferents. Touch stimuli were applied using von Frey-filaments steered by a computer-controlled device for standardized somatosensory stimulation. Presentation of pain and touch stimuli was pseudorandomly varied. Reaction times were measured as time from stimulus onset until the release of a button pressed with the stimulated hand. Perceptual ratings were obtained on numerical rating scales from 0 to 100. ISI, interstimulus interval.

Figure 2.

Perceptual ratings and reaction times to pain and touch stimuli in patients and controls. Mean ratings and reaction times for pain and touch stimuli of low, medium, and high intensities are shown. Error bars indicate the SE of individual means. Ratings increased and reaction times decreased with increasing intensity of pain and touch stimuli in patients and healthy control subjects.

Figure 3.

Moderated multi-level mediation analyses of the relationships between stimulus, behavior, and perception in patients and controls. Left, The perception-behavior-model reflecting the traditional view of stimulus-perception-behavior relationships (upper panel) and the behavior-perception-model reflecting an extension of the traditional view of relationships (lower panel). Middle, Results of the moderated multi-level mediation analyses for both models for pain and touch in chronic pain patients. In the perception-behavior-model (upper panel), the DE of stimulus on behavior and the ME of perception did not differ between modalities. In the behavior-perception-model (lower panel), the ME of behavior was significantly stronger for pain than for touch. Right, Results of the moderated multi-level mediation analyses for both models for pain and touch in healthy control subjects. Again, in the perception-behavior-model (upper panel), the DE of stimulus on behavior and the ME of perception did not differ between modalities. Like in chronic pain patients the behavior-perception-model (lower panel) shows that the ME of behavior was significantly stronger for pain than for touch. All effects are quantified in original units (milliseconds for behavior in the upper panel and ratings on the NRSs for perception in the lower panel) so that coefficients reflect the estimated average effects of a one level stimulation increase on the respective dependent variable. Effect sizes are further coded by the thickness of arrow lines. Significant differences between stimulus modalities (pain vs touch) are marked with asterisks. n.s., not significant; ***p < 0.001.