Rethinking pain threshold as a zone of uncertainty

Abstract

Background. The pain threshold is traditionally conceptualised as a boundary that lies between painful and non-painful events, suggesting a reasonably stable relationship between stimulus and response. In two previous experiments, participants received laser stimuli of various intensities and rated each stimulus on the Sensation and Pain Rating Scale (SPARS), which includes ranges for rating painful and non-painful events and clearly defines the presumed boundary between them. In the second experiment, participants also provided ratings on the conventional 0-100 Numerical Rating Scale for pain (NRS) and a new rating scale for non-painful events. Those data showed the SPARS to have a curvilinear stimulus-response relationship, reflecting that several different intensities may be rated as painful and non-painful in different trials. This suggests that participants were uncertain about painfulness over a range of intensities and calls into question the idea of a boundary between non-painful and painful events. The current study aimed to determine the number of different stimulus intensities across which each participant provided 'painful' and 'non-painful' reports in different trials. Methods. We undertook novel exploratory analyses on data from the aforementioned two experiments (n = 19, 11 female, 18-31 years old; n = 7, 5 female, 21-30 years old). We used the binomial test to formally determine the width of this 'zone of uncertainty' about painfulness, using ratings on the SPARS and the comparator scales, and data visualisation to assess whether trial-totrial change in stimulus intensity influences ratings. Results. We found that the width of the zone of uncertainty varied notably between individuals and that the zone was non-continuous for most participants. Plots of group-level data concealed the inter-individual variability apparent in the individual plots, but still showed a wide zone of uncertainty on both the SPARS and the NRS, but a narrow zone on the scale for non-painful events. There was no evidence that trial-to-trial change in stimulus intensity influenced ratings. Conclusions. The variability revealed by this study has important design implications for experiments that include initial calibration of repeatedly delivered stimuli. The variability also stands to inflate the size of sample that is required for adequate statistical powering of experiments, and provides rationale for the use of statistical approaches that account for individual variability in studies of pain. Finally, the high variability implies that, if experimental stimuli are to be used in clinical phenotyping, many trials may be required to obtain results that represent a single patient's actual response profile.

The Sensation and Pain Rating Scale: easy to use, clear to interpret, and responsive to clinical change

“…Some studies have reported that scales with more than 7-9 points offer too many response options for users to meaningfully distinguish between isolated stimuli, compromising test-retest reliability (Miller, 1994). Our own previous data showed considerable variance in within-participant ratings of experimental stimuli on both the SPARS and on a 101-point NRS (Kamerman et al, 2018;Madden et al, 2019;Madden et al, 2021). However, the 7-9-option threshold for meaningful differentiation may apply only to unidimensional stimuli: studies of multidimensional experiences suggest a capacity to meaningfully distinguish between more response options (Euasobhon et al, 2022;Preston & Colman, 2000).…”

Variability in experimental pain studies: nuisance or opportunity?

“…That is, does variability have utility as a risk factor for chronicity? We recently speculated that the variability with which an individual rates different, intensity-matched stimulation trials could reflect the flexibility of his/her perceptual processing 15 and could represent a Selected data (from Madden and colleagues 15 ) used to illustrate the consequences of averaging. (a) Rating given by six participants at each stimulus intensity.…”

“…Fig 1.Selected data (from Madden and colleagues15 ) used to illustrate the consequences of averaging. (a) Rating given by six participants at each stimulus intensity.…”

Movement-related pain as a learned response: an investigation of the possible mechanisms underlyng movement-related pain persistence and recovery in subjects with chronic musculoskeletal pain

“…Current evidence suggesting that pain can be conditioned to visual (282,283), visceral (296,297), vibrotactile (250,280,281) and proprioceptive stimuli (298) and that expectancy and threat anticipation play an important role in pain modulation by influencing pain threshold and producing hyperalgesia (156, 235-237, 268-270, 282, 283, 299, 300) strengthen the idea that the pain threshold is a zone of uncertainty that signals the possibility that body may be under threat (136,137,301). Further evidences suggesting that learning to predict harmful situations may be an important aspect to understand how individual's may modulate pain and behavior (25,142) and that pain anticipation through cognitive and associative mechanisms may bias pain perception by facilitating its occurance (135, 138-140, 296, 302, 303), raises the need to understand perception as an contextual representation of the body and the world.…”