Abstract
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 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. The SPARS has a curvilinear stimulus-response relationship, reflecting that several different intensities may be rated as painful and non-painful in different trials. Here, we used the binomial test to 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-to-trial change in stimulus intensity influences ratings. We found that the width of the zone of uncertainty varied between individuals and was wide at the group level on both the SPARS and the NRS, but narrow on the scale for non-painful events. These findings have important implications for experiment design, sample sizes and clinical phenotyping procedures.
Footnotes
Funding and disclosures: This project was supported by a project grant from the Australian National Health & Medical Research Council (ID 1047317). VJM was supported by the Oppenheimer Memorial Trust and is now supported by the National Research Foundation of South Africa. VJM receives speaker’s fees for lectures on pain and rehabilitation. PK is on retainer for Partners in Research, and receives speaker’s fees for lectures and professional development courses on pain. GLM has received support from Pfizer, Kaiser Permanente, Workers’ Compensation Boards in Australia, Europe and North America, the International Olympic Committee, the Port Adelaide Football Club and the Arsenal Football Club. He receives royalties for books on pain and speaker’s fees for talks and professional development courses on pain and rehabilitation. The authors declare no other competing interests related to this work.