Defining the nociceptive flexion reflex (NFR) threshold in human participants: a comparison of different scoring criteria

Abstract

Despite the widespread use of the nociceptive flexion reflex (NFR) paradigm in clinical and experimental pain research, there is currently no consensus on how best to define NFR threshold. Accordingly, the present studies were designed to assess the accuracy and reliability of different NFR threshold scoring criteria. Study 1 compared 13 scoring criteria in their accuracy for identifying the presence of the NFR, then generated empirically derived cut-points for the best criteria, and examined the test-retest reliability of NFR thresholds derived from these cut-points. Study 2 evaluated the replicability of these findings in an independent sample. Results from the two studies suggested that standardized peak (NFR Interval Peak z score) and mean (NFR Interval z score) biceps femoris electromyogram (EMG) activity were accurate and reliable criteria for defining NFR threshold. Acknowledging that cut-points may need to be adjusted for different research designs, graphs depicting sensitivity and specificity across a range of cut-points have been provided to facilitate researcher's decision-making. It is hoped that the results of these studies will promote a standard NFR threshold assessment methodology, and further encourage the application of the NFR paradigm in the investigation of mechanisms and characteristics of both painful and non-painful diseases.

Figure 1

Example trials in which raters agreed an NFR was present or absent. Panels on the left-hand side are biceps femoris electromyogram (EMG) from single trials of 3 different subjects in which both raters agreed a NFR was present. Panels on the right are trials from the same 3 subjects that were one stimulus intensity lower and raters agreed a NFR was absent. The bottom panels illustrate stimulation onset/offset.

Figure 2

Plots of sensitivity and specificity values (y-axis) by threshold values (x-axis) for the NFR Interval Peak z score (top panel) and NFR Interval z score (bottom panel) criteria. These graphs are based on data from Study 1 and can be used to adjust the threshold values for each criterion to alter accuracy rates to improve sensitivity or specificity.