Habituation of the Nociceptive Flexion Reflex is Dependent on Inter-stimulus Interval and Stimulus Intensity

The nociceptive flexion reflex (NFR) has become a popular tool in experimental and clinical pain research. However, the gradual decrease of the reflex size during repeated application of stimuli, which is termed habituation, may reduce its validity and the comparability of studies. We investigated the degree of habituation at different inter-stimulus intervals (ISI) commonly used in clinical studies and the dependency of habituation on stimulus intensity. Thirty subjects each received 12 sets of 50 stimuli to elicit the NFR at ISI of 1, 3, 6 and 10s, each at stimulus intensities of 1 ×, 1.5 × and 2 × the individual NFR-threshold. For each of the sets, the habituation was calculated as the relative size of the average of the last 10 reflexes compared to the average of the first 10 reflexes. The influence of the factors ISI and stimulus intensity, as well as the factor interaction, was tested using a two-way analysis of variance (ANOVA). Analysis proved that influences of ISI and stimulus intensity on reflex habituation were significant (p < 0.01, two-way ANOVA), while the interaction between the two factors had no significant influence (p = 0.99). We conclude that habituation of the NFR is dependent on ISI and stimulus intensity. Lower stimulus intensities and shorter ISI lead to stronger habituation. Therefore, to ensure habituation is avoided during repeated elicitation of the NFR, stimulation should be conducted according to the ISI for the respective stimulus intensities presented here.