Background & Aims

Expectations influence the perception of pain, as shown by extensive research on placebo and nocebo effects. These effects can shape perceived pain intensity, the outcomes of medical procedures, as well as the emergence of side effects of various medical treatments. Central sensitization, a key factor in persistent pain, can be temporarily induced in healthy volunteers through high-frequency electrical stimulation (HFS). Here, high frequency electrical stimulation is delivered to the skin of the volar forearm, which results in the increased sensitivity of the skin sorrounding the site of stimulation (secondary hypersensitivity), which is a psychophysical correlate of central sensitisation.However, the literature on the impact of expectations on secondary hypersensitivity is still sparse. This study aimed to replicate and extend previous findings on the role of negative expectations in the development of secondary hypersensitivity and associated physiological responses.

Methods

Sixty healthy female volunteers participated in a single experimental session. Participants were randomly assigned to a nocebo group or a control group. Secondary mechanical hypersensitivity was induced in both groups through the administration of HFS to the right volar forearm. Both groups received written information about HFS, but the nocebo group were specifically informed that HFS would increase the sensitivity of their skin. Baseline (T0) and 20 mins post-HFS (T1) skin sensitivity to mechanical pinprick stimuli were measured using a 128mN pinprick stimulator. The pinprick stimulation was delivered using a custom built robot, and both, subjective ratings of pinprick intensity as well as pupil dilation responses to the pinprick stimulation were measured. Manipulation checks were conducted before HFS administration where participants rated whether they expected their skin at T1 to be more sensitive, same sensitivity or less sensitive.

Results

Manipulation checks revealed that more participants in the nocebo group expected their skin sensitivity to be increased at T1, however there was no statistically significant differencein the expectation ratings between the groups. Pinprick intensity ratings significantly increased post-HFS (T1) compared to baseline (T0), however, there were no significant group differences in the extent of increase in skin sensitivity to mechanical stimulation. The proximal-distal spread of hypersensitivity and resting pupil diameter also showed no significant differences between groups. Cluster-based permutation tests on pupil dilation responses indicated a significant effect of HFS with a minimal data processing pipeline as used in previous studies.

Conclusions

The manipulation of negative expectations did not significantly influence pain perception or physiological responses. This lack of difference may have been due to the fact that we only used written instructions to manipulate expectations. Additionally, given that participants were aware that they are participating in an experiment with electrical stimulaiton, all participants may have been predisposed to having negative expectations about the effect of the HFS stimulation, thus biasing the expectations of the entire sample. We were unable to replicate previous findings from a similar paradigm, however, this may have been due to the fact that the previous study was a clinical study conducted in a hospital context while the current study was conducted in a lab context, particularly, among a sample that predominantly consisted of psychology students.

References

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Presenting Author

Ketan Jaltare

Poster Authors

Ketan Jaltare

MSc

City University of London

Lead Author

E-mail

Topics

  • Mechanisms: Psychosocial and Biopsychosocial