A test of psychological and electrodermal changes immediately after the delivery of 3 analgesic treatment messages

The Two Sides of Placebo Analgesia: Differential Functional Connectivity Reveals Mechanisms of Placebo Analgesic Response

Background

Previous research has demonstrated that placebo induction manipulations can reduce an individual's pain through non-specific mechanisms, such as expectancy manipulations. However, despite robust research characterizing these effects, individual differences in predicting placebo analgesic responses are not well understood.

Methods

Fifty-four healthy pain-free adults over 18 (M=22.8, SD=7.82) were recruited (66.7% women). Participants completed a baseline followed by a placebo session involving the application of an inactive cream in the context of an expectancy-enhancing instruction set while undergoing a functional magnetic resonance imaging scan (fMRI). Painful heat stimuli were applied to the thenar eminence of the right palm. Stimulus intensity was individually calibrated to produce pain ratings of approximately 40 on a 100-point visual analog scale. Generalized psychophysiological interaction (gPPI) was used to assess the group differences in functional connectivity during painful stimulation compared to warmth stimulation.

Results

About 68.5% showed a reduction in pain in the placebo condition with an average decrease of 30.3%. Non-responders showed an increase in pain in the placebo condition, with an average increase of 18.6%. Repeated measures ANOVA demonstrated a significant within-subjects interaction between expectancy and responder type (F(1,49)=4.27, p=0.04, ηp2=0.08). Expected pain was significantly associated with pain in the placebo session for the responders (b=0.37, R2=0.29, p<0.001), but not for the non-responders (b=0.11, R2=0.04, p=0.42). gPPI analysis revealed three clusters exhibiting greater increases in FC in areas related to attention and sensory integration in placebo responders compared to non-responders. One cluster was identified where greater increases in functional connectivity were associated with non-responders compared to responders in regions associated with attention and motor processing.

Conclusion

Our results provide evidence that responders and non-responders have differential behavioral and functional responses to acute pain during a placebo analgesic task.

Task-dependent functional connectivity of pain is associated with the magnitude of placebo analgesia in pain-free individuals

BACKGROUND

Task-based functional connectivity (FC) of pain-related regions resulting from expectancy-based placebo induction has yet to be examined, limiting our understanding of regions and networks associated with placebo analgesia.

METHODS

Fifty-five healthy pain-free adults over 18 (M = 22.8 years, SD = 7.75) were recruited (65.5% women; 63.6% non-Hispanic/Latino/a/x; 58.2% White). Participants completed a baseline followed by a placebo session involving the topical application of an inactive cream in the context of an expectancy-enhancing instruction set. Noxious heat stimuli were applied to the thenar eminence of the right palm using an fMRI-safe thermode. Stimulus intensity was individually calibrated to produce pain ratings of approximately 40 on a 100-point visual analogue scale.

RESULTS

A total of 67.3% of the participants showed a reduction in pain intensity in the placebo condition with an average reduction in pain across the whole sample of 12.7%. Expected pain intensity was associated with reported pain intensity in the placebo session (b = 0.32, p = 0.004, R2  = 0.15). Voxel-wise analyses indicated seven clusters with significant activation during noxious heat stimulation at baseline (pFDR  < 0.05). Generalized psychophysiological interaction analysis suggested that placebo-related FC changes between middle frontal gyrus-superior parietal lobule during noxious stimulation were significantly associated with the magnitude of pain reduction (pFDR  < 0.05).

CONCLUSIONS

Results suggest that stronger expectancy-based placebo responses might be underpinned by greater FC among attentional and somatosensory regions.

SIGNIFICANCE

This article provides support and insight for task-dependent functional connectivity differences related to the magnitude of placebo analgesia. Our findings provide key support that the magnitude of expectation-based placebo response depends on the coupling of regions associated with somatosensory and attentional processing.

Psychological mechanisms of offset analgesia: The effect of expectancy manipulation

A frequently used paradigm to quantify endogenous pain modulation is offset analgesia, which is defined as a disproportionate large reduction in pain following a small decrease in a heat stimulus. The aim of this study was to determine whether suggestion influences the magnitude of offset analgesia in healthy participants. A total of 97 participants were randomized into three groups (hypoalgesic group, hyperalgesic group, control group). All participants received four heat stimuli (two constant trials and two offset trials) to the ventral, non-dominant forearm while they were asked to rate their perceived pain using a computerized visual analogue scale. In addition, electrodermal activity was measured during each heat stimulus. Participants in both intervention groups were given a visual and verbal suggestion about the expected pain response in an hypoalgesic and hyperalgesic manner. The control group received no suggestion. In all groups, significant offset analgesia was provoked, indicated by reduced pain ratings (p < 0.001) and enhanced electrodermal activity level (p < 0.01). A significant group difference in the magnitude of offset analgesia was found between the three groups (F[2,94] = 4.81, p < 0.05). Participants in the hyperalgesic group perceived significantly more pain than the hypoalgesic group (p = 0.031) and the control group (p < 0.05). However, the electrodermal activity data did not replicate this trend (p > 0.05). The results of this study indicate that suggestion can be effective to reduce but not increase endogenous pain modulation quantified by offset analgesia in healthy participants.