Expectations of increased and decreased pain explain the effect of conditioned pain modulation in females

Figures & data

Figure 1 The experimental design.

Notes: All subjects were presented with seven test stimuli (TS), ie, noxious heat stimuli (+46°C) to the nondominant forearm for 150 seconds each. The first TS was administered to familiarize the subjects with the noxious stimulus and the numerical rating scales, and was not included in the analyses. During three of the stimulations the dominant hand was submerged in cold water (+8°C), ie, the conditioning stimulus (CS). For half the subjects the conditioning stimulus was applied during the last three stimuli. For the other half of the subjects the conditioning stimulus was applied during the first three stimuli. Three types of information were given about the effect of the conditioning stimulus on the heat pain: information that the conditioning stimulus would reduce heat pain (analgesia group), information that the conditioning stimulus would increase heat pain (hyperalgesia group), and no information (no info group). Half the participants in each group were females. Measures of pain, stress, and arousal were taken during each test stimulus between 120 and 150 seconds. Heart rate was measured for the entire 150 seconds of each test stimulus. The subjects reported their expected reduction/increase in pain just prior to the application of the conditioning stimulus.

Table 1 Descriptive data for the main outcome measures for males and females in the experimental groups

Outcome	Analgesia group			Hyperalgesia group			No info group
	TS	TS + CS	Mean % change	TS	TS + CS	Mean % change	TS	TS + CS	Mean % change
Males
Pain intensity	4.97 ± 1.94	4.63 ± 2.35	−10.60 ± 6.02	4.89 ± 1.80	4.06 ± 1.60	−12.76 ± 7.68	4.94 ± 1.65	5.00 ± 1.56	8.10 ± 13.43
Pain unpleasantness	4.60 ± 1.70	4.70 ± 2.08	1.8 ± 8.47	5.08 ± 1.82	4.28 ± 1.89	−11.38 ± 12.06	4.79 ± 2.00	5.00 ± 1.73	9.94 ± 11.30
Subjective stress	3.08 ± 1.95	4.05 ± 2.05	35.53 ± 9.82	3.75 ± 1.54	3.97 ± 1.42	10.72 ± 8.52	3.98 ± 1.30	4.76 ± 1.36	23.09 ± 7.40
Heart rate	72.94 ± 8.71	72.69 ± 8.45	−0.23 ± 1.11	65.09 ± 8.05	69.06 ± 10.34	7.29 ± 2.87	61.87 ± 8.76	68.96 ± 15.67	4.76 ± 3.15
Females
Pain intensity	5.94 ± 2.50	4.94 ± 2.25	−15.75 ± 5.80	4.58 ± 1.36	5.42 ± 2.04	21.33 ± 13.91	4.64 ± 2.09	4.56 ± 1.77	11.09 ± 16.00
Pain unpleasantness	6.36 ± 2.17	5.83 ± 1.73	−3.65 ± 6.95	4.97 ± 1.83	6.08 ± 1.81	35.08 ± 19.27	5.18 ± 2.09	5.28 ± 2.19	14.54 ± 16.07
Subjective stress	4.43 ± 1.90	4.90 ± 1.69	40.94 ± 28.90	2.89 ± 1.37	4.68 ± 2.14	71.71 ± 21.60	2.87 ± 1.88	3.96 ± 1.82	64.59 ± 18.17
Heart rate	68.47 ± 8.22	70.48 ± 7.51	3.25 ± 1.88	74.35 ± 10.86	76.78 ± 9.01	3.75 ± 1.39	69.43 ± 10.22	70.43 ± 9.84	1.66 ± 1.45

Note: Mean percentage of change was computed by calculating the percentage of change in pain due to conditioning stimulation for each subject and computing the mean of all subjects.

Abbreviations: TS, test stimulus; CS, conditioning stimulus.

Figure 2 Change in pain intensity from test stimulus alone to conditioning stimulation. Negative scores indicate a reduction in pain during conditioning stimulation. Error bars depict standard error of the mean. (A) Change in pain intensity across the experimental groups including both males and females. (B) Change in pain intensity across the experimental groups in males. (C) Change in pain intensity across the experimental groups in females.

Figure 3 Change in pain intensity from test stimulus alone to conditioning stimulation as a function of order of presentation of conditioning stimulation.

Notes: Error bars depict standard error of the mean. Conditioning stimulation reduced pain when presented after test stimulus alone, but not when presented first.
Abbreviation: cs, conditioning stimulation

Figure 4 Change in stress from test stimulus alone to conditioning stimulation. Positive scores indicate an increase in stress. Error bars depict standard error of the mean. (A) Change in stress across the experimental groups including both males and females. (B) Change in stress across the experimental groups in males. (C) Change in stress across the experimental groups in females.

Figure 5 (A) Relationship of expectation to the effect of conditioning stimulation on pain intensity. Only subjects in the analgesia and hyperalgesia groups were asked about expectations (n = 46). (B) Relationship of change in stress to the effect of conditioning stimulation on pain intensity (n = 70).

Notes: Negative scores on the y-axis indicate a reduction in pain during conditioning stimulation compared to during test stimulus alone. Negative scores on the x-axis indicate expectations of reduced pain or reduction in stress during conditioning stimulation compared to during test stimulus alone.