Table 1.
Pain-Classification Performance, According to Study.*
| Study | Discrimination between Pain and No Pain† | Effect Size‡ | P Value | Performance on Forced-Choice Test§ | ||||
|---|---|---|---|---|---|---|---|---|
| Signature-Response Threshold | Sensitivity | Specificity percent (95% CI) |
Positive Predictive Value | AUC | Discriminability | (percent 95% CI) | ||
|
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| Study 1 | ||||||||
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| Painful vs. warm¶ | 1.40 | 95 (86–100) | 95 (86–100) | 95 (85–100) | 0.95 | 2.69 | < 0.001 | 100 (100–100) |
|
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| Pain vs. pain anticipation | 0.36 | 100 (100–100) | 99 (96–100) | 95 (86–100) | 0.99 | 3.69 | < 0.001 | 100 (100–100) |
|
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| Pain vs. pain recall | 0.54 | 95 (85–100) | 94 (89–98) | 79 (64–92) | 0.96 | 2.35 | < 0.001 | 100 (100–100) |
|
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| Study 2 | ||||||||
|
| ||||||||
| Painful vs. warm ||** | 1.32 | 93 (84–100) | 93 (84–100) | 93 (84–100) | 0.92 | 1.54 | < 0.001 | 100 (100–100) |
|
| ||||||||
| Painful vs. near pain threshold†† | 2.50 | 88 (77–97) | 85 (72–95) | 85 (73–96) | 0.88 | 1.74 | < 0.001 | 100 (100–100) |
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| High vs. low warmth | 1.00 | 56 (36–75) | 100 (100–100) | 100 (100–100) | 0.79 | 1.31 | 0.001 | 100 (100–100) |
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| Study 3 | ||||||||
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| Painful vs. warm | 1.40‡‡ | 85 (76–94) | 78 (67–89) | 80 (68–89) | 0.86 | 1.64 | < 0.001 | 93 (86–98) |
|
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| Painful vs. rejecter | 1.40‡‡ | 85 (76–94) | 73 (61–84) | 76 (65–86) | 0.88 | 1.83 | < 0.001 | 95 (89–100) |
|
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| Rejecter vs. friend | 1.40‡‡ | 27 (16–38) | 88 (79–95) | 69 (50–88) | 0.57 | 0.31 | 0.22 | 56 (43–69) |
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| Study 4 | ||||||||
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| Painful vs. warm, before drug treatment | 1.40‡‡ | 90 (79–100) | 81 (65–95) | 83 (67–95) | 0.89 | 1.61 | < 0.001 | 90 (79–100) |
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| Painful vs. warm, during drug treatment | 1.61 | 86 (73–96) | 62 (42–80) | 69 (52–84) | 0.74 | 1.01 | 0.003 | 76 (61–90) |
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| Painful before vs. during drug treatment | 1.61 | 86 (72–96) | 62 (43–79) | 69 (54–83) | 0.74 | 1.01 | 0.003 | 76 (60–92) |
Study 1 included 12 trials each in painful and warm conditions. Study 2 included a mean (±SD) of 24±13 trials for pain and 36±9 trials for warmth, depending on the ratings. Study 3 included 8 trials each in painful and warm conditions. Study 4 included 3 trials for pain and 3 for warmth in the before-drug-treatment condition and in the condition with peak drug concentration. CI denotes confidence interval.
The tradeoff between sensitivity and specificity at different thresholds was assessed by means of receiver-operating-characteristic (ROC) plots; the signature-response threshold that minimized overall classification errors is reported here.
For the area under the ROC curve, chance is 0.5. Discriminability is a measure of effect size under a gaussian model. Performance varied across studies, according to the number of trials averaged to form the condition maps.
For the two-choice (forced-choice) discrimination test, the classification threshold for the difference between paired observations is 0. The sensitivity, specificity, and positive predictive value are the same and are equal to the decision accuracy.
Painful conditions were defined as temperatures greater than 44.5°C and as ratings of more than an average of 5.80 points on a visual-analogue scale (VAS), and warm conditions as temperatures of less than 44.5°C and ratings of less than 3.34 points on the VAS.
Study 2 was conducted with the use of a scanner with a different field strength (3 T), so the threshold was reestimated.
Participants made judgments of painful versus nonpainful conditions for each trial.
Participants rated pain or warmth intensity on a continuous VAS, with scores ranging from 0 to 99 points for warmth and from 100 to 200 points for pain. Pain was defined as a score of more than 125 points, near the pain threshold as a score of 75 to 125 points, high warmth as a score of 50 to 100 points, and low warmth as a score of O to 50 points.
The threshold derived from study 1 was applied.