Table 2.
Characteristics of studies included in the systematic review.
| Author and year | City, county | # Evaluated participants | # Males | Age at enrolment: Mean (SD) | Trial design | Was there any form of baseline assessment/calibration? | Type of effect aimed to be induced by OL (as specified by authors) | The way the pain-related information is presented to the participant | Sex of the model | Type of a placebo | Type of placebo | Nature of noxious stimuli | Study included in the meta-analysis | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Bajcar et al., 20207 | Kraków, PL | 96 | 36 | 22.00 (2.67) | Parallel groups | No/yes | Placebo | In-person | Direct | F | Color | Nonmedical | Electrocutaneous | Yes |
| Bajcar et al., 20226 | Kraków, PL | 150 | 99 | 23.06 (3.43) | Parallel groups | Yes/yes | Placebo | Pain ratings from others | Indirect | Both | White circle | Nonmedical | Thermal (heat) | Yes* |
| Bieniek and Bąbel, 202214 | Kraków, PL | 60 | 28 | 23.38 (2.55) | Parallel groups | No/yes | Placebo | Videotaped | Indirect | M | Color | Nonmedical | Electrocutaneous | Yes |
| Brączyk and Bąbel, 202117 | Kraków, PL | 60 | 24 | 23.65 (2.54) | Parallel groups | No/yes | Placebo | Videotaped | Indirect | M | Color | Nonmedical | Electrocutaneous | Yes |
| Colloca and Benedetti, 200921 | Turin, IT | 16 | 0 | 21.70 (3.40) | Parallel groups | No/yes | Placebo | In-person | Direct | M | Color. Sham electrode | Medical | Electrocutaneous | Yes |
| Egorova et al., 201527 | Boston MA | 20 | 8 | 23.00 (2.00) | Within-subjects | No/yes | Both | Videotaped | Indirect | Both | Shapes | Nonmedical | Thermal (heat) | Yes† |
| Helsen et al., 201133 | Leuven, BE | 62 | 0 | 19.80 (1.80) | Cross-over | No/NA | Nocebo | Videotaped | Indirect | F | Color | Medical | Thermal (CPT) | Yes |
| Helsen et al., 201334 | Leuven, BE | 60 | 0 | 19.88 (2.68) | Within-subjects | No/NA | Nocebo | Videotaped | Indirect | F | Colored water | Nonmedical | Thermal (WWT) | Yes |
| Helsen et al., 2015; exp 135 | Leuven, BE | 49 | 0 | 20.47 (3.76) | Within-subjects | No/NA | Nocebo | Videotaped | Indirect | F | Color | Nonmedical | Thermal (cold bars) | Yes |
| Helsen et al., 2015; exp 235 | Leuven, BE | 43 | 0 | 20.16 (1.65) | Within-subjects | No/NA | Nocebo | Videotaped | Indirect | F | Color | Nonmedical | Thermal (cold bars) | Yes |
| Hunter et al., 201438 | London, UK | 45 | 0 | 28.37 (7.46) | Parallel groups | No/yes | Placebo | Both live and videotaped | Both | F | Color. Sham electrode | Medical | Electrocutaneous | Yes |
| Raghuraman et al., 201956 | Baltimore MD | 31 | 12 | 23.40 (4.00) | Within-subjects | No/yes | Placebo | Pictures | Indirect | M | Color. Colored cream | Medical | Thermal (heat) | Yes |
| Schenk and Colloca, 202060 | Baltimore MD | 38 | 15 | 28.10 (8.50) | Within-subjects | No/yes | Placebo | Videotaped | Indirect | M | Color. Colored cream | Medical | Thermal (heat) | Yes |
| Świder and Bąbel, 201365 | Kraków, PL | 84 | 42 | 22.70 (2.69) | Parallel groups | No/yes | Placebo | In-person | Direct | Both | Color | Nonmedical | Electrocutaneous | Yes |
| Świder and Bąbel, 201666 | Kraków, PL | 65 | 0 | 22.23 (2.64) | Parallel groups | No/yes | Placebo | In-person | Direct | F | Color. Geometric shape | Nonmedical | Electrocutaneous | Yes |
| Vögtle et al., 201369 | Göttingen, DE | 53 | 0 | 22.50 (4.40) | Parallel groups | No/NA | Nocebo | Videotaped | Indirect | F | Cream | Medical | Pressure | Yes |
| Vögtle et al., 201670 | Göttingen, DE | 97 | 0 | 43.05 (15.51) | Parallel groups | No/NA | Nocebo | Videotaped | Indirect | F | White cream | Medical | Pressure | Yes |
| Vögtle et al., 201971 | Göttingen, DE | 80 | 0 | 22.40 (4.80) | Parallel groups | No/NA | Nocebo | Videotaped | Indirect | F | White cream | Medical | Pressure | Yes |
| Lorber et al., 200746 | Mansfield CT | 86 | 35 | 18.99 (1.02) | Parallel groups | Yes/NA | Nocebo | In-person | Direct | F | Sham toxin | Medical | n/a | No‡ |
| Mazzoni et al., 201047 | Hull, UK | 119 | 60 | 20.67 (4.63) | Parallel groups | No/NA | Nocebo | In-person | Direct | Both | Sham toxin | Medical | n/a | No‡ |
| Tu et al., 201967 | Boston MA | 21 | 9 | 25.00 (3.90) | Within-subjects | No/yes | Both | Videotaped | Indirect | Both | Neutral male faces | Nonmedical | Thermal (heat) | No§ |
| Zhang et al., 201773 | Chongqing, China | 40 | 0 | 20.57 (1.46) | Parallel groups | No/yes | Both | Videotaped | Indirect | M | Red/green dot. Metal ring | Nonmedical | Electrocutaneous | No‖ |
*
Pain ratings from others were presented in 2 ways across groups: without any observation of the model (ratings only) and with indirect observation of the model (ie, on a picture). Only the subgroups in which pain ratings were combined with a picture were included in the meta-analysis in the subgroup “indirect observation of the model.”
†
Both placebo and nocebo were induced; study excluded from certain subgroup analyses.
‡
No pain stimulation comparable to other studies; headache assessed as a result of placebo manipulation.
§
Observation was reinforced by a conditioning procedure.
‖
Participants also underwent a conditioning procedure that was intermixed with OL so the pure OL effect could not be isolated.
CPT, cold pressor task; F, females; M, males; NA, not applicable; OL, observational learning; WWT, warm water task.