Participant characteristics
The patients with ASD and healthy controls were matched for age (ASD 25.8 ± 9.2-year old, control 26.3 ± 7.5-year old, U = 103.0, z = −0.395, p = 0.7) and sex (male/female = 2/3). Additional characteristics of the individuals with ASD were as follows: education: 13.6 ± 2.6 years, full-scale IQ 101.8 ± 14.3, and total scores on the Japanese version of the Asperger’s Questionnaire (AQ-J; n = 11): 31.7 ± 7.4. These data were not available for the controls.
Sensory thresholds
We measured the minimum detection threshold, pain detection threshold, and pain tolerance by having each participant push a button at each sensory point (Fig. 1). The minimum detection thresholds for electrical and cold stimulation occurred significantly earlier in the ASD group than in the control group (electrical: ASD mean ± standard error (SE) = 8.30 ± 1.71 μA, control = 10.87 ± 3.50 μA, U = 57.50, z = −2.28, p = 0.023; cold: ASD = 29.64 ± 0.93 °C, control = 28.66 ± 1.48 °C, U = 54.00, z = −2.43, p = 0.015; Fig. 1a, c). These results suggest that the ASD group was hypersensitive to electrical and cold stimulation. Alternatively, they may have responded to these stimulations by pushing the button earlier than the controls. The minimum detection thresholds for heat stimulation did not differ between the groups (ASD mean ± SE = 34.71 ± 0.82 °C, control = 34.99 ± 0.96 °C, U = 93.00, z = −0.81, p = 0.42; Fig. 1b). Therefore, the ASD group most likely pushed the button appropriately during the experiment. No significant differences were detected between groups with regard to the pain detection thresholds for each stimulation (electrical: ASD mean ± SE = 20.58 ± 10.72 μA, control = 26.44 ± 16.48 μA, U = 94.00, z = −0.77, p = 0.44; heat: ASD = 40.54 ± 2.68 °C, control = 40.56 ± 3.07 °C, U = 109.00, z = −0.15, p = 0.89; cold: ASD = 19.59 ± 6.92 °C, control = 17.60 ± 8.64 °C, U = 98.00, z = −0.60, p = 0.55; Fig. 1). In addition, no significant between-group differences were observed with regard to pain tolerance for each stimulation (electrical: ASD mean ± SE = 104.48 ± 52.23 μA, control = 92.86 ± 55.99 μA, U = 95.00, z = −0.73, p = 0.47; heat: ASD = 46.12 ± 2.23 °C, control = 46.35 ± 3.16 °C, U = 102.00, z = −0.44, p = 0.66; cold: ASD = 2.50 ± 7.22 °C, control = 3.74 ± 11.24 °C, U = 102.00, z = −0.44, p = 0.66; Fig. 1). These results suggest that the sensory thresholds for pain in the ASD group were not impaired.
Subjective pain sensitivity measured using the VAS
The VAS pain scores for the ASD group were significantly lower than those for the control group for every stimulation (electrical: ASD mean ± SE = 3.55 ± 2.43, control = 5.47 ± 2.44, U = 64.00, z = −2.01, p = 0.044, Fig. 1a; heat: ASD = 5.51 ± 2.37, control = 7.15 ± 2.06, U = 63.50, z = −2.03, p = 0.042, Fig. 1b; cold: ASD = 3.44 ± 2.59, control = 5.77 ± 2.06, U = 51.00, z = −2.55, p = 0.011, Fig. 2c). For the electrical stimulation, the VAS discomfort score of the ASD group was lower than that of the control group (ASD mean ± SE = 2.50 ± 1.82, control = 5.50 ± 2.68, U = 44.50, z = −2.82, p = 0.0048; Fig. 2a). Although the mean discomfort scores for heat and cold stimulation within the ASD group were lower than those within the control group, significant differences were not found between the groups (heat: ASD mean ± SE = 4.69 ± 2.51, control = 6.15 ± 2.18, U = 75.50, z = −1.54, p = 0.13, Fig. 2b; cold: ASD mean ± SE = 3.49 ± 2.90, control = 4.50 ± 1.97, U = 80.50, z = −1.33, p = 0.18, Fig. 2c). These results suggest that the stimulations were less painful for the ASD group than for the controls (Fig. 2a–c). They also felt greater discomfort than the controls with regard to painful electrical stimulations. However, they felt the same amount of discomfort as the controls with regard to painful cold and heat stimulation. Therefore, subjective pain processing pathways in individuals with ASD may differ from those in controls.
Subjective pain sensitivity measured using the SF-MPQ
The affective pain sensitivity scores associated with electrical and cold stimulation in the ASD group were lower than those in the control group (electrical: ASD mean ± SE = 1.07 ± 1.22, control = 2.73 ± 2.12, U = 49.50, z = −2.69, p = 0.0071, Fig. 3a; cold: ASD = 1.20 ± 1.74, control = 2.20 ± 1.66, U = 65.00, z = −2.03, p = 0.042, Fig. 3c). The mean score for affective pain sensitivity to heat stimulation was lower in the ASD group than in the control group; however, no significant differences were observed between the groups (ASD mean ± SE = 1.60 ± 1.72, control = 2.53 ± 1.60, U = 71.00, z = −1.76, p = 0.079; Fig. 3b). In contrast, the subjective pain sensitivity scores for the ASD group were not significantly different from those in the control group for any stimulation (electrical: ASD mean ± SE = 8.40 ± 3.22, control = 8.47 ± 3.87, U = 107.00, z = −0.23, p = 0.82, Fig. 3a; heat: ASD = 8.73 ± 4.83, control = 10.60 ± 5.79, U = 93.50, z = −0.79, p = 0.43, Fig. 3b; cold: ASD = 8.53 ± 4.49, control = 8.80 ± 4.72, U = 112.00, z = −0.021, p = 0.98, Fig. 3c). The mean total pain scores for the ASD group were lower than those for the control group (Fig. 3a–b). However, no significant difference in total pain was observed between the groups for any type of stimulation (electrical: ASD mean ± SE = 9.47 ± 3.56, controls = 11.20 ± 5.21, U = 86.50, z = −1.09, p = 0.28, Fig. 3a; heat: ASD = 10.33 ± 6.16, control = 3.13 ± 6.73, U = 84.50, z = −1.17, p = 0.24, Fig. 3b; cold: ASD = 9.73 ± 5.98, control = 11.00 ± 5.94, U = 95.00, z = −0.73, p = 0.47, Fig. 3c). These results suggest that the participants with ASD were impaired in their emotional evaluation of painful electrical and cold stimulations.
Correlation between AQ scores and pain sensitivities
No significant differences were observed between the total AQ scores and any of the variables among individuals with ASD (data not shown).