There was a significant main effect of odorant type on the intensity ratings (F2,90 = 35.74, p < 0.001). The main effect of group was significant, with healthy controls rating odours as more intense than did the schizophrenia patients (F1,45 = 4.38, p = 0.042). Pair-wise comparisons revealed that ambient air was judged to be significantly less intense than hydrogen sulfide (t46 = 7.55, p < 0.001) and vanillin (t46 = 3.32, p = 0.004). Vanillin was rated as less intense than hydrogen sulfide (t46 = 6.11, p < 0.001). None of the between-group t tests for individual odours yielded significant results.
Pleasantness scores revealed a significant main effect of odorant type (F2,90 = 79.38, p < 0.001). Pair-wise comparisons showed that vanillin was rated as more pleasant (t46 = −2.38, p = 0.023) and hydrogen sulfide was rated as less pleasant than ambient air (t46 = 9.20, p < 0.001). Vanillin was rated significantly more pleasant than hydrogen sulfide (t46 = 13.82, p < 0.001) ().
Fig. 2 Mean intensity (A) and pleasantness (B) ratings for vanillin, ambient air and hydrogen sulfide (H2S). Ratings were converted from the Self-Assessment Manikin32 rating scales filled out by the participants after completion of the task. Error bars indicate (more ...)
There was no significant effect of group on the pleasantness ratings, either as a main effect of the ANOVA or in a post hoc t test.
The Sniffin’ Sticks score did not covary significantly with performance, so this factor was excluded from the model. Logistic regressions revealed a trend for a 3-fold interaction of odour, face and group (χ4 = 9.28, p = 0.054). There was also a significant 2-way interaction between face and odour (χ4 = 16.61, p = 0.002) and a main effect of face (χ2 = 17.64, p < 0.001). Between-group contrasts showed that for disgust faces, there was no performance difference between controls and schizophrenia patients under ambient air stimulation, but a difference emerged when disgust faces were presented with a pleasant (χ2 = 16.57, p < 0.001) or unpleasant (χ2 = 22.55, p < 0.001) odour ().
Fig. 3 Accuracy of emotion recognition among healthy participants and schizophrenia patients. We excluded nonresponses. The x-axis indicates the condition separately by odour and emotional face presented. Error bars indicate +/− 1 standard error. The (more ...)
Exploratory post-hoc t tests showed that healthy controls were significantly more accurate in recognizing disgust expressions during the unpleasant (t23 = 2.25, p = 0.034) and pleasant (t23 = 5.41, p < 0.001) conditions than when no odour was presented. The pleasant and the unpleasant odour conditions did not differ significantly from each other. The recognition of happy and neutral faces was not significantly influenced by the presentation of an odorant stimulus. In the patient group, recognition of disgust faces during ambient air trials did not differ from either the pleasant or unpleasant odour condition. There was, however, a significant difference between the 2 odour stimulation conditions for disgust expressions (t23 = 2.51, p = 0.02), with patients performing worse at recognizing disgust faces during unpleasant olfactory stimulation than during than pleasant stimulation. No significant effect of odour was found for happy or neutral faces.
We did not find an effect of Sniffin’ Sticks score; we thus excluded this covariate from the model. Reaction times of the correct responses revealed a significant 3-fold interaction between face, odour and group (F4,8151 = 2.87, p = 0.022), as well as interactions between face and odour (F4,8151 = 5.99, p < 0.001) and between face and group (F2,8151 = 18.03, p < 0.001). We also found main effects of group (F1,8151 = 178.36, p < 0.001) and face (F2,8151 = 14.38, p < 0.001). Between-group contrasts revealed that patients were significantly slower than controls in all conditions (all below p = 0.005).
The separate 2 × 3 model for neutral faces revealed a significant interaction between odour and group (F2,2760 = 5.34, p = 0.005). There was also a significant main effect of group (F1,2760 = 39.76, p < 0.001).
Post-hoc t tests for mean accuracy ratings showed that in the healthy control group, responses to disgusted faces were decreased relative to ambient air stimulation when a pleasant (t23 = 2.07, p = 0.038) or an unpleasant odour (t23 = 2.12, p = 0.034) was presented. There was no significant difference between the odours. At the same time, response times to happy faces were increased relative to the ambient air condition when participants were exposed to a pleasant (t23 = 2.79, p = 0.005) or an unpleasant odour (t23 = 2.18, p = 0.029). Again, there was no difference between the 2 odours.
In the patient group, we observed faster response times for pleasant odours relative to neutral odours for the disgust faces (t23 = 2.82, p = 0.005). The reverse was true for neutral faces (t23 = 3.02, p = 0.003), where pleasant odour led to slower response times relative to the ambient air condition ().
Fig. 4 Log-transformed reaction times to the facial emotion recognition task (correct trials only). The left graph shows results for controls, the right graph shows results for patients. The x-axis indicates the condition separately by odour and emotional face (more ...)
Influence of psychopathology and neurocognitive performance
There were no significant correlations of the PANSS scores with accuracy (positive: r = 0.042, p = 0.85; negative: r = −0.045, p = 0.84; general psychopathology, r = −0.048, p = 0.82; total score: r = 0.005, p = 0.98) or reaction times (positive: r = −0.19, p = 0.36; negative: r = −0.18, p = 0.41; general psychopathology: r = −0.013, p = 0.95; total score: r = −0.037, p = 0.86). There was no significant correlation between chlorpromazine equivalents and accuracy (r = −0.22, p = 0.37) or reaction time (r = −0.29, p = 0.23). Reaction time and accuracy were not significantly correlated with cigarette consumption (r = 0.03, p = 0.84 and r = −0.21, p = 0.16, respectively). We did not find any significant positive or negative correlation between MWT-B scores and reaction time or accuracy (r = 0.29, p = 0.06 and r = 0.21, p = 0.17). There was also no significant negative correlation of digit span score with reaction time (r = −0.25, p = 0.10) and a positive correlation with accuracy (r = 0.39, p = 0.01). The amount of time required to solve the TMT was positively correlated with reaction time, separately for TMT-A (r = 0.56, p < 0.001) and TMT-B (r = 0.55, p < 0.001). For accuracy, there was a significant negative correlation with TMT-A (r = −0.38, p = 0.008) but not with TMT-B (r = −0.21, p = 0.15). To rule out the possibility that cognitive deficits accounted for the observed findings, we reanalyzed the data with the TMT and digit span scores as covariates. For reaction times, the adjusted model with all 3 correlated test scores included showed significant effects for TMT-A, TMT-B and digit span. All main effects and interactions remained significant; however, only the 3-fold interaction between odour, face and group dropped to borderline significance (p = 0.054).
Correlation of valence and intensity ratings with reaction time and accuracy gains
Difference scores for pleasantness ratings averaged 1.13 (SD 1.84) for the pleasant odour and −2.91 (SD 1.86) for the unpleasant odour for the controls and 0.58 (SD 2.99) for the pleasant odour and −3.42 (SD 2.78) for the unpleasant odour for the patients. Because these values were computed from a bipolar scale, values higher than 0 indicate that the odour was rated more pleasant than the neutral odour, and values below 0 indicate that the odour was rated less pleasant than the neutral odour.
For intensity ratings, difference scores averaged 1.04 (SD 2.55) for the pleasant odour and 3.17 (SD 2.85) for the unpleasant odour in the controls and 1.04 (SD 2.35) for the pleasant odour and 2.96 (SD 2.77) for the unpleasant odour in the patients. Higher values indicate higher intensity ratings of odorants relative to ambient air.
For reaction time, a negative value of the difference score equaled a reaction time gain for odorant relative to ambient air, whereas for accuracy, a positive value was descriptive of a processing benefit.
For intensity ratings, there was no significant correlation for any odour and face combination for healthy controls or patients.
For pleasantness ratings, a number of significant and borderline significant correlations emerged. For neutral faces, we found a negative correlation among controls between reaction time difference score and H2S difference score; however this was not significant (r = −0.37, p = 0.07). This indicated that controls who rated H2S as more unpleasant relative to ambient air showed reaction time benefits during odour presentation relative to neutral faces. The reverse was observed for the schizophrenia patient group. Whereas the schizophrenia patients on average showed worse performance for neutral faces with an unpleasant odour, we found a positive correlation between the reaction time difference score and the H2S difference score (r = 0.42, p = 0.04), indicating that their difficulties were greater for H2S trials, the lower the perceived difference in pleasantness rating for neutral odour and hydrogen sulfide.
For disgusted faces, we found a nonsignificant correlation between difference scores for vanillin and accuracy difference scores in the control group (r = −0.35, p = 0.09). This indicated that the more participants preferred vanillin over the neutral odour, the more they were affected in their accuracy of recognizing disgust relative to the ambient air condition. Patients were affected in the reaction times, and their pattern matched that observed in the control group. It showed that the more patients preferred vanillin over ambient air, the longer it took them to respond to disgust trials in comparison to those primed with an ambient air stimulus (r = 0.62, p = 0.001).