IBS patients rated the intensity and unpleasantness of the abdominal stimulation and the unpleasantness and intensity of the biceps stimulation significantly greater than controls ().
Context threat – The effects of stimulation pad attachment
shows ASR magnitude during the first and second set of six baseline startle trials and the six context threat trials. ASR declined due to habituation from baseline 1 to baseline 2 and increased again during context threat, resulting in a significant Condition main effect (F(2, 295) = 24.84, p < .001) in the 2 Group (IBS; controls) × 3 Condition (baseline 1; baseline 2; context) × 6 Trials (per condition) ANOVA. This was confirmed by post-hoc contrasts showing ASR in baseline 1 and context was significantly greater than that during baseline 2 (p’s < .001). No significant difference was found between baseline 1 and context (p > .05). Both groups showed significant habituation of ASRs across startle trials within each condition and an initial increase in ASR magnitude on the first context trial following stimulation pad placement. This resulted in a main effect for Trial, Number (F(5, 762) = 13.77, p < .001) and a Condition × Trial Number interaction (F(10, 811) = 2.35, p = .0099). There were no significant effects involving Group.
Figure 2 Context manipulation results. ASR magnitude during the first and second set of six baseline startle trials and the six context threat trials. Values are estimated means (with standard error bars) for each startle trial. sqrt(μV) = square root (more ...)
Cued threat – the effects of safety, anticipation, and imminent threat
The 2 Group (IBS; Control) × 3 Condition (safe; anticipation; imminent threat) × 2 Threat Site (abdominal; arm) × 3 Block (first; second; third) mixed model ANOVA initially revealed no differences as a function of threat location (abdominal or arm). Therefore, Threat Site was collapsed in subsequent analyses. For both groups there were significantly larger ASRs during the imminent threat condition compared to both the anticipation and safe conditions (p’s < .001), but no significant difference between the safe and anticipation condition (p > .05). This was reflected in the 2 Group (IBS; Control) × 3 Condition (safe, anticipation, imminent threat) × 3 Block (first; second; third) ANOVA showing a significant main effect for Condition (F(2, 2031) = 17.37, p < .001). There was also a significant Condition × Group interaction (F(2, 2031) = 4.01, p = .018). A post-hoc contrast of condition effects across the groups indicated that the IBS group showed a significantly greater difference in ASR magnitude between the safe and imminent threat conditions relative to controls (p = .01). The IBS group also showed a significantly greater difference in ASR magnitude from the safe to the anticipation condition relative to controls (p = .05). These results are shown in .
Figure 3 ASRs for the cued threat procedure averaged over Blocks. Values are estimated means (with standard error bars) for each condition, Anticipation and Threat conditions are averaged over threat type (abdominal and arm). Safe includes both the early and late (more ...)
These effects however did differ over the three blocks in the cued threat component. shows the estimated ASRs during the safe, anticipation and imminent threat conditions for the IBS and control subjects during the three blocks. There were no significant Block main effects, or interactions between Block and Condition; however, there was a significant interaction between Group and Block (F(2, 964) = 3.73, p = .024) as well as a three-way interaction between Group, Condition, and Block (F(4, 2104)= 2.39, p = .049). Post-hoc contrasts of condition effects across groups demonstrated that IBS patients compared to controls showed a significantly greater increase in ASR magnitude from safe to imminent threat for Blocks 2 (p = .0074) and 3 (p = .0081) but not Block 1 (p > .1). Similarly IBS patients tended to show a significantly greater increase in ASRs from safe to anticipation during Blocks 2 (p = .027) and 3 (p = .082) but not in Block 1 (p > .1). Thus it appears the major group differences emerged after subjects experienced an actual stimulation, regardless of site.
Figure 4 ASRs for the cued threat procedure including Block. Values are estimated means (with standard error bars) for each condition over the three blocks. Anticipation and Threat conditions are averaged over threat type (abdominal and arm). Safe includes both (more ...)
Moderation of ASR potentiation as a function of neuroticism and anxiety symptoms
As expected, the IBS patients reported significantly greater anxiety symptoms and neuroticism compared to controls despite selection for absence of psychiatric disorders (). Mixed model analyses of covariance (ANCOVAs) were first used to examine the effects of neuroticism and anxiety scores on ASR modulation across the groups. After inclusion of neuroticism or anxiety symptoms, the significant Group × Condition interactions found in the analyses described above remained, indicating the primary group differences in ASR were not due to elevated anxiety symptoms or neuroticism in the IBS subjects. To further explore the impact of these variables on ASR, Condition × Block mixed model ANCOVAs were performed for each group separately. For IBS there was a greater positive slope between neuroticism and ASRs during both the safe and anticipation conditions than during the imminent threat condition (p’s < .005). This resulted in a significant Neuroticism × Condition interaction (F(2, 1192) = 7.03, p<.001). illustrates this interaction by showing the estimated ASRs at three levels of neuroticism: mean neuroticism, one standard deviation above the mean, and one standard deviation below the mean. A similar analysis for the HAD anxiety scale in IBS () also yielded a significant Anxiety × Condition interaction (F(2, 1290) = 15.15, p < .001). However this was due to there being a greater negative slope between anxiety and ASRs during the imminent threat condition compared to both the safe and anticipation conditions (p’s < .005), indicating increased ASR with lower anxiety symptoms. For controls, the same mixed model ANCOVAs did not yield any significant neuroticism or anxiety symptom effects or interactions (p’s > .05).
Figure 5 Estimated mean ASRs (and standard errors) for IBS patients within 1 standard deviation of the mean and those > 1 standard deviation above and below the mean on neuroticism (EPQ-R) for the three cued threat conditions. sqrt(μV) = square (more ...)
Figure 6 Estimated mean ASRs (and standard errors) for IBS patients within 1 standard deviation of the mean and those > 1 standard deviation above and below the mean on anxiety symptoms HAD-A) for the three cued threat conditions. sqrt(μV) = square (more ...)