summarizes the demographic and clinical data. Control subjects were slightly younger (mean age=48.29, SD=11.50) than the low symptom PTSD patients (mean age=53.21, SD=6.29) and the high symptom PTSD patients (mean age=53.85, SD=7.30); however, this difference was not significant (F(2,54)=2.09, P>0.1). Race was distributed equally across the three groups (χ2(4)=6.55, P>0.1). The control subjects had more years of education (mean=15.43, SD=1.87) than the low symptom PTSD group (mean=13.36, SD=1.08; F(2,52)=8.70, post-hoc P<0.01) and the high symptom PTSD group (mean=13.96, SD=1.42; F(2,52)=8.70, post-hoc P<0.01).
Demographic and symptom characteristics of subject groups.
3.2. Clinical Assessment
The median CAPS score in the PTSD patients was 82 and the low and high symptom PTSD groups were defined by this criterion. Thus the two symptom groups had significantly different CAPS total scores, as well as CAPS frequency and intensity scores, and CAPS cluster scores. However, the two groups did not differ on severity of combat exposure as assessed by the Combat Exposure Scale (CES).
Twenty-five of the 27 PTSD patients were taking psychiatric medication at the time of the study, with the greatest proportion of patients taking antidepressants in the selective serotonin reuptake inhibitor (SSRI) class (11 of 27). A Chi-square analysis indicated that the distribution of prescriptions did not differ between the two PTSD groups (the most common pharmacotherapies were: SSRIs, χ2
>0.1; atypical antipsychotics, χ2
>0.1; benzodiazepines, χ2
>0.1; and anxiolytics, χ2
>0.1). Benzodiazepines have been found to reduce baseline startle without affecting fear-potentiated startle (Baas et al., 2002
). In our sample, the five patients who were taking benzodiazepines did not have significantly lower baseline startle compared to the other patients (F(1,26)<1, P
3. 4. Aversiveness Ratings
The subjects rated the airblast as more aversive (mean=3.09, SE=0.26) than the startle probe (mean=2.57, SE=0.21), (F(1, 30)=6.59, P<0.05). While the airblast was equally aversive to all participants, the PTSD patients rated the startle probe as more aversive (low symptom mean=2.90, SE=1.29; high symptom mean=3.05, SE=1.21) than the control subjects (mean=1.77, SE=1.17), F(2,32)=3.91, P<0.05. Post hoc analyses showed that both patient groups differed from the control group; however, the low symptom patients found the startle equally aversive as the high symptom patients.
3.5. Fear-Potentiated Startle
Baseline startle magnitude, i.e. startle to the sound probe alone, without the presence of lights, did not differ between the three groups (F(2,54)=0.09, P>0.1).However, startle magnitude was robustly potentiated in the presence of the reinforced stimulus, NA vs. AX (Trial Type F(1,53)=59.93, P<0.001, η2=0.49). shows fear-potentiated startle in the three groups.
Startle amplitude (mean + SEM) to noise alone (NA) and in the presence of the reinforced stimulus (AX+) in controls, low symptom, and high symptom PTSD subjects. Brackets indicate significant contrasts. *P<0.05. **P<0.01.
There was no interaction effect of Trial Type and Group; i.e., startle was potentiated in the control group (F(1,27)=19.89, P<0.001, η2=0.42), the low symptom PTSD group (F(1,13)=15.50, P<0.01, η2=0.54), and in the high symptom PTSD group (F(1,12)=14.89, P<0.01, η2=0.55).
shows the percent startle potentiation from noise alone in the presence of the four trial types (AX, BX, AB, and AC) in the controls and the two PTSD groups.
Percent potentiation (mean + SEM) from noise alone to AX+, BX-, AB, and AC trial types in control, low symptom, and high symptom PTSD subjects. Brackets indicate significant contrasts. *P<0.05. **P<0.01. †P<0.1
Analyses of percent potentiation to the different trial types revealed a significant main effect of Trial Type (F(3, 156)=8.08, P<0.001, η2=0.13), and Group (F(2, 52)=4.26, P<0.05, η2=0.14), but there was no Group by Trial Type interaction. Given our apriori hypotheses that high symptom patients will show impaired fear inhibition, we tested planned comparisons for Trial Type separately in each group. The overall effect of trial type was significant in the controls (F(3, 81)=2.96, P<0.05, η2=0.10). However, among the patients, only the low symptom group had a significant effect of trial type (F(3, 39)=4.75, P<0.01, η2=0.27), while the high symptom group did not (F(3, 36)=1.91, P>0.1, η2=0.14). Follow-up contrasts comparing the different trial types within each group showed that startle was potentiated more in the presence of AX than BX in the controls (F(1,27)=4.54, P<0.05, η2=0.14) and the low symptom PTSD group (F(1,13)=6.27, P<0.05, η2=0.32), with a trend for discrimination in the high symptom PTSD group (F(1,12)=3.28, P=0.09, η2=0.21). Potentiation in the presence of AB was significantly inhibited relative to AX in the controls (F(1,27)=5.67, P<0.05, η2=0.17) and the low symptom PTSD group (F(1,13)=9.35, P<0.01, η2=0.42), but not in the high symptom PTSD group (F(1,12)=1.68, P>0.1, η2=0.12). Unexpectedly, in the control and low symptom group startle potentiation was also decreased in the presence of AC relative to AX (F(1,27)=5.39, P<0.05, η2=0.17 and F(1,13)=7.00, P<0.05, η2=0.35, respectively). On the other hand, in the high symptom group, startle to AC trials was not inhibited relative to AX trials (F(1,12)<1, P>0.1, η2=0.00). In all three groups, startle was equally potentiated in the presence of AB and AC trials.
There was a significant between-groups effect of Group on percent potentiation (F(2,52)=4.26, P<0.05, η2=0.14). Follow-up one-way ANOVAs with LSD post-hoc tests indicated that the high symptom PTSD group potentiated more to AX+ compared to the control group (P<0.05), and had less fear inhibition to AB than either the control group (P<0.05) or the low symptom group (P<0.05). Finally, the high symptom group also potentiated more to the AC trials compared to the control group (P<0.05) and the low symptom PTSD group (P<0.05) (see ). The low symptom PTSD group did not differ from the control group on any trial types (see ).
Since the clinical presentation of PTSD is characterized by 3 different symptom clusters, we wanted to see whether fear inhibition in patients with high symptoms was evident for each of the individual clusters. Thus we compared percent potentiation in the presence of AX to potentiation to AB in patients who were categorized as high using the median split on the Re-experiencing, Avoidance, and Hyperarousal subscores of the CAPS. Patients with high Hyperarousal symptoms (F(1,14)=10.73, P<0.01, η2=0.43) demonstrated fear inhibition, while patients with high Re-experiencing (F(1,14)=1.93, P>0.1, η2=0.12) and Avoidance symptoms (F(1,13)=2.79, P>0.1, η2=0.18) showed a lack of fear inhibition.
3.6. Expectancy ratings
shows the subjects’ US-expectancy ratings for each trial type. An analysis of subject’s keypad responses to the final conditioning block (i.e., AX+ and BX-) showed that all groups discriminated between AX and BX, i.e., they expected to receive an airblast on AX trials, and did not expect an airblast on BX trials (Controls: F(1,27)=17.56, P<0.01); low symptom PTSD: F(1,12)=5.73, P<0.05); and high symptom patients: F(1,12)=20.53, P<0.01). Furthermore, on the first block of AB trials the expectancy of the airblast was reduced relative to AX in all groups (Controls: F(1,27)=17.34, P<0.01); low symptom PTSD: F(1,12)=7.92, P<0.05); and high symptom patients: F(1,12)=28.96, P<0.01). Finally, on the first block of AC trials, the subjects had a reduction in airblast expectancy relative to AX; however, this difference was significant only in the controls (F(1,27)=22.39, P<0.01) and the high symptom group (F(1,12)=23.11, P<0.01). We used chi-square analyses in order to examine the subjects’ expectancy on the first presentation of B and C. We found that in all groups, a majority of subjects rated B as “safe” (Controls: 78%; low symptom PTSD: 75%; high symptom PTSD: 89% of the responders). On the other hand, C was rated as “don’t know” in a majority of the controls (55% of responders) and high symptom PTSD patients (63% of responders); while the low symptom PTSD patients were more likely to rate C as “threat” (56% of responders), although this difference was only at trend level χ2(4)=8.32, P=0.08.
Figure 4 Expectancy ratings (mean + SEM) for AX+, BX-, AB, and AC trial types in low symptom and high symptom PTSD subjects. Positive score indicate airblast expectancy, negative scores indicate that there is no expectancy of airblast, and zero indicates uncertainty. (more ...)