An initial assessment of the reliability of the Peritraumatic Dissociation Index was undertaken because it was modified from the published version of the Peritraumatic Dissociative Experiences Questionnaire—Rater Version (5
). Cronbach’s alpha was 0.75 for the eight items constituting the dissociation index, indicating relatively good internal consistency for these items. The range of possible scores on this index was 0–32. The scores were approximately normally distributed (). On the basis of the scores from this index, subjects who scored more than 1 SD above the mean (scores from 21 to 32) were placed in the high dissociation group (N=16); those who scored more than 0.50 SD below the mean (scores from 1 to 10) were placed in the low dissociation group (N=31). Subjects with scores falling in the moderate range (scores of 11–20) were excluded from further analyses (N=38). The definitions for the high and low dissociation groups were based on the desire to have a high dissociation group that had clearly been dissociating most of the time during the trauma; hence our selection of a group that had a score 1 SD above the mean on the Peritraumatic Dissociation Index. We decided to use only 0.50 SD below the mean to define the cutoff for the low dissociation group because the distribution was positively skewed (skewness index=0.52; a score of 0.00 would indicate a perfectly symmetrical distribution). This procedure ensured that a group who clearly had experienced peritraumatic dissociation was compared with a group who clearly had not.
Frequency Distribution of Scores of 85 Rape Victims on the Peritraumatic Dissociation Index
The frequencies of PTSD symptoms for the high and low dissociation groups are presented in . Chi-square tests with Yates’s correction indicated a significant difference between expected and observed cell frequencies. The high dissociation group had a disproportionate number of subjects who met the symptom criteria for PTSD.
Self-Report and Diagnostic Interview Data of Rape Victims With Low or High Levels of Peritraumatic Dissociation
Before analysis the physiological data were examined for univariate and multivariate outliers. Examination of detrended plots revealed two subjects with data for skin conductance responses that were more than 3.5 SD above the mean and two subjects with skin conductance amplitude that was more than 4.0 SD above the mean. These data were deemed to be outliers and were considered as missing. These subjects had peritraumatic dissociation scores that placed them in the low dissociation group. Evaluation of multivariate normality through Mahalanobis distance and homogeneity of variance-covariance matrices (17
) after these deletions was satisfactory.
The main factors for the laboratory analyses were laboratory phase (initial resting baseline, neutral, second resting baseline, trauma, final resting baseline) as a repeated measures factor and group (high or low peritraumatic dissociation) as a between-subjects factor. A 2×5 mixed-design multivariate analysis of variance (MANOVA) was performed on the physiological variables: skin conductance responses, skin conductance amplitude, and heart rate. The independent variables were peritraumatic dissociation group and laboratory phase. The combined physiological variables showed a significant group-by-phase interaction (F= 2.1, df=12, 666, p<0.01; Pillai’s criterion =0.11). Univariate analysis of variance for each physiological variable indicated a significant group-by-phase interaction for heart rate (F=14.6, df=4, 176, p<0.001), skin conductance responses (F=4.5, df=4, 176, p<0.005), and skin conductance amplitude (F=2.9, df=4, 176, p< 0.05). Follow-up analyses of simple main effects revealed significant differences between groups during the trauma phase (talk about the rape) and the final resting baseline (). A separate 2×5 mixed-design MANOVA was performed on the nonspecific subject movement data. Findings indicated no significant group main effects or group-by-phase interactions for the movement data.
Physiological Data Across Laboratory Study Phases of Rape Victims With Low or High Levels of Peritraumatic Dissociationa
Analyses of race effects were conducted to compare African American and white subjects on the physiological variables. The data indicated that there were no significant race main effects or race-by-dissociation-level interactions for each of the three physiological variables.
Subjects’ scores on the Subjective Units of Distress Scale at the end of each phase of the laboratory assessment were significantly higher in the high dissociation group than in the low dissociation group (). We also compared the Subjective Units of Distress Scale scores with the physiological measure that provided the greatest difference between the high dissociation and low dissociation groups—skin conductance responses—by converting the data to z scores and then performing t tests with Bonferroni corrections for alpha level. For the high dissociation group, the results indicated a significant difference between the skin conductance response measure of distress and the self-reported distress scale scores during the trauma phase (talk about the rape) and the final baseline phase (t=5.7, df=15, p<0.001, and t=4.6, df=15, p<0.001, respectively) (). Both results indicated that these subjects’ self-report ratings of distress were significantly higher than their physiological measures of arousal. The low dissociation group’s self-reports of distress were in close agreement with their physiological measures of arousal except during the neutral phase, when self-reports of distress were significantly lower than the measures of physiological arousal (t=4.8, df=30, p<0.01) ().
Transformed Scores (z) on Skin Conductance Responses and the Subjective Units of Distress Scale Across Laboratory Study Phases of Rape Victims With Low or High Levels of Peritraumatic Dissociation
Analyses of self-report data indicated significant group main effects on several measures of distress (). The MANOVA of the subscales of the PTSD Symptom Scale revealed that the high dissociation group scored significantly higher than the low dissociation group on the combination of PTSD subscales (F=4.0, df=3, 43, p<0.05; Pillai’s criterion=0.22). Follow-up univariate analyses indicated that each subscale score (reexperiencing, avoidance, and arousal) was significantly greater for the high dissociation group. The high dissociation group also scored significantly higher on measures of global distress from the Rape Aftermath Symptom Test, the Beck Depression Inventory, and the index of perception of life threat during the rape.
To examine the role of peritraumatic dissociation in PTSD symptoms more closely, we conducted additional analyses of data from only the subjects who met the symptom criteria for PTSD. These subjects were divided into those with high dissociation scores (N=15) and those with low scores (N=17). This information is presented in . A 2×5 mixed-design MANOVA was performed on the physiological data. The combined physiological variables showed a significant group-by-phase interaction (F=2.1, df=12, 666, p<0.01; Pillai’s criterion=0.11). Univariate analyses revealed significant group-by-phase interactions for heart rate (F=14.6, df=4, 176, p<0.001) and skin conductance response frequency (F=5.6, df=4, 176, p<0.005). For heart rate, examination of simple main effects revealed significant differences between groups during the trauma phase and final baseline phase, with the PTSD/high dissociation group displaying a significantly lower heart rate during these phases than the PTSD/low dissociation subjects. The high dissociation group displayed a significantly lower number of skin conductance responses than the low dissociation group during each phase except for the neutral phase. There was a trend for an overall group main effect on skin conductance amplitude.
Physiological and Self-Report Data of Rape Victims With PTSD Symptoms and Low or High Levels of Peritraumatic Dissociation