Altered oscillatory brain dynamics after repeated traumatic stress

194 subjects participated in the study: 97 healthy controls (mean age 30.6 years, SD = 10.3, age range 22–66; 56 male, 41 female) and 97 patients (mean age 31.9 years, SD = 7.9, age range 16–53; 64 male, 33 female) diagnosed with PTSD according to DSM-IV [37]. All persons were right-handed as measured by the Edinburgh handedness questionnaire [38].

Participants with posttraumatic stress disorder (PTSD) were refugees who came for treatment or expert opinion to the Psychotrauma Research and Outpatient Clinic for Refugees, located at the Center for Psychiatry, Reichenau, Germany. Refugees had the following ethnicities: 74 Turks (71 Kurds), 6 Albanians, 3 Algerians, 3 Romanies, 1 Amharic (Ethiopian), 1 Bosnian, 1 Cameroonian, 1 Georgian, 1 Iranian, 1 Liberian, 1 Serb, 1 Sierra Leonian, and 1 Tamil, and 2 Germans who had fled from prisons of the former German Democratic Republic. In clinical interviews with trained translators, trained psychologists completed the Posttraumatic Stress Diagnostic Scale [PDS, 39], the Hopkins Symptoms Checklist-25 [HSCL-25, 40], or the interviewer-rated 21-item version of the original Hamilton Depression Scale [HAM-D, 41]. See Table 1 for mean questionnaire scores and standard deviations. For 40 patients HAM-D scores were available as a measure of depressive symptoms and for 57 patients HSCL scores were available. Fifty-five percent of participants with PTSD had received blows in the face, 24 were severely beaten on the head and sustained injuries, so that concussions could not be excluded, and 18 were not beaten on the head. Two subjects abused alcohol, 2 subjects abused drugs, and 1 subject abused both alcohol and drugs. Major depression is highly comorbid with PTSD [42], in particular in very severe cases of PTSD such as in survivors of torture. Due to substantial symptom overlap between severe PTSD and depression (disordered sleep, anhedonia/emotional numbing, concentration problems, irritability), comorbid depression diagnosis should be treated with caution and therefore quantitative depression scores were included in the analyses. In fact, nearly all participants would have additionally met DSM-IV criteria for major depression.

Controls were recruited by public newspaper announcements and on campus bulletin boards. In order to participate in the study, controls had to have no current or past history of psychiatric disorders and had to be free of psychiatric medication. All participants provided written informed consent and the procedures were approved by the ethics committee of the University of Konstanz. Controls were paid 5 EUR per hour for participation.

The magnetoencephalogram (MEG) was measured in supine position with a 148-channel whole-head neuromagnetometer (MAGNES™ 2500 WH, 4D Neuroimaging, San Diego, CA, USA) during a 5 min resting period before other experimental investigations (not reported in this manuscript) were conducted. Subjects were instructed to relax but stay awake and fixate a mark on the ceiling of the room throughout the recording period, in order to avoid eye and head movements. A video camera monitored subjects' behavior and assured compliance throughout the experiment. In order to define a subject-related headframe coordinate system and head shapes of subjects, five index points were digitized with a Polhemus 3Space^® Fasttrack (Polhemus, Colchester, VT, USA) prior to each measurement. The subject's head position relative to the pickup coils of the sensor was estimated before and after each measurement.

Data acquired during the 5 min resting period were reduced by a factor of 16 (antialias filters were applied automatically in the same processing step). Altogether this results in (678.17 Hz × 300 s)/16 = 12715 sampling points. Data were digitally filtered in the delta (1.5–4.0 Hz) frequency range with a Butterworth filter of order 2. In each one of five standard channel groups (as defined by 4D Neuroimaging: anterior, center, posterior, left, and right), a single equivalent current dipole (ECD) was fitted for each time point in the selected artifact free segments. Groups did not differ significantly in the number of artifact free sampling points. Dipoles were assigned to 1331 2 × 2 × 2 cm³ voxels (for details on the method see [43]). Fitted dipoles had to satisfy the following criteria: a) goodness of fit (GoF) > 90%, b) source intensity of 10–100nAm around a focal point source (equivalent to 0.1–1 cm² of activated cortex). Further analysis comprised two different strategies: voxel-based and mask-based.

For data analysis, linear mixed effects models [46] were implemented using SAS 9.1 (SAS Institute Inc.). In all analyses of variance (ANOVAs), Subjects served as a random effect nested in Gender and Group [47], whereas all other factors were fixed effects. Differences in dipole density solutions between groups were evaluated by means of a 2 × 2 × 4 × 2 ANOVA with between factors Group (PTSD patients, controls), Gender (male, female), and repeated measures factors Region (frontal, central, temporal, parieto-occipital) and Hemisphere (left, right).

Because Max and AvgAbThre values cannot be assumed to be normally distributed, ANOVA F statistics will not be F distributed for these dependent variables. In order to detect significant effects, permutation tests were conducted on the residuals of submodels for each factor and interaction [48, 49], holding all other factors and interactions constant by use of restricted permutations. For example, when investigating the significance of Group × Region × Hemisphere, the vectors of eight residuals corresponding to the eight combinations of Region and Hemisphere were permuted within each subject before being added to the unpermuted values predicted by the submodel defined by excluding Group × Region × Hemisphere from the full model. Next, subjects' group designations were permuted among male and female subjects separately, in order not to include gender effects. The resulting resampled dependent variable datasets were analyzed using the full model. In each case, 1000 permutations were conducted, and the original F value was inserted in the empirical distribution of F values from the resampled ANOVAs. p values as reported below are the difference between 1 and this percentile, such that an original F value falling at the 95^th percentile in the resampled F value distribution is considered significant at the .05 level and is reported as p = .05. Degrees of freedom are irrelevant in permutation tests and are not reported below. Significant effects were further analyzed by calculating contrasts, applying the same permutation procedure as reported above. For significant contrasts, Cohen's d was calculated as a measure of effect size, using pooled variances [50, 51].

Enhanced abnormal slow wave activity was observed in voxels in left temporal areas in the region of the insula in individuals with PTSD compared to controls, whereas in voxels in parieto-occipital areas fewer slow waves were observed in the PTSD compared to the control group (Figure 1).

The insula lies deep inside the lateral sulcus in the sylvian fissure under the operculum. It is an important site of multimodal convergence, directly receiving afferent auditory, visual, somatosensory, gustatory and olfactory information. The insula is involved in attention, pain perception, emotion, and in the processing of verbal, motor and musical information, as well as visceral sensory and vestibular functions. It has direct connections not only with the primary and secondary sensory areas but also with the hippocampus, the amygdala, the cingulate gyrus and Broca's area among others (for an overview see [20, 53]). Functional neuroimaging studies have linked insula activation to modulation of affective processing, cognitive and affective processing during learning, and aversive interoceptive processing [19].

The role of the insula in speech and language processing has long been noted [54], while there is now broad clinical and functional imaging evidence for a participation of the left anterior insula in speech motor control [55]. The left insula is notably larger than the right in most humans [56], consistent with a left-hemispheric dominance in language. Mutism has been frequently observed in cases of insular pathology: Transient mutism is found in cases of left inferior motor cortex damage extending to the insula [57, 58], whereas lasting mutism appears to be associated with bilateral lesions of the frontal operculum and anterior insula [59‐61]. Functional magnetic resonance studies have revealed significant blood flow increases at the level of intrasylvian cortex during overt speech in the left anterior insula [62, 63], suggesting that the left insula plays a role in the coordination of speech articulation. Because of its role in both affective and speech processing, the insula has been assumed to influence verbal affect [54]. It has been proposed [55] that the insula might fuse linguistic data structures with affective-prosodic information on a moment-to-moment basis into a smooth motor innervation pattern during speech production.

Recently, the role of the insula in anxiety disorders [19] and other neuropsychiatric disorders has received more attention [20]. Indeed, studies investigating volume changes in the brains of individuals with PTSD found evidence for reduced gray matter density in the left insula [64, 65].

This insular dysfunction may account for the nonverbal nature of traumatic memory recall in PTSD subjects (compare [66]) and the difficulty to identify, verbally express, and regulate one's emotional states in response to various kinds of trauma reminders, which has been called posttraumatic alexithymia [67]. Frewen et al. [67] suggested that alexithymic individuals with PTSD may experience intense emotional-physiological states such as fear, anger and dysphoria that are poorly integrated with, and modulated by, higher-order verbal cognitive processing. Therefore these individuals may report that they either do not know what they feel or cannot feel anything at all. Indeed, Frewen et al. found positive correlations between the Toronto alexithymia scale (TAS-20) and PTSD symptoms, dissociation, and childhood abuse and neglect. Furthermore, in a functional magnetic resonance imaging (fMRI) study they reported positive correlations between TAS-20 scores and insula activation in individuals with PTSD when exposed to trauma script imagery. However, Frewen et al. investigated insula activity in response to a trauma script induction procedure and did not directly measure emotional identification or expression abilities and associated neural network dysfunction. Thus, it is possible that the positive correlation they find is due to the fact that more severe traumatization leads both to stronger alexithymia and more anxiety, and therefore insula activation when confronted with trauma reminders.

Furthermore, the hypoperfusion of Broca's area (motor speech) during symptom provocation in PTSD is a replicated finding [18, 68]. Broca's area is necessary for the labeling of emotions, therefore, its deactivation under symptom provocation eventually may be due to insular dysfunction in connecting verbal affect with smooth motoric articulation patterns.

Alternatively, given the central role of the insula in multimodal processing, the connection between structural and functional changes in the insula and PTSD may be more complex and may touch on more facets of brain function than emotion identification and verbalization. Thus, future work should focus on elucidating the role of the insula in PTSD, along with other neuropsychiatric disorders, in which the relationship to the insula has only recently come under scrutiny [20].

The enhanced regional slow wave clusters (AvgAbThre) in the right frontal cortex are consistent with the large body of literature on structural, neurochemical, and functional abnormalities in medial PFC, including anterior cingulate cortex (ACC) and medial frontal gyrus, in PTSD [13]. The right PFC has been associated with negative affect, behavioral inhibition and vigilant attention, whereas left-sided PFC regions are particularly involved in approach-related, appetitive goals [69].

The most prevalent functional neuroimaging finding is that of a relatively diminished responsivity in medial PFC (decreased activation and/or failure to activate) in PTSD [13], which would be consistent with the present results showing PFC dysfunction as indicated by enhanced abnormal slow waves in right frontal areas. Current models on the neurocircuitry of PTSD assume the medial PFC, which plays an important role in fear extinction, to be hyporesponsive, leading to diminished extinction of conditioned fear and together with a hyperresponsive amygdala to augmented fear responses and hyperarousal symptoms [13].

The meaning of differences in parieto-occipital delta dipole densities in PTSD subjects compared to controls in the resting state deserves further investigation. Of particular interest is the present finding of lower levels (z values) but higher left-hemispheric peak (Max) values and a general trend toward more abnormal slow waves over parieto-occipital areas (compare Figures 2 and 3) in PTSD patients compared to controls. This clearly points to the existence of slow wave foci that do not appear in averaged values but differentiate between severely traumatized and non-traumatized individuals. However, currently the functional significance of these findings remains unclear and should be further investigated in future studies.