An electrocortical investigation of emotional face processing in military-related posttraumatic stress disorder
Introduction
Of the more than 2 million U.S. soldiers deployed to Afghanistan and Iraq, 14–16% have developed posttraumatic stress disorder (PTSD) (Moring et al., 2014) Consequently, PTSD represents one of the most prevalent injuries incurred during Operations Enduring Freedom (OEF), Iraqi Freedom (OIF), and New Dawn (OND). Given the profound mental, physical, occupational and functional costs associated with PTSD (Jackson et al., 2016, Tanielian et al., 2008) considerable emphasis has been placed on identifying the biological substrates that underlie the disorder (Michopoulos et al., 2015).
Against this backdrop, two primary types of neuroimaging have been used in a complementary fashion to elucidate the underlying pathophysiology of PTSD. Perhaps most well known is work from functional magnetic resonance imaging (fMRI) studies that have indicated disruptions in prefrontal, limbic and interactive brain function are most commonly associated with the disorder (Hayes et al., 2012a, Hayes et al., 2012b). In addition to fMRI, electroencephalography (EEG) and, more specifically, event related potentials (ERPs) have been used to identify additional neural biomarkers of the disorder (Lobo et al., 2015). A complementary method of neural measurement, ERPs offer superior temporal resolution, thereby providing information that cannot be ascertained by fMRI alone. One ERP component that may be particularly relevant to examining PTSD is the late positive potential (LPP).
The LPP is a centro-parietal, positive-going ERP component that appears approximately 400 ms after stimulus onset and is larger for emotional (e.g., threatening) stimuli than neutral stimuli (Foti et al., 2009, Schupp et al., 2000). Specifically, the LPP is considered to be a means of tracking motivated attention that is particularly sensitive to emotionally salient stimuli (Hajcak et al., 2013) Because of its relation to emotional processing and motivated attention, the LPP has been examined in disorders of affect dysregulation, such as depression (e.g., Foti et al., 2010) and anxiety disorders (e.g. MacNamara and Hajcak, 2010, MacNamara et al., 2016) For example, a study that examined LPP response in individuals with generalized anxiety disorder (GAD) as compared to healthy controls found that, for individuals with GAD, the LPP was enhanced to aversive targets relative to their healthy counterparts (MacNamara and Hajcak, 2010). Given the emotion and attentional deficits associated with PTSD, the LPP may provide important information as a putative neural measure of PTSD.
To date, however, it has been infrequently studied in relation to PTSD. In the limited studies evaluating the LPP in PTSD populations, findings are mixed such that some research indicates the disorder is associated with increased LPP reactivity to threat (Lobo et al., 2014) and other studies demonstrate an attenuated LPP response, suggesting emotional disengagement to threat (MacNamara et al., 2013). The reason for these discrepant findings is unclear. One plausible reason for the discrepancies may be attributable to individual differences in reactivity. Another plausible explanation is that the differences may be due to different subtypes of PTSD. For example, it is conceivable that dysregulation may be linked to either over- or under-responsiveness to threat in distinct subtypes of PTSD—and recent research has suggested there are likely divergent biological profiles of PTSD (Michopoulos et al., 2015). Finally, the heterogeneity of findings may be attributable to methodological differences; while both studies used emotionally provocative stimuli, the former used disturbing images of mutilated bodies and the latter used emotional faces (i.e., happy, fearful, angry).
Notably, all prior studies examining the relation between PTSD and LPP response have used binary classifications (Fitzgerald et al., 2016; MacNamara et al., 2013; Lobo et al., 2014). For example, MacNamara and colleagues conducted their analyses based on 33 veterans, 19 who met full diagnostic criteria for PTSD and 14 who did not as determined by a clinical cutoff of 40 on the Clinician-Administered PTSD Scale (CAPS-IV). However, one challenge associated with traditional diagnostic categories is the exclusion of many veterans experiencing subthreshold PTSD symptoms, which thereby limits the generalizability to the larger veteran population. Of note, prior work that has examined trauma in conjunction with other neurobiological markers has found important differences in brain functioning associated with subclinical PTSD that are not seen in individuals without PTSD symptoms (Garrett et al., 2012, Peres et al., 2011). Such research suggests that a dimensional approach to PTSD has important consequences for understanding brain-behavior relationships in the context of trauma's sequelae. Further pressing the need to take a dimensional approach to PTSD symptoms is recent work that has highlighted the profound clinical and functional implications (e.g., heightened suicide risk, greater health problems) associated with subclinical symptoms (Eekhout et al., 2016; Jakupcak et al., 2011; Pietrzak et al., 2009).
Instead of examining groups (i.e., PTSD patients vs. a non-PTSD group), this study sought to address these gaps by examining PTSD symptom severity as a continuous predictor of LPP in order to extend our understanding of the role of individual differences and psychophysiology in OEF/OIF/OND veterans. In the current study, we hypothesized that the extent of LPP reactivity in response to threatening (i.e., fear and/or angry faces but not happy) faces would be negatively related to military-related PTSD symptoms given prior evidence of blunted LPP reactivity to social signals of threat (MacNamara et al., 2013).
Section snippets
Methods
This study was approved by the Institutional Review Boards at Jesse Brown VA Medical Center, Chicago IL and its university affiliate, the University of Illinois at Chicago. Research was conducted in accordance with the Helsinki Declaration.
Behavioral
Participants performed well on the EFMT as average accuracy was 95% SD: ±3.7. There was an effect of condition on reaction time (F(2.38, 168.82) = 461.8, p < 0.001, η2 = 0.87). Post-hoc analyses indicated that trials with faces elicited slower reaction times than those with shapes. Specifically, angry faces elicited the slowest reaction time (i.e., angry > fear > happy > shapes; Bonferroni-corrected p < 0.001). Similarly, there was an effect of condition on accuracy (F(1.65, 117.08) = 111.05, p
Discussion
The primary aim of the current study was to examine a neural measure of socio-emotional processing (i.e., LPP to angry, fearful, and happy faces) as a potential predictor of military-related PTSD symptoms in a sample of OEF/OIF/OND veterans. Results revealed that higher PTSD symptoms were related to an attenuated LPP response to angry facial expressions. This finding was not observed for happy or fearful faces and was maintained when adjusting for current depression symptoms, suggesting that
Financial disclosures
The authors report no financial interests or potential conflicts of interest.
Acknowledgements
This material is based on work supported by Veterans Affairs Merit Review Program Awards (I01BX007080 to KLP) from Clinical Sciences Research and Development, Office of Research and Development of the U.S. Department of Veterans Affairs.
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