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. Author manuscript; available in PMC: 2020 Oct 1.
Published in final edited form as: Neuropsychology. 2019 Jun 13;33(7):913–921. doi: 10.1037/neu0000560

The Impact of PTSD and mTBI on the Relationship Between Subjective and Objective Cognitive Deficits in Combat-Exposed Veterans

Elsa K Mattson 1, Nathaniel W Nelson 2, Scott R Sponheim 1,3, Seth G Disner 1
PMCID: PMC6763389  NIHMSID: NIHMS1035955  PMID: 31192654

Abstract

Objective:

Cognitive complaints, such as attentional or memory concerns, are commonly reported by veterans diagnosed with post-traumatic stress disorder (PTSD) or a history of mild traumatic brain injury (mTBI). The degree to which those complaints actually map on to measurable cognitive deficits is unclear and is likely complicated by the severity of trauma-related sequelae. In the present study we sought to characterize the degree to which PTSD symptoms and mTBI accounted for the relationship of subjective cognitive complaints to objective cognitive performance, with the goal of facilitating the accurate assessment of trauma-exposed veterans complaining of cognitive decline.

Method:

A sample of 203 US military veterans previously deployed to Iraq and/or Afghanistan were assessed for PTSD severity, history of blast and impact mTBI, objective cognitive function, and subjective cognitive complaints. Separate mediation analyses were conducted to explore the degree that PTSD severity, blast mTBI severity, and impact mTBI severity influenced the association between subjective cognitive complaints and objective cognitive performance. Models reflecting significant mediation were followed by post-hoc moderated mediation analyses.

Results:

Subjective cognitive complaints and objective cognitive performance were significantly associated (β [SE]= −6.49[2.85]; p=.03), but this relationship was mediated by PTSD severity (β [SE]= −2.95[2.86]; p=.30). PTSD mediation was not moderated by either blast or impact mTBI.

Conclusion:

The present results delineate the prominent impact of PTSD symptoms, relative to blast and impact mTBI, on cognition following combat. These findings highlight the importance of assessing for trauma-related psychopathology in those seeking neuropsychological assessment or rehabilitative care for cognitive complaints.

INTRODUCTION

Military personnel who serve in combat areas are at greater risk for both mild traumatic brain injury (mTBI) and post-traumatic stress disorder (PTSD) than those without such service (Combs et al., 2015). The incidence of mTBI and PTSD in returning combat veterans is roughly similar, around 12%–17% (Hoge, Terhakopian, Castro, Messer, & Engel, 2007), and both conditions have been associated with cognitive difficulties, including impairments in attention, memory, and processing speed (Combs et al., 2015; Spencer, Drag, Walker, & Bieliauskas, 2010). The two conditions also frequently co-occur in combat veteran samples, with the presence of mTBI often (though not always) linked to worse PTSD outcomes, such as higher anxiety, functional limitations, and cognitive impairments (cf. Kontos et al., 2013; Ragsdale, Neer, Beidel, Frueh, & Stout, 2013; Brewin, Kleiner, Vasterling, & Field, 2007). The frequency and complexity of comorbid PTSD and mTBI place considerable burden on clinicians tasked with assessing and treating veterans who report cognitive symptomatology.

The difficulty of this task is compounded by the observed differences between subjective cognitive complaints (e.g. self-reported symptoms) and objective cognitive performance (e.g. results from a neuropsychological battery; Drag, Spencer, Walker, Pangilinan, & Bieliauskas, 2012; Vasterling, Brailey, Constans, & Sutker, 1998). Objective assessments of cognitive ability are typically time- and resource-intensive, meaning that self-reports are often the primary source of information available to clinicians (Spencer et al., 2010). However, the reliability of self-reported cognitive complaints has been called into question because the relationship between subjective cognitive complaints and objective cognitive performance is inconsistent (Spencer et al., 2010). There are many possibilities as to why the relationship between objective and subjective cognition remains unclear. Firstly, it is possible that unaccounted comorbidity, such as PTSD, depression, or chronic pain, could be confounding results (Gervais, Ben-Porath, & Wygant, 2009). There is also evidence that the context in which testing takes place (e.g., research vs. forensic) can impact results (Nelson et al., 2010). Additionally, while neuropsychological tests have been designed to identify cognitive impairment, they are typically administered in quiet environments free from distraction and are not designed to predict how individuals’ function in real-world settings (Limond & Adlam, 2001). Subjective cognitive complaints that were not corroborated by objective cognitive impairment have been observed in a multitude of health conditions, including depression (Chamelian & Feinstein, 2006; Srisurapanont, Suttajit, Eurviriyanukul, & Varnado, 2017), multiple sclerosis (Middleton, Denney, Lynch, & Parmenter, 2006), bipolar disorder (Svendsen, Kessing, Munkholm, Vinberg, & Miskowiak, 2012), and mTBI (French, Lange, Brickell, & Psych, 2014; Nelson et al., 2012; Spencer et al., 2010). Similar findings have been found in veteran samples, with many who report cognitive complaints performing well within normal limits on measures of objective cognition (Drag et al., 2012; Spencer et al., 2010; French et al., 2014). These studies support the notion that subjective cognitive complaints are tenuously coupled with objective cognitive performance, which may undermine rehabilitative interventions targeting cognition (Basford & Malec, 2015).

Understanding the factors that mediate the relationship between subjective and objective cognition is a necessary step to help clinicians provide more effective and efficient interventions. Growing evidence suggests that trauma-related sequelae, such as the symptoms associated with PTSD and mTBI, may play a crucial role. Seal and colleagues (2016) found that perceptions of cognitive problems were strongly associated with post-traumatic cognitions about the self, world, and trauma coping self-efficacy, suggesting that negative perceptions of cognitive functioning may represent a specific type of post-traumatic appraisal. In a sample of Operation Iraqi Freedom/ Operation Enduring Freedom (OEF/OIF) veterans, self-reports of cognitive complaints were significantly correlated with psychological distress and were not significantly correlated with overall neurocognitive functioning (French et al., 2014). Moreover, in a sample of 167 OEF/OIF veterans, psychological distress has been shown to mediate the relationship between injury characteristics and subjective ratings of cognition (Drag et al., 2012). These results suggest that trauma-related psychopathology can play a prominent role in perceived cognitive deficits, though the precise nature of these contributions is not well defined, nor is it known what additive role mTBI may play.

The present study aimed to address this uncertainty by examining the role that PTSD and history of mTBI have on the equivocal relationship between subjective cognitive complaints and objective cognitive performance. Based on the literature that links mTBI to worse cognitive symptoms in cases of PTSD, we aimed to establish a mechanism by which mTBI and PTSD symptoms may combine to influence subjective and objective cognition. This approach will help examine how psychological and physiological processes work independently and in conjunction to influence the presentation of cognitive symptoms in combat veterans. Identifying a moderating or mediating role in this sample would provide clinicians with critical evidence to help in the conceptualization of cognitive impairment, particularly when subjective complaints are central to the patient’s presentation.

METHOD

Participants

The study initially enrolled 221 combat exposed OEF, OIF, and Operation New Dawn (OND) veterans (mean age= 33.40, SD=0.24, 94% male) who completed study procedures at the Minneapolis VA Health Care System (MVAHCS). Overall, 72% of participants identified as Caucasian, 3% African American, 3% Native American, 2% Hispanic, and 20% Mixed Race/Other. Participants who self-reported experiencing mTBI were specifically recruited, alongside a more general population of service members and veterans. Following a review of computerized records and a clinical screening (including the Structured Clinical Interview for DSM-IV-TR, First & Gibbon, 2004), participants were excluded if they had lifetime history of moderate or severe TBI, a significant non-TBI neurological condition (e.g., epilepsy or seizures, history of stroke, neurodegenerative disease), current psychotic symptoms, an unstable medical condition that could affect brain function (e.g. diabetes), or current substance abuse or dependence. The study protocol was approved by the MVAHCS institutional review board.

Measures

Post-traumatic Stress Disorder

PTSD symptoms were assessed using the Clinician Administered PTSD Scale for DSMIV (CAPS; Blake et al., 1995). The CAPS was administered through a structured clinician-administered interview covering 17 PTSD symptoms as outlined in DSM-IV (American Psychiatric Association, 2000). Participants were asked to identify an index trauma which was not limited to deployment-related events. Questions corresponding to DSM-IV PTSD symptoms were then asked, with answers scored separately for frequency and intensity using a 5-point scale (0–4). Symptom severity for each item is determined by summing both frequency and intensity scores. Continuous scores of current PTSD severity were used in all analyses. Lifetime vs. current PTSD symptoms are reported for descriptive purposes only.

Assessment of mTBI

Symptoms of mTBI across the lifespan were assessed by interview using the Minnesota Blast Exposure Screening Tool (MN-BEST; Nelson, Hoelzle, McGuire, Ferrier-Auerbach, et al., 2011) to provide a total symptom profile of primary blast-related and primary impact-related mTBI. An mTBI was considered to be blast-related if it followed a direct or indirect exposure to an explosion (e.g., improvised explosive device or mortar explosion), whether or not there was a secondary impact with an object (e.g. hitting the head on the ground following an explosion). An mTBI was considered impact-related if it involved concussive force between the head and an object in the absence of a blast (e.g., physical assault or automotive accident). Severity of mTBI was classified based on the rating scheme outlined by the American Congress of Rehabilitation Medicine (ACRM; Kay, Harrington, & Adams, 1993) including loss of consciousness (LOC) for no more than 30 minutes and post-traumatic amnesia for no more than 24 hours. Information was also gathered regarding time since injury, approximate proximity to blast, and neurological symptoms following injury such as headaches, nausea, and sensitivity to light or noise. Trained neuropsychologists then assigned ratings for each possible concussive event ranging from 0 (no brain injury) to 3 (severe concussion symptoms). Separate severity scores for blast mTBI and impact mTBI were then calculated by summing the three most significant blast and impact events.

Subjective Cognitive Complaints

The Minnesota Multiphasic Personality Inventory-2-Restructured Form (MMPI-2-RF; Gervais et al., 2009) cognitive complaints (COG) scale was utilized to assess self-reported cognitive difficulties. The COG scale consists of 10 items describing various symptoms such as difficulty with memory, concentration, forgetfulness, reading comprehension, frustration, and poor tolerance for stress. Scores of 65–80T are associated with reports of diffuse cognitive difficulties, while scores at or above 81T indicate more pronounced cognitive complaints such as confusion, impaired memory, and poor concentration (Ben-Porath & Tellegen, 2008).

Objective Cognitive Performance

Participants completed a neuropsychological battery to evaluate cognitive performance. Assessments in this battery included the California Verbal Learning Test (CVLT-II; Delis, Kramer, Kaplan, & Ober, 2000) which measures episodic verbal learning and memory, the Wechsler Adult Intelligence Scale (WAIS-III; Wechsler, 1997) Coding & Digit Span subsets which measure working memory, visual processing speed, verbal and performance IQ. The Trail Making Test (TMT; Bowie & Harvey, 2006) was included to measure visual attention (TMT-A) and task switching (TMT-B). Each measure was included in a confirmatory factor analysis (CFA) to form a single latent factor of general cognition with excellent overall model fit (RMSEA < 0.001, CFI>0.999, SRMR = 0.020).

Validity

Previous studies have shown insufficient effort can account for a significant proportion of variance in overall cognitive test performance in OEF/OIF veterans with mTBI (Nelson et al., 2010). To assess for engagement during cognitive testing, four effort measures were administered, and participants whose performance failed to meet effort thresholds on more than one of the tasks were also excluded from subsequent analyses. Effort measures and thresholds were as follows: Victoria Symptom Validity Test (43 or fewer correct answers; Jones, 2013); WAIS-III Digit Span (scaled score of 5 or lower; Babikian, Boone, Lu, & Arnold, 2006; DeJong & Donders, 2009; Young, Sawyer, Roper, & Baughman, 2012); CVLT-II Forced Choice measure (raw score of 14 or lower; Moore & Donders, 2009; Root, Robbins, Chang, & Van Gorp, 2006) TMT-A task (48 seconds or higher; (Iverson, Lange, Green, & Franzen, 2002). In addition, participants were excluded from subsequent analyses if their MMPI-2-RF profiles were invalid based on any of the following criteria: VRIN-r > 80, TRIN-r > 80, F-r = 120, Fp-r > 100, L-r > 80, K-r > 70 (Ben-Porath 2012).

Of 221 participants who originally completed the cognitive battery, 13 (5.80%) were excluded due to invalid MMPI-2-RF profiles. Five participants (2.37%) were excluded due to failing more than one effort measure, with 3 failing two effort measures and 2 failing three effort measures. After excluding these 18 participants for invalidity or insufficient effort, 203 participants were included in the final analyses.

Analytic Approach

PTSD severity and variables reflecting each participant’s blast mTBI and impact mTBI experience were tested separately for possible mediation using 10,000 bootstrapped samples to assess the indirect effect of mediators on the relationship between objective cognitive performance and subjective cognitive complaints. Mediation analyses that reached statistical significance were followed by post hoc moderated mediation analysis using the PROCESS macro (Hayes, 2018) in SPSS, where the remaining variables were tested to determine their impact on the mediational relationship. Importantly, the present study uses well-validated measures, and all analyses were limited to veterans who demonstrated acceptable symptom and performance validity, which increases the reliability of the findings.

RESULTS

Assessment Outcomes

Results are based on the 203 valid participants. Based on the CAPS interview, 104 participants (51%) met criteria for current or lifetime history of PTSD. Based on the MN-BEST assessment, 156 participants endorsed at least one mTBI, with 22 endorsing two, 42 endorsing three, and 82 endorsing four or more mTBIs. Both mTBI and PTSD were assessed across the entire lifetime, and were not limited specifically to deployment-related events. For the MMPI-2-RF COG scale, 63 participants scored above the 65-point cutoff (reflecting diffuse cognitive difficulties) and 29 scored above the 81-point cut off (reflecting pronounced cognitive problems; Ben-Porath & Tellegen, 2008). The 18 participants that were excluded due to invalidity or insufficient effort did not differ significantly in terms of age, race, years of education, CAPS score, or MN-BEST score from those included in the analyses. See Table 1 for complete summary statistics of participants who were included in the analyses.

Table 1.

Descriptive statistics for measurements included in regression and mediation analyses.

Demographic Characteristics N % M SD Observed range
Gender
 Male 191 94
 Female 12 6
Age, range in years 33.33 7.97 22–60
 20–30 95 46
 31–40 68 34
 41–50 32 15
 51–60 8 4
Ethnicity
 Caucasian 147 72
 African American 6 3
 Native American 6 3
 Hispanic 4 2
 Other/Mixed Race 40 20
Education 14.61 1.82 12–20
Subjective Cognitive Complaints (MMPI-2-RF COG T score) 64.71 14.69 40–96
 ≥ 64 pt. COG score cut off 63 28.13
 ≥ 81 pt. COG score cut off 29 12.94
CAPS Severity 33.79 28.41 0–120
 Current DSM-IV PTSD Diagnosis 60 29.56
 Lifetime DSM-IV PTSD Diagnosis 100 49.26
Blast mTBI 1.5 2.19 0–17
Impact mTBI 1.88 2.52 0–21
Zero mTBI events 48 23.65
One mTBI event 9 4.43
Two mTBI events 22 10.84
Three mTBI events 42 20.96
Four or more mTBI events 82 40.39
Time since injury (in months)1 65.96 41.90 10–331
Time since deployment (in months)2 43.57 30.59 0–185
Loss of consciousness (in minutes) 1.64 4.61 0–30
Objective Cognitive Performance 0.03 0.31 −0.97–0.94
 2.5 SD below norm. 6 2.68
CVLT-II Trails 1–5 (z) 0.06 0.98 −2.90–2.50
WAIS-III Coding (SS) 9.87 2.53 3–16
WAIS-III Digit Span (SS) 10.03 2.46 4–17
Trails A (t) 10.05 4.24 2–17
Trails B (t) 10.26 4.21 5–19
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Note: For standardized cognitive assessments, score type is included in parentheses. SS= scaled score, t= t score, z= z score. Objective Cognitive Performance reflects the score on the General Cognition latent factor score.

1

Time since injury reflects time since most recent mTBI.

2

Time since deployment reflects months since the participants’ most recent deployments.

Mediation Models

PTSD severity, blast mTBI, and impact mTBI were tested as individual mediators of the objective - subjective cognition relationship. As depicted in Figure 1, objective cognitive performance was significantly associated with PTSD symptoms (Path A) and with subjective cognitive complaints (Path C). Participants with greater PTSD severity were likely to report greater cognitive complaints (Path B). The indirect effect of objective cognition and subjective cognitive complaints through PTSD was significant (Path AB). Consequently, when PTSD severity was included in this model, the relationship between objective and subjective cognition was no longer significant (Path C′). The overall bootstrap analysis of the indirect effect was significant, indicating PTSD severity mediated the relationship between objective and subjective cognition.

Figure 1.

Figure 1.

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Overall mediation model for the role of PTSD in the relationship between objective cognitive performance and subjective cognitive complaints. Results show a mediatedmodel. Path C′ reflects the direct effect of Objective Cognitive Performance on Subjective Cognitive Complaints with Path AB (via PTSD) included.

The same steps were repeated in examining the mediating effect of blast mTBI on subjective and objective cognition. The analysis indicated the relationship between objective cognition and blast mTBI was not significant (β [SE]= −0.13[0.45], p=.77; 95% CI [−1.18, 0.51]) and blast mTBI was not associated with subjective cognitive complaints (β [SE]= −0.18[−0.45], p= .69; 95% CI [−14.59, 2.82]). Furthermore, a bootstrap analysis indicated that blast mTBI did not have a significant mediating effect (95% CI [−0.54, 1.20]).

The possible mediating effects of impact mTBI between objective and subjective cognition was analyzed next. The relationship between objective cognition and impact mTBI was not significant (β [SE]= 0.43[0.50], p=.33; 95% CI [−0.50, 1.48]). The association between impact mTBI and subjective cognitive complaints was not significant [SE]= 0.23[0.35], p=.51; 95% CI [−0.46, 0.93]). The bootstrap analysis revealed impact mTBI did not mediate the relationship between objective and subjective cognition (95% [−12.51, −0.72]).

We next aimed to examine the hypothesis that the mTBI variables would moderate the mediating relationship of PTSD severity on subjective and objective cognition. When blast mTBI was added to the model as a moderator (see Figure 2), there was no significant interaction between blast mTBI and objective cognitive performance on PTSD severity (Interaction 1), nor was there a significant interaction between blast mTBI and PTSD severity on subjective cognitive complaints (Interaction 2). These results indicate that blast mTBI did not have a moderating effect on the mediating role of PTSD (Paths A and B).

Figure 2.

Figure 2.

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Overall moderated mediation model detailing the degree to which the mediating effect of PTSD is moderated by blast mTBI. The interactions reflect no significant moderation of this model.

Next, impact TBI was tested for moderated mediation using the same procedure (see Figure 3). When impact mTBI was added to the model as a moderator, there was no significant interaction between impact mTBI and objective cognitive performance on PTSD severity (Interaction 1), nor was there a significant interaction between impact mTBI and PTSD severity on subjective cognitive complaints (Interaction 2). As such, there is no indication that impact mTBI moderated the mediating role of PTSD (Paths A and B).

Figure 3.

Figure 3.

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Overall moderated mediation model detailing the degree to which the mediating effect of PTSD is moderated by impact mTBI. The interactions reflect no significant moderation of this model.

DISCUSSION

The aim of this study was to examine what role common sequalae of combat exposure have on the relationship between actual cognitive performance and perceptions of cognitive ability in OEF/OIF veterans. We demonstrated that although self-reported cognitive symptoms were in fact associated with lower performance on objective measures of cognition, this relationship could be accounted for by PTSD symptoms. Even though previous studies have found that a history of mTBI may impact the severity of cognition-related PTSD symptoms (Donnelly, Donnelly, Warner, Kittleson, & King, 2018) we failed to identify any significant moderation of mTBI (impact or blast) on the mediating effect PTSD severity has between subjective cognitive complaints and objective cognitive performance.

Previous research suggests mTBI may exacerbate PTSD severity, specifically in the cognitive domains of concentration, memory, decision-making and processing speed (Donnelly et al., 2018; Nelson et al., 2015), but that relationship was not observed in the present study. The lack of moderation or mediation involving mTBI indices could be due to multiple reasons. It is possible that any cognitive symptoms following an mTBI may dissipate in the acute recovery stage and therefore have minimal impact on subjective cognitive complaints. Therefore, it is likely that any lingering cognitive impairment may be driven more so by psychological, rather than physiological factors. PTSD severity has been shown to impact negative self-focused thoughts including self-criticism and beliefs about self-efficacy (Samuelson et al., 2017; Spencer et al., 2010) either of which could bias self-assessment of cognitive ability. Samuelson and colleagues (2017) suggested that negative posttraumatic appraisals may influence one’s perceptions of memory and attention functioning. Although there is considerable evidence suggesting that the effects of mTBI dissipate within approximately 3 months (McCrea et al., 2009), a recent review incorporating complicated and uncomplicated mTBI suggests that recovery time may be longer in many cases (McInnes et al., 2017). Future research investigating psychological processes following mTBI, such as how self-efficacy and self-referential processes influence self-reported cognition, could help clinicians understand the etiological factors contributing to cognitive complaints, thus improving the efficiency of clinical services by cutting down on unnecessary assessments and treatments.

The mediating role of PTSD severity between subjective and objective cognition highlights the importance of recognizing the distinction that exists between cognitive symptoms (i.e. those that impact an individual’s life) and cognitive impairments (i.e. those assessed by neuropsychological battery). By recognizing the mediating role of PTSD severity, we hope to emphasize that the two constructs are discrete phenomena that, while frequently coinciding, ought not to be conflated. Both symptoms and impairments have a place in the clinical conceptualization process. This can serve as an important point of psychoeducation for clinicians as they assist veterans to understand their difficulties during the therapeutic process. For example, clinicians might encourage patients that they can anticipate improvement in cognitive symptoms coincident with greater management of PTSD symptoms.

Several limitations of this study should be noted. First, the cross-sectional design of this study makes it difficult to infer causality from the relationships presented. Many of the reported mTBIs took place during incidents that would meet criteria for a traumatic event, thus complicating the ability to retrospectively discern the relative contributions of each event. Future longitudinal studies may allow for modeling the impact of both processes over time, which could then facilitate more causal interpretations. Second, mTBI data was based on retrospective self-report of events that may have taken place many years prior to the time of the assessment. Although this limitation was offset by the use of trained neuropsychologists, the reliability of self-reported recollections can be compromised, particularly when alterations to cognition (such as loss of consciousness) are involved. Third, data was not collected reflecting each veteran’s military and post-military experience, such as length of service, duty status, age at deployment, and service-connected disability. Such factors may plausibly influence cognitive performance, though we expect that any cases of exaggerated or underreported symptoms would have likely been excluded based on validity criteria. Lastly, information regarding chronicity of PTSD symptoms was not collected. Previous research has linked cognitive performance to PTSD duration and course (Johnsen & Asbjornsen, 2008; Vasterling et al., 2018) and inadequate termination of stress responses may result in biological alterations resulting in long-term consequences (Yehuda, 2000; Kitayama et al., 2005). However, data on time since deployment was collected, and since most index traumas occurred during deployment, this is at least a rough proxy of the association we might expect to see between chronicity and cognitive symptoms. When examined, however, there was no association between time since deployment and either objective cognition (p= .94) or subjective cognition (p= .45). Future research would benefit from investigating longitudinal changes in cognitive performance, with emphasis on how changes relate to chronicity of PTSD symptoms.

Nevertheless, the present results can play a key role in the assessment and case conceptualization of those experiencing comorbid mTBI and PTSD. Such patients frequently report cognitive problems, and providers often struggle with how to interpret self-reports due to potential bias and inconsistencies. Some patients may also be aware of how their diagnosis can impact their memory and attention and be especially vigilant of any perceived deficits. The present results reaffirm the importance of a broad and comprehensive assessment for those reporting cognitive complaints after trauma, with a particular emphasis on the diagnosis and treatment of trauma-related psychopathology as a primary intervention.

Public Significance Statement:

This study outlines how the relationship between cognitive complaints and cognitive performance may be attributable to trauma-related psychopathology, such as posttraumatic stress disorder (PTSD) and mild traumatic brain injury (mTBI), in combat veterans. This result emphasizes the importance of assessing for the psychological impact of trauma as part of a comprehensive neuropsychological assessment, as it may prove to be a more salient and promising target for rehabilitative care for those reporting subjective complaints.

Acknowledgements:

The authors would like to thank the participants for generously sharing their time and experiences with us. We would also like to acknowledge the efforts of Carly Anderson, Bridget Doane, Greg Lamberty, Craig Marquardt, Kathryn McGuire, Matt Margraff, Jessica Jones, Daniel Goldman, and James Hoelzle for their assistance with consensus review of mTBI and clinical diagnoses. The views expressed in this article are those of the authors and do not necessarily represent the views of the Department of Veteran Affairs.

Funding:

This work was supported by Congressionally Directed Medical Research Program (W81XWH-08-2-0038: PI-Sponheim) and the Department of Veterans Affairs, Rehabilitation R&D Program (101RX000622: PI-Sponheim; 1IK1RX002325: PI-Disner).

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