Neurological Reactivity of Combat Veterans with PTSD Following Exposure to Trauma-related Auditory and Olfactory Cues: An fNIRS Study
November 22, 2017
In recent years, national agencies and clinical research centers began advocating for integrative approaches that capture behavioral, genetic, and neurophysiological mechanisms to better understand complex and chronic disorders such as posttraumatic stress disorder (PTSD). This initiative in psychiatric research has followed models of precision medicine evident in other medical fields, such as oncology (Collins & Varmus, 2015). At this time, clinicians assessing PTSD among military personal rely on diagnostic or self-report instruments, which are vulnerable to bias or malingering (Frueh et al., 2003). A biological or objective indicator of PTSD may strengthen diagnostic screening, tailor treatment planning, and improve detection of remission. Neurophysiological markers such as functional near-infrared spectroscopy (fNIRS) may provide incremental validity beyond self-report and advance allocation of resources to U.S. service men and women with PTSD.
Contemporary theories on neural activity of PTSD posit disruptions across the prefrontal cortex (PFC), amygdala, and hippocampus (Ross et al., 2017). Specifically, these regions govern emotion regulation, fear reactivity, and memory consolidation (Koenigs & Grafman, 2009; Shin, Rauch, & Pitman, 2006). The PFC is within the range of depth for fNIRS imaging (Tian & Liu, 2014), and as such, this brain region is applicable to fNIRS investigation. In addition, fNIRS is a portable device that allows naturalistic body positioning, emits no acoustic noise, and is less susceptible to motion artifacts than functional magnetic resonance imaging.
U.S. service men and women encounter horrific or life-threatening experiences during combat that are often associated with certain sounds, such as explosions. Exposure to combat-related sounds is a promising method to identify neurophysiological markers associated with PTSD (Bremner et al., 1999; Liberzon et al., 1999). In contrast to auditory cues, olfaction remains an understudied sensory modality with trauma despite evidence for odors eliciting more evocative and emotional responses over auditory, visual, and verbal cues (Herz, 2004). A recent review linked olfactory cues and traumatic memories to emotional processing regions of the PFC and limbic system (Daniels & Vermetten, 2016). As such, our study attempted to push the field of behavioral neuroscience further by measuring neurological reactivity among combat veterans with and without PTSD during presentation of trauma-related sounds and odors.
This investigation collected neurological reactivity to trauma-related auditory and olfactory cues across three groups – combat veterans with PTSD (n = 16), combat veterans without PTSD (n = 16), and nonmilitary participants without PTSD (n =16). All participants included in the study scored acceptable smell acuity using the University of Pennsylvania Smell Identification Test (Doty, Shaman, & Dann, 1984) and no hearing difficulty was observed during the experiment. We screened for PTSD using the Clinician-Administered PTSD Scale (CAPS; Blake et al., 1995). Participants received the following stimuli in a randomized counterbalanced fashion: trauma-related sound (explosion), trauma-related odor (diesel fuel), neutral sound (phone ringing), neutral odor (n-Butanol), negative sound (dentist drill), and negative odor (rotten egg). Following presentation of each negative, neutral, and trauma-related stimulus, participants’ provided hedonic and intensity ratings.
Results indicated combat veterans with PTSD displayed significant neural activation during presentation of the trauma-related sound compared to combat veterans and nonmilitary participants without PTSD. Specifically, this increased activation was located approximately at the right superior medial PFC (Broadmann’s areas 9/10). In addition, combat veterans with PTSD rated the trauma-related sound as significantly more unpleasant than combat veterans and nonmilitary participants without PTSD, whereas no group differences emerged between the latter two groups. During presentation of the trauma-related sound, the following PTSD severity scores demonstrated significant positive correlations with neural activation in the right superior medial PFC: CAPS total severity score, hyperarousal total subscale score, and avoidance total subscale score. Although combat veterans with PTSD rated the trauma-related odor as significantly more unpleasant than nonmilitary participants, no significant group differences emerged for neural activation after correction for multiple testing.
Increased activation of the right superior medial PFC is consistent with other symptom provocation analyses which found increased activation in the right superior/middle frontal gyrus between individuals with PTSD and trauma-exposed controls (Sartory et al., 2013; Vermetten, Schmahl, Southwick, & Bremner, 2007). The right medial superior PFC is associated with experiencing negative or threat-related stimuli (Kalisch & Gerlicher, 2014), as well as emotional detachment (Falquez et al., 2014; Koenigsberg et al., 2010). Avoidance of thoughts, feelings, or situations related to the traumatic event is a hallmark symptom of PTSD. Combat veterans with PTSD might have displayed increased activation of the right superior medial PFC to emotionally detach from unpleasant stimuli, whereas combat veterans and nonmilitary participants without PTSD did not experience the explosion sound as unpleasant, and as such, did not feel the need to detach.
We believe this study has direct implications for the assessment and treatment of combat-related PTSD. At this time, neurological measurement cannot replace observable signs and symptoms of PTSD. Nevertheless, it could foster an individualized and comprehensive assessment that promotes treatment of this complex and chronic disorder.
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- How can clinicians incorporate fNIRS assessment of trauma-related cues with first-line interventions for PTSD such as exposure therapy?
- What are some potential reasons fNIRS measurement during presentation of the trauma-related odor failed to differentiate combat veterans with PTSD compared to combat veterans and nonmilitary participants without PTSD?
Gramlich, M.A., Neer, S. M., Beidel, D. C., Bohil, C. A., & Bowers, C. A. (2017). A functional near-infrared spectroscopy study of trauma-related auditory and olfactory cues: Posttraumatic stress disorder or combat experience? Journal of Traumatic Stress.
Michael A. Gramlich, M.S. is a fourth-year clinical psychology doctoral student at the University of Central Florida (UCF). His research focuses on the relationship between trauma-related stimuli and posttraumatic stress disorder (PTSD) in an effort towards better understanding the neurological markers of PTSD. He is currently a graduate student clinician at the UCF RESTORES clinical research center in the UCF Psychology Department.
Sandra M. Neer, Ph.D. is a clinical psychologist on the faculty of the Psychology Department at the University of Central Florida (UCF). She is Director of Clinical Services at UCF RESTORES, a clinical research center for trauma. Her research focuses on the outcome of evidence-based treatments for posttraumatic stress disorder (PTSD).
Deborah C. Beidel, Ph.D., ABPP is Trustee Chair and Pegasus Professor of Psychology and Medical Education and Director of UCF RESTORES at the University of Central Florida. Her recent work focuses on developing effective treatments for PTSD for veterans, active duty personnel, first responders, and other victims of trauma utilizing technology to enhance effective treatments and translate them into standard clinical practice.