Searching for Signatures of PTSD in the EEG

PTSD is a constellation of symptoms resulting from traumatic stress and is difficult to study. However some directional clues and new use of EEG could change that.

Many people across the world experience traumatic events in their lives – war, natural disaster, violent crime, severe accidents, terrorism to name a few. But whilst many people are simply left with an unpleasant emotional imprint in their memory, a subset of individuals will experience prolonged post traumatic stress that can hinder their normal functioning.

Challenges of Understanding PTSD

Like most mental health ‘disorders’ Post Traumatic Stress Disorder or PTSD is a constellation of symptoms that are typically self- reported through questionnaires.  PTSD is typically characterized by an anxiety which is most often associated with symptomatic “flashbacks” of the traumatic experience and heightened emotional reactivity, but individuals can also have disturbances to their mood, as well as deficits in their memory and attentional processing, as well as other neurobiological changes (see here for a recent review). Some estimates suggest that the prevalence of PTSD  is ~7% in the US although this likely varies globally and could be much higher in some populations.

In effect PTSD can provide a view into the long term impact of significant trauma and stress on the brain. However, since typically the brains of individuals with PTSD have not been studied prior to the traumatic event, and given large individual variability and difficulty controlling for other influencers, it is exceptionally difficult to determine how the trauma has changed it.  Nonetheless there are some clues that point to future directions of study with EEG.  With the prospect of easier long term monitoring, EEG could become a useful tool to discover changes associated with PTSD, determine why some individuals are more susceptible to PTSD than others after experiencing trauma, and help develop therapeutic interventions.

ERP Studies in PTSD

There is a considerable body of evidence looking at the modulation of different task-evoked potentials (e.g. see here for a meta analysis of ERP studies). For example, one recent study published in Scientific Reports from researchers in the UK and the Netherlands, studied modulations in neural activity whilst patients with PTSD (n=13) were presented with an auditory oddball paradigm. They showed that the deviant tones (compared to the regular tones) led to increased theta power (5-7 Hz) over frontal sites and a stronger suppression of upper alpha activity (13–15 Hz) over right centro-parietal and occipital sites in patients vs a matched control group (n=13). They also showed a greater mismatch negativity potential in the patients vs controls for the deviant tones vs the regular tones. These findings potentially reflect an enhancement of involuntary attention towards unexpected auditory events, in line with the hypervigilant state which is often symptomatic of patients with PTSD. However, although this study did include an age and education matched control group, it had a relatively small sample size and only included male participants.

Resting State Studies

Other studies have examined resting-state EEG in patients with PTSD compared to healthy controls. One relatively consistent finding across a number of studies is that patients with PTSD display suppressed alpha activity especially across posterior sites, which is also in line with the finding above. This has been demonstrated in studies employing both eyes-open and eyes-closed procedures. For example, research from Florida State University, published in the journal Brain, showed suppressed alpha power across posterior brain sites when patients (n=25) rested with their eyes open (M-RS) and when they passively viewed scenes (S-RS) compared to normal controls (n=20), as well as a comparison group of patients with generalized anxiety disorder (B&C panels below). This was accompanied by increased gamma power across frontal sites (F&G panels below).

Alpha Activity

Gamma activity

Together the authors concluded that PTSD is characterized by a resting state of sensory hyperactivity, and a deficient inhibition of sensory projections to the frontal cortex, which leads to excessive frontal activity. They observe that this is different to emotional disturbances observed in generalized anxiety disorder.

Other abnormalities which have been found in some studies include increased beta activity over frontal regions, demonstrated by researchers in Croatia (see here and here), and changes in peak alpha frequency (see here) in patients with PTSD compared to healthy controls. For example, researchers from Oregon Health & Science University in the USA found that veterans with PTSD (n=59) exhibited higher peak alpha frequency with an eyes closed procedure, compared to a control group of veterans without PTSD (n=27).

See related post Alpha Oscillations and Attention

Frontal Asymmetry and PTSD?

In addition, another debate in the literature is the degree to which PTSD is associated with the frontal asymmetry which is often exhibited in patients with depression and other types of anxiety.  The data is still relatively inconsistent, potentially due to differences in methodological approaches across studies (see here for a recent review). For example, methods which simply employ resting-state EEG measures have typically failed to find robust evidence of frontal asymmetry when comparing between patients with PTSD compared to healthy controls or when looking for correlations between frontal asymmetry and scores from clinical PTSD scales (e.g. see here).

However, studies utilizing emotional interventions such as showing patients pictures which arouse fear or depict trauma have more successfully demonstrated frontal asymmetry. For example a study by researchers at Dresden University of Technology in Germany which used pictures to evoke fear and trauma, found greater right anterior and posterior activity in patients with PTSD after a motor vehicle accident (n=22) compared to controls who had also experienced a motor vehicle accident but who did not have a clinical diagnosis of PTSD, and healthy controls who had not experienced a motor vehicle accident (n=23).

Still a relatively inconsistent picture

EEG studies of PTSD so far display a mixed picture of effects about how the disorder may change the intrinsic properties of the brain. Modulations in alpha, gamma and beta frequencies have all been reported, as well as changes in frontal asymmetry and peak alpha frequency but there is no real clear understanding about how these fit together and manifest as PTSD relative to other conditions that have similar effects.  In addition, the relatively low number of studies and the small sample sizes of the studies conducted so far, particularly in the context of large individual variability, means that our ability to use EEG to identify at-risk individuals is still relatively limited to date. However further research that takes advantage of new EEG technology and analytics with larger scale studies could eventually tackle this emotionally debilitating disorder.

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