A novel comprehensive model tackles arcane PTSD differences in neural activity

July 19, 2020

Toshinori Chiba (ATR) and his collaborators have proposed an innovative new "Reciprocal Inhibition Model" of post-traumatic stress disorder (PTSD), which may aid considerably in its treatment. This model comprehensively explains the inhomogeneous nature of PTSD by addressing inter- and intra-patient variabilities at the neural, attentional, and symptom levels. This model may therefore pave the way for individual-tailored clinical PTSD treatment.

In the current climate where COVID-19 is pervasive, this new model of PTSD may be particularly pertinent. This is because patients recovering from COVID-19 are beginning to display "PTSD-like" adverse psychological effects related to fear and anxiety. Some example patients who have recovered from COVID-19, such as Stuart Gannaway (pseudonym), age 20, report symptoms such as increased fearfulness and mentally re-living their time in hospital over and over again. Others, such as Anabelle Hendy (pseudonym), age 34, report a different subset of psychological symptoms. Anabelle reflects "(after my COVID-19 diagnosis) I don't feel like myself anymore... I kind of feel like there is a veil between myself and reality, if that makes sense? I keep zoning out. I avoid talking about corona with people from work- I am worried they would be scared of me if they found out I had it, even though I am recovered now". A comprehensive explanation of why, after trauma, different patients develop different psychological symptoms is provided by the new "Reciprocal Inhibition Model". The proposals of this model pave the way for novel individual-specific clinical treatments. This model may therefore be of particular use under the current circumstances, where the number of physically-recovered yet mentally-still-traumatized Covid19 patients is increasing.

In movies, patients with PTSD are commonly portrayed as people who remain on high alert for threat and who often mentally re-live past traumatic events (similar to the symptoms reported by Stuart, above). While this is often true, paradoxically some patients with PTSD do their best to avoid threat and its associated memories, resulting in a dissociation from reality and a numbing of their emotions (similar to the symptoms reported by Anabelle, above). In line with these two clusters of symptoms, current clinical diagnostic criteria divide patients into two PTSD subtypes. However, the newly proposed "Reciprocal Inhibition Model" submits that differences between patients with PTSD may not be this black and white, but instead span the whole range of grey in between. It further proposes something not taken into account under current diagnostic criteria- that differences likely occur within individual patients so that each individual does not always consistently experience the same symptoms, but can switch between different states with different symptoms. Indeed, when the two recovered COVID-19 patients described above were further questioned, Stuart confides that he does sometimes find himself "zoning out" and Anabelle admits that she does sometimes find herself replaying her "darkest times with corona", indicating that these patients do sometimes switch between different subsets of symptoms.

What does "reciprocal inhibition" mean? To comprehend this, imagine two children fighting for your attention- each tries to cover the mouth of the other so that only the child who is currently winning can be heard clearly at any given time. This is similar to what happens in the brain during "reciprocal inhibition", where two neural regions alternate between dominating each other. The newly proposed model is called the "Reciprocal Inhibition Model" of PTSD, because, in this model, reciprocal inhibition between two distinct regions of the brain- the amygdala and the ventrolateral prefrontal cortex- is proposed to be what causes individual patients to switch between different states with different symptoms.

The "Reciprocal Inhibition Model" of PTSD predicts that when participants are in an amygdala dominant state, their attention will be biased towards threat causing them to experience relatively more fear-related symptoms, such as increased mental re-living of past traumatic events. Recovered COVID-19 patients, who display these kind of symptoms (such as Stuart, reported above), may be overly spending time in this state. On the other hand, when participants are in a ventrolateral prefrontal cortex dominant state, their attention will be biased away threat causing them to experience relatively more avoidance-related symptoms, such as a dissociation from reality and emotional numbing. Recovered COVID-19 patients who display these kind of symptoms (such as Anabelle, described above) may be overly spending time in this state. This model supposes that PTSD subtype diagnoses, that are still being used in the clinic today, might reflect categorical judgements based on the ratio of time that patients spend in each of these states.

In the paper which proposes the Reciprocal Inhibition Model of PTSD, experimental evidence is provided that supports the idea that attention alternates- even within the same individual patient- so that sometimes it is biased towards and sometimes it is biased away from threat. Furthermore, experimental and meta-analysis evidence is provided that supports the predicted relationship between attention and symptoms, and the predicted relationship between amygdala activity and symptoms. Other aspects of this model still need to be tested directly, but are well supported by existing literature.

If individual patients do alternate between states, as predicted in this model, then this has big implications for individual-tailored clinical treatments of PTSD. This is because different clinical treatments for PTSD have been shown to be more or less effective dependent on the amygdala reactivity of the patient (e.g. exposure therapy is less effective when amygdala reactivity is high). Until now, clinicians may have selected treatments for an individual patient with PTSD based on their diagnosed subtype (e.g. amygdala reactivity is thought to be exaggerated in the non-dissociative subtype, so people with this subtype might be less likely to be prescribed exposure therapy). However, if the predictions of the Reciprocal Inhibition Model are true, then this means that different treatments might be more or less effective for the same individual patient with PTSD depending on the state that they are currently in. This may also apply to patients experiencing emotional distress after COVID-19. Quick measurement of the patient's current state (symptoms could be used as a proxy) might inform clinicians of the most appropriate treatment for an individual patient during any given therapy session.

The Reciprocal Inhibition Model of PTSD includes novel proposals about the neural mechanisms that underlie PTSD. Elucidation of such mechanisms is vital if, for the treatment of PTSD, one wishes to use methods such as neurofeedback (where certain regions of the brain, or the way that certain regions of the brain work together, are "retrained") or transcranial magnetic stimulation (where, essentially, magnets are used to induce or decrease activity in targeted regions of the brain). Cutting-edge research is showing much benefit from these kinds of treatments and so further studies to confirm the neural mechanistic proposals of this model are imperative.
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ATR Brain Information Communication Research Laboratory Group

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