ミラーニューロンと解離 24

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Neurological basis of dissociative symptoms

 The difficult problem that we discussed above, as to whether we should understand personalities in DID as independent individual or “partial” or “incomplete” might be easier to handle if there is any neurological basis or “neural correlate” of these personalities. If a personality A happens to be “located” in one side of the hemisphere whereas the personality B resides in the other without any overlapping area or communication between them, for example, we could assume that they are different and independent consciousnesses. Unfortunately, no study has so far indicated that each personality in DID has its distinct localization or neural network.
    However, there has been a considerable number of studies that are suggestive of biological basis of trauma-related or dissociative mechanism of PTSD and dissociative disorders. In 1980s, Bessel van der Kolk discussed biological basis for the formation of traumatic memories which underlies clinical manifestation of flashbacks and dissociated memories. He stressed that intense emotional experiences affect amygdala and hippocampus, and trauma-related memories are dissociated on the body/visceral level on a traumatized individual, in a way quite different from the ordinary formation of episodic memories.

　More recently, dissociative symptoms in PTSD has been the focus of study, which led to the notion of dissociative subtype of PTSD (PTSD + DS). The study suggests that PTSD+DS has the mean prevalence of 20.35% among PTSD population (Hansen et al.,2016) and are found to be related to increased re-experiencing symptoms, male sex, history of childhood trauma, history of trauma prior to the index trauma (Stein, et al, 2013). Studies found that PTSD+DS is to be associated with greater activity of areas of the frontal cortex that are involved in inhibiting brain areas that coordinate fear responses, such as the amygdala, as the study of van der Kolk suggested, but also it spans multiple brain areas, particularly those involved in sensory integration, giving rise to the complex subjective sense of dissociation (National Center for PTSD, 2018).  

Hansen, M., Műllerová, J., Elklit, A., & Armour, C. (2016). Can the dissociative PTSD subtype be identified across two distinct trauma samples meeting caseness for PTSD? Social Psychiatry and Psychiatric Epidemiology, 51, 1159-1169.
National Center for PTSD (2018) PTSD Research Quarterly. VOLUME 29/ NO. 3. ISSN: 1050 -1835.Stein, D. J., Koenen, K. C., Friedman, M. J., Hill, E., McLaughlin, K. A., Petukhova, M., . . . Kessler, R. C. (2013). Dissociation in posttraumatic stress disorder: Evidence from the world mental health surveys. Biological Psychiatry, 73, 302-312.

Along with these studies, Polyvagal theory proposed by Porges (2011) made a contribution in elucidating close relationship between autonomic nervous system and dissociation. He proposed three branches of our autonomic nervous system, including what he calls ventral vagal system (VC) which has been developed in mammalian animals but has never been delineated until his discovery. When in real crisis, this VC shuts down and if fight-flight response based on the sympathetic nerve fails, the dorsal vagal system activates which is largely responsible for dissociative process. Recent biological studies including Porges’ research help us understand how much dissociative mechanism in involved in our traumatic response and this led to the proposal of dissociative type of PTSD which now appears in the last version of DSM (DSM-5, 2013).
　Although this view helps clinicians understand the involvement of different area of the brain in the formation of dissociative experiences it remain still unclear how this mechanism is translated to the way massive entity such as a personality structure can be dissociated in the case of DID.