Chapter 9. Dissociation and Neuroscience (2) the Mechanism of Switching (2)

Since the time that hysteria gained the new name, i.e., dissociation, it is not so much regarded as feigned or simulated as before, but there is still a tendency that dissociative disorder is not regarded as a well substantiated condition, and therefore, not a “real” illness. Whether DID really exists or not is still argued among European and American clinicians (Kluft, 2003).
Kluft, R.P. (2003) Current Issues in Dissociative Identity Disorder. Journal Bridging Eastern and Western Psychiatry, Vol. 1, pp. 71-87.
I stated before that hysteria could have been always there in the history of human beings, and its roots can even be found in the evolutionally process. If attacked, some animals such as opossum, armadillo, insects etc. display a feigned death in order to avoid receiving any further attack. This reaction, also called freezing response (see chapter 4) is regarded as a genuine physiological response. Human equivalent of the freezing response is tonic immobility and it is also equivalent to dissociation (see also chapter 4). In tonic immobility, there are a loss of voice, bradycardia, hypoventilation and other physiological characteristics (Schmahl, C, Bohus, M, 2007).
Schmahl, C, Bohus, M (2007) Translational Research Issues in Dissociation.. in Traumatic Dissociation: Neurobiology and Treatment edited by Eric Vermetten, Martin J. Dorahy, and David Spiegel. American Psychiatric Publishing.
Direct stimulus to the brain and the use of drugs can elicit dissociative switching
One of the biological bases for the switching is that there are locations in the brain which are involved in switching. This makes us assume that sudden activation of these locations gives rise to dissociative reaction. As I stated in the previous chapter, Penfield (1952) stimulated various locations in the cerebral cortex and yielded dissociative-like experiences. Recently, stimulus to the temporo-parietal junction is found to be related to out of body experiences. One study reported （De Ridder, et al. 2007）that a patient in whom electrodes had been implanted to suppress tinnitus experienced an out-of-body experience that was elicited during stimulation of the posterior part of the right superior temporal gyrus (see figure #). Also recently by the use of TMS (transcranial Magnetic Stimulus) researchers gave direct stimulus to the brain from outside of the cranium and gained the similar result（Blanke, Mohr, et al 2007）。
 De Ridder, D., Van Laere, K., Dupont, P., Menovsky, T., Van de Heyning, P. (2007) Visualizing out-of-body experience in the brain. N Engl J Med.;357(18):1829-33.
Blanke, O., Mohr, C., Michel, C.M., Pascual-Leone, A., Brugger, P., Seeck,, M., Landis, T., and Thut, G. (2007) Linking Out-of-Body Experience and Self Processing to Mental Own-Body Imagery at the Temporoparietal Junction. Science. Vol. 317. no. 5841, pp. 1096-1099.



(Figure # temporo-parietal junction (bilateral) )　

   It is well known that administration of some medications can cause dissociation-like experiences, and they are even called dissociative drugs. NMDA antagonists, serotonergic hallucinogens, morphine are among them. Especially well known dissociative drugs are PCP (phencyclidine) and ketamine. These NMDA antagonists are medically used as anesthetic drugs and are known to elicit out of body experiences, distortion of body image, etc. As NMDA receptors are abundant in hippocampus, it is hypothesized that stress acts on hippocampus to cause dissociation.
Brain’s involvement according to the “structural dissociation” theory
The theory that I mentioned in the Prologue, “structural dissociation” proposes brain’s implication in dissociation (van der Hart, et al. 2006). According to this theory, dissociation has two states, each having different neurophysiological characteristics. These are EP（emotional personality） and ANP（apparent normal personality）.
In ANP, daily living is typically devised to avoid dangerous stimuli. Frontal, occipital, parietal lobes are all activated for the individual to be able to focus on current perception and cognitive activities while amygdala’s involvement in emotion is suppressed and traumatic memories are ignored.
On the other hand, in the state of EP, the situation opposite to ANP occurs, where amygdala and insula are activated while hippocampus, anterior cingulate, medial prefrontal area are all suppressed.