The neurocognitive mechanisms underlying dissociative amnesia
Dissociative amnesia refers to the loss of autobiographical memory with a presumed psychological cause. This can involve memory loss for traumatic experiences, such as in PTSD. It can also involve more extensive amnesia for several years, or, in its most extreme form, a fugue state involving total loss of memory and sense of identity. These latter, ‘generalised’ forms of dissociative amnesia are rare, and the mechanisms underlying the memory loss are poorly understood. It has been theorised that the amnesia results from a prefrontally-mediated inhibition of memory systems, which is presumed to occur sub or semi-consciously (Kopelman, 2000, 2002). A parallel body of experimental work has defined a prefrontally-mediated ‘memory control' network, which can be engaged to inhibit memory retrieval voluntarily. These findings offer a candidate neurobiological mechanism for the inhibition of retrieval in dissociative amnesia, and provide a set of specific, testable predictions regarding the putative role of this mechanism in the memory loss.
In the current thesis, we systematically reviewed the literature on the neurophysiological correlates of dissociative amnesia, evaluating the existing evidence in support of this proposed neurobiological mechanism. Chapter 3 reports a reanalysis of existing fMRI data from two patients with dissociative amnesia (Kikuchi et al. 2010), in which we formally tested whether the patterns of neural activation associated with dissociative amnesia aligned with those observed during voluntary retrieval inhibition. We found strong evidence for the hypothesised prefrontally-medicated inhibition of memory systems. Thus, we attempted to replicate and extend these findings in an ongoing prospective case series. By combining qualitative, quantitative and neuroimaging methods, we aimed to develop a more cohesive understanding of the bio-psycho-(social) mechanisms underlying dissociative amnesia. Chapters 4-7 report the results from the first 3 patients, recruited to the study over a period of 2 ½ years, with neuroimaging completed in one case. Chapter 8 provides a discussion of these findings in relation to the broader clinical understanding dissociative amnesia, and in relation to current understanding of dissociative symptoms more broadly.