Neuroimaging and Neuropsychological Mechanisms in Selective Dissociative Amnesia

By: Mehrun Nisa 

Dissociative amnesia is a controversial psychiatric disorder, and in the past two decades, interest in understanding its pathophysiology has significantly increased. Dissociative amnesia is characterised by functional impairment. Additionally, preliminary data suggest that affected people have an increased and possibly underestimated suicide risk. The prevalence of dissociative amnesia differs substantially across countries and populations. Symptoms and disease courses also vary, indicating a perhaps heterogeneous disorder. Clinical features vary across cultural groups. Most dissociative amnesias are retrograde, with memory impairments mainly involving the episodic-autobiographical memory domain. Anterograde dissociative amnesia occurring without significant retrograde memory impairments is rare. 

Functional neuroimaging studies of dissociative amnesia with prevailing retrograde memory impairments show changes in the network that subserves autobiographical memory. There have been some neuroimaging studies of dissociative amnesia, but the findings are diverse 

and the interpretation of the data is still controversial. Some PET studies with a single patient have reported reduced activity in the right hemisphere (Markowitsch et al., 1997), decreased activity in the right frontal and temporal regions (Markowitsch et al., 1997)), and reduced activity of the hippocampus with increased activity of the anterior medial-temporal lobe(MTL) including the amygdala (Yasuno et al., 2000) during memory retrieval tasks. 

In fMRI experiments, Yang et al. (2005) investigated a patient with dissociative amnesia and showed deactivation in the MTL when the patient was viewing faces that were unrecognizable to her because of her memory impairment. Botzung et al. (2007) reported a patient who showed greater activity in the posterior cortical regions and reduced activity in the frontal networks and left MTL, associated with the retrieval of autobiographical memory stemming from the forgotten period. Hennig-Fast et al. (2008) showed that during the patientʼs acute state, the temporo-occipital part of the network known to be related to autobiographical memory was not activated. 

Although the findings of the fMRI experiments tend to indicate dysfunction of the MTL in patients with dissociative amnesia, the reason why the deactivation occurs in the regions responsible for memory processing has not been fully clarified. According to one notable theoretical model, frontal control systems precipitate memory inhibition in dissociative amnesia (Kopelman, 2000, 2002). This theory, together with fMRI studies with healthy participants (Mendelsohn et al., 2008; Depue et al., 2007; Anderson et al., 2004), led us to hypothesize that prefrontal regions play an important role in dissociative amnesia. 

In the present study, we conducted fMRI experiments combined with face and name recognition tasks in two patients with dissociative amnesia: Patient 1, a 27-year-old man, and Patient 2, a 52-year-old man. Both patients had profound and time-limited retrograde amnesia; for Patient 1, it involved the period between his last half year at college and onset (approximately 4.5 years), and for Patient 2, it involved the period from his time at university to onset (approximately 35 years). Both patients could easily retrieve autobiographical memories for the years between childhood and high school. 

After therapeutic sessions of interview using intravenous injection of sodium thiopental (Kopelman, 2000, 2002), Patient 1 could recover most of the memories he had lost due to amnesia, whereas the retrograde amnesia of Patient 2 remained unchanged. These rare cases enabled us to systematically investigate the neural mechanisms underlying dissociative amnesia with a well-controlled experimental design. 

To our knowledge, the present study is the first to provide direct evidence that dissociative amnesia is associated with prefrontal activation and hippocampal deactivation. The present fMRI study revealed that increased activity in the pFC and decreased activity in the hippocampus are associated with dissociative amnesia. After treatment for retrograde amnesia, these activation changes disappeared in one patient, as his retrograde memories were recovered, whereas they remained unchanged in the other patient, as his retrograde memories were not recovered. 

These results strongly indicate that the changes in activation were specific to the patientsʼ's deficit in retrieving retrograde memories. 

References 

● Staniloiu, A., & Markowitsch, H. J. (2014). Dissociative amnesia. The Lancet Psychiatry, 1(3), 226–241. 

● Kikuchi, H., Fujii, T., Abe, N., Suzuki, M., Takagi, M., Mugikura, S., Takahashi, S., & Mori, E. (2010). Memory repression: Brain mechanisms underlying dissociative amnesia. Journal of Cognitive Neuroscience, 22(3), 602-613.