Objectives/Aims The neurocognitive mechanisms underlying dissociative amnesia are poorly understood. It has been proposed that personal memory retrieval is inhibited via top- down cognitive control mechanisms. We developed a behavioural paradigm to test this hypothesised mechanism in a prospectively-recruited case series.

Methods Participants included 3 patients with dissociative amnesia, alongside two groups of 12 healthy controls. Patients were shown photographs of people from their personal past who were either recognisable, or who were forgotten due to their amnesia. We hypothesised that seeing forgotten faces would lead to global downregulation of the hippocampus/medial temporal memory systems, which would temporarily impair hippocampally-mediated encoding and stabilisation processes. To test this, we presented novel ‘bystander’ images immediately before and after presentation of personal reminders. These included photographs of objects in particular location, and participants were asked to generate a story for how the object came to be in the given location. Later, a surprise memory test assessed memory for these bystander objects. We predicted that recall of bystanders presented before/after forgotten faces would be impaired relative to those presented before/after recognised faces. Healthy controls were shown i) faces from their recent vs remote past, and ii) familiar vs novel faces, and were predicted to show no difference in bystander recall across conditions.

Results In line with our hypothesis, all three patients showed impairments in later recall of novel bystander items that were presented immediately before and/or after forgotten faces, relative to recognisable faces. Neither group of healthy control showed differences across conditions, confirming that the deficit was not merely a consequence of seeing an unrecognisable face. The task was repeated with one patient following memory recovery, at which point he recognised all of the faces. We no longer observed any differences in bystander encoding between conditions.

Conclusions Results indicate that memory systems are reflexively and globally downregulated when people with dissociative amnesia are reminded of people they no longer recognise, impairing encoding of novel information.