The neurocognitive basis of autobiographical memory

Modern conceptualisation of autobiographical memory distinguishes between episodic and personal semantic components. In essence, episodic autobiographical memory is the system that allows conscious reliving of personal experiences, a subjective travelling back and then forwards in time to recreate the unfolding of a specific past event. Recollecting autobiographical events is reconstructive and subjective, influenced by many factors including the age and nature of the event and the characteristics of the individual retrieving it. In contrast, personal semantic memory refers to factual information about oneself and in this sense is not event based. The distinction is of central importance in ongoing debates concerning the neural bases of long-term memory storage and retrieval.

Much of my recent work has focused on the neural bases of autobiographical memory, specifically the role of medial temporal lobe (MTL) structures in encoding, storage, and retrieval. Neuropsychological studies of amnesic patients with focal brain lesions and neuroimaging studies of healthy participants indicate at least partly independent neural networks serving semantic and autobiographical episodic memory. There are two main theoretical claims in the literature. According to classic consolidation theory, successful encoding and storage of episodic and personal semantic information is initially dependent on the hippocampal formation but over time becomes represented in the neocortex, independently of MTL structures. In contrast, multiple trace theory (MTT) claims that recollection of genuinely episodic autobiographical memories is always dependent on MTL structures (irrespective of the age of those memories). Although the hippocampus is also critically important in the formation and assimilation of semantic information, recollection of more remote semantic memories is claimed to operate independently of the hippocampal complex.

Many of the published studies are interpreted as strongly supportive either of the consolidation or of the MTT theoretical framework, suggesting that neither model can fully account for the patterns of episodic and semantic memory performance in amnesic patients or for the neural correlates observed across different neuroimaging studies of the healthy brain. Our work, which employs detailed lesion delineation and quantification and the development of highly sensitive tests of episodic and semantic memory, also challenges current models. We are now developing a new body of research focusing on interdependencies among types of memory at all stages from initial encoding to conscious retrieval.

A working hypothesis is that a single flexible binding system serves both autobiographical episodic memory and conceptual knowledge. In this sense, MTL structure(s) serve a function whereby temporal as well as perceptual, affective and factual information about an event is integrated to provide a coherent autobiographical memory. Damage would effectively loosen this integration such that the episodic quality of an autobiographical memory is impaired. If we can detect evidence for a general binding process that encompasses episodic as well as conceptual binding, it should render obsolete notions of episodic and semantic memory as independent and dissociable functions.