According to Schacter and Tulving (1994), human memory is subserved by a number of mind–brain systems including the episodic and semantic systems (see Gardiner, this volume, Chapter 1). The episodic system encodes and retrieves personally experienced event memories, which consist of complex collections of features including spatial, temporal and contextual information. Conversely, the semantic system is largely responsible for factual memory and general knowledge. These memories are not generally associated with spatial, temporal or contextual information. Crucially, retrieval from each system is associated with a distinct form of conscious awareness: episodic retrieval – or ‘remembering’ – involves autonoetic (self-knowing) consciousness, whereas semantic retrieval – or ‘knowing’ – involves noetic consciousness. Thus, semantic retrieval might involve, for example, bringing a known fact to mind, whereas episodic retrieval might involve remembering the particular episode during which that fact was learned. The episodic system underlies the capacity for ‘mental time travel’ (Suddendorf & Corballis, 1997; Wheeler, Stuss & Tulving, 1997) – either mentally projecting oneself into the past to re-experience an event or mentally ‘projecting oneself into the future to pre-experience [italics added] an event’ (Atance & O'Neill, 2005, p. 127).
Individuals with an autistic spectrum disorder (ASD) generally perform well on tasks that require semantic (or other) memory processes, but perform poorly on tasks that demand the support of the episodic system (see this volume, Chapters 12, 14 and 17; also Ben Shalom, 2003; Bennetto, Pennington & Rogers, 1996; Minshew & Goldstein, 2001).