Sleep and semanticisation
An important issue in memory research is the question of why recently formed representations depend on the hippocampus while older ones do not. The related questions of how this transition occurs, and how it relates to the transformation from episodic memory to semantic knowledge are also of interest. One account, the complementary learning systems framework (CLS)  posits that the hippocampus learns new information rapidly and this gradually transfers to the cortical system through overnight replay. The resulting cortical representations code stable decontextualised memories, forming the basis of semantic knowledge .
Two studies have recently supported the CLS [2,3] by showing that activity in hippocampus and ventromedial prefrontal cortex (VMPFC) have a reciprocal relationship in retrieval, with hippocampal involvement decreasing over a 6 month period while VMPFC involvement increases in parallel (figure 1). Importantly, aspects of this transfer correlate with slow wave sleep after training  and the process is disrupted if subjects are sleep-deprived on the post-training night . These findings suggest that post-training sleep is critical for the transfer of information from hippocampal to cortical representations, however they do not relate specifically to semantic memory since the tasks involved are episodic (picture identification  and word-pair learning ).
Work in NARU has shown that the anterior temporal lobe (ATL) is essential for semantic memory, and particularly for abstract representations bridging multiple modalities . We therefore expect increased involvement of ATL as memories transition from episodic to semantic, and more so for multimodal representations. Testing this hypothesis is an important aim of current work in the sleep and memory group.
- McClelland J.L., McNaughton BL, O'Reilly RC (1995) Why there are complementary learning-systems in the hippocampus and neocortex: Insights from the successes aand failures of connectionist models fo learning and memory. Psychological Review 102: 419-457
- Gais S, Albouy G, Boly M, ng-Vu TT, Darsaud A, Desseilles M, Rauchs G, Schabus M, Sterpenich V, Vandewalle G, Maquet P, Peigneux P (2007) Sleep transforms the cerebral trace of declarative memories. Proc Natl Acad Sci U S A 104: 18778-18783
- Takashima, A., Petersson, K. M., Rutters, F., Tendolkar, I., Jensen, O., Zwarts, M. J., et al. (2006). Declarative memory consolidation in humans: A prospective functional magnetic resonance imaging study. Proceedings of the National Academy of Sciences of the United States of America, 103(3), 756-761
- Dumay N, Gaskell MG (2007) Sleep-associated changes in the mental representation of spoken words. Psychol Sci 18: 35-39
- Spencer RM, Sunm M, Ivry RB (2006) Sleep-dependent consolidation of contextual learning. Curr Biol 16: 1001-1005
- Barnes JM, Underwood BJ (1959) Fate of first-list associations in transfer theory. J Exp Psychol 58: 97-105
- Rogers, T. T., & Mayberry, E. (2008). Sleep promotes memory for atypical or idiosynchratic semantic information. unpublished
- Lambon Ralph, M. A., & Patterson, K. (2008). Generalization and differentiation in semantic memory: insights from semantic dementia. Annals of the New York Academy of Sciences, 1124, 61-76
- Meeter, M. Murre, JM. (2004). Consolidation of long term memory: evidence and alternatives. 130(6), 843-57