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Scientific Understanding of Consciousness |
Memory Consolidation and Associative Changes in Neocortical Code
Science 7 November 2008: Vol. 322. no. 5903, pp. 960 - 963 Spontaneous Changes of Neocortical Code for Associative Memory During Consolidation Kaori Takehara-Nishiuchi and Bruce L. McNaughton Arizona Research Laboratories, Division of Neural Systems, Memory, and Aging, University of Arizona, Tucson, AZ 85724–5115, USA. After learning, the medial prefrontal cortex (mPFC) gradually comes to modulate the expression of memories that initially depended on the hippocampus. We show that during this consolidation period, neural firing in the mPFC becomes selective for the acquired memories. After acquisition of memory associations, neuron populations in the mPFC of rats developed sustained activity during the interval between two paired stimuli, but reduced activity during the corresponding interval between two unpaired stimuli. These new patterns developed over a period of several weeks after learning, with and without continued conditioning trials. Thus, in agreement with a central tenet of consolidation theory, acquired associations initiate subsequent, gradual processes that result in lasting changes of the mPFC's code, without continued training. The hippocampus is necessary for rapid association among elements of an event, and is initially also critical for retrieval of these associations; however, its necessity for retrieval is time-limited. Consolidation of memory is presumed to involve gradual reorganization of cortical networks. Such synaptic modification might be triggered by the replay of task-related neural activity patterns during sleep. The hippocampus may direct the mPFC to refine its memory-related neural activity, as hypothesized in standard consolidation theory. Memories of associations between various elements of an event are distributed over wide areas of cortex. The hippocampus may store a memory index for a unique array of neocortical modules representing each experienced event. Direct cortico-cortical connections that are gradually established during consolidation may render the hippocampal index codes for the original memory no longer necessary. The mPFC might take over the linking function from the hippocampus by storing a similar, but perhaps more efficient, index code. The context-dependent, cross-modal associations in the mPFC are a critical prerequisite for the mPFC to function as a storage site of memory indices for consolidated memory. The role of the mPFC in the retrieval of consolidated memory may thus parallel the putative role of the hippocampus in retrieving recently formed memories by completing the intercortical neural pattern from a partial cue.
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