This activity is likely to be sparse and anatomically distributed, with different brain regions contributing to the quality and strength of the recall. Retrieval is hence considered as a reconstructive rather than a replicative process. The study of memory retrieval in the mammalian brain assumes that the process involves reactivation of patterns of neural activity associated with the original experience, although not necessarily identical with the activity patterns that represented the original experience. We will begin with selected studies of memory retrieval in the rodent brain and proceed to discuss aspects of retrieval of episodic memory in the human brain. In this work, we will review some of these developments. The development of novel paradigms, model systems, and new tools in molecular genetics, electrophysiology, optogenetics, in situ microscopy and functional imaging, have contributed markedly in recent years to our ability to investigate retrieval and understand part of its processes and mechanisms from the cellular to the behavioral level. This was owing to a multitude of hindrances, including difficulties in teasing apart retrieval from encoding, limited knowledge on localization of specific candidate memory circuits in humans and animals, and lack of neurobiological methods with the proper spatiotemporal resolution that permits monitoring and manipulation of these circuits to observe, block, trigger or enhance retrieval. In fact, once encoding is over, memory unretrieved, whether naturally or by experimental manipulations, is undetected, hence retrieval of the engram or part of it is an essential part of the proof that the specific engram exists.ĭespite its central importance in the study of memory and the abundance of data and models of retrieval in human experimental psychology, until fairly recently, retrieval in complex neural circuitry remained mostly an uncharted terrain in the neuroscience of memory. Retrieval is critical to understanding memory. These sequential and parallel processes can be completed within a fraction of a second (e.g., Thorpe et al. In memories encoded and stored in more complex circuits, such as distributed memories in the mammalian brain, retrieval is posited to involve distinct processes, including selection, reactivation or reconstruction of the target representation, and assessment of the outcome ( Tulving 1983 Dudai 2002). In simple modified reflex behavior, it refers to the postexperience readout of the experience-induced change in behavior and in its underlying synaptic efficacy ( Kandel and Schwartz 1982 Byren and Hawkins 2015). Retrieval is the use of learned information, induced by sensory or internal cues. These representations are driven largely by the activity patterns shaped during encoding, but are malleable, subject to the influence of time and interaction of the existing memory with novel information. The picture that emerges is that retrieval involves coordinated fast interplay of sparse and distributed corticohippocampal and neocortical networks that may permit permutational binding of representational elements to yield specific representations. We review selected developments in the study of explicit retrieval in the rodent and human brain. The development of novel paradigms, model systems, and new tools in molecular genetics, electrophysiology, optogenetics, in situ microscopy, and functional imaging, have contributed markedly in recent years to our ability to investigate brain mechanisms of retrieval. Retrieval, the use of learned information, was until recently mostly terra incognita in the neurobiology of memory, owing to shortage of research methods with the spatiotemporal resolution required to identify and dissect fast reactivation or reconstruction of complex memories in the mammalian brain.
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