• N Samuel, A H Taub, R Paz, and A Raz.
• Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel; Department of Pediatric Emergency Medicine, Rambam Health Care Campus, Haifa, Israel. Electronic address: nir.samuel@weizmann.ac.il.
• Br J Anaesth. 2018 Jul 1; 121 (1): 219-232.

AbstractExplicit memory after anaesthesia has gained considerable attention because of its negative implications, while implicit memory, which is more elusive and lacks patients' explicit recall, has received less attention and dedicated research. This is despite the likely impact of implicit memory on postoperative long-term well-being and behaviour. Given the scarcity of human data, fear conditioning in animals offers a reliable model of implicit learning, and importantly, one where we already have a good understanding of the underlying neural circuitry in awake conditions. Animal studies provide evidence that fear conditioning occurs under anaesthesia. The effects of different anaesthetics on memory are complex, with different drugs interacting at different stages of learning. Modulatory suppressive effects can be because of context, specific drugs, and dose dependency. In some cases, low doses of general anaesthetics can actually lead to a paradoxical opposite effect. The underlying mechanisms involve several neurotransmitter systems, acting mainly in the amygdala, hippocampus, and neocortex. Here, we review animal studies of aversive conditioning under anaesthesia, discuss the complex picture that arises, identify the gaps in knowledge that require further investigation, and highlight the potential translational relevance of the models.Copyright © 2018 British Journal of Anaesthesia. Published by Elsevier Ltd. All rights reserved.

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