SUMMARYWe investigated whether visuoauditory association can be artificially implanted in rodents and then retrieved in a behaviorally relevant context. Rats were trained to approach the left or right hole of a behavioral apparatus to retrieve a reward depending on the side of electrical stimulation of the auditory cortex (EAC) they received and mice were fear-conditioned to EAC. Next, an irrelevant visual stimulus (VS) was repeatedly paired with EAC in the presence of cholecystokinin (CCK) or with activation of terminals of entorhinal CCK neurons in the auditory cortex. In subsequent behavioral testing with VS, rats approached the hole associated with reward availability and mice showed a freezing response to the VS. A CCK antagonist blocked the establishment of visuoauditory association, whereas a CCK agonist rescued the deficit of association. Our findings provide a scientific foundation for "memory implantation" and indicate that CCK is the switching chemical for formation of visuoauditory association.peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/241141 doi: bioRxiv preprint first posted online Dec. 30, 2017; 3The hippocampal system consists of the hippocampus and adjacent entorhinal, perirhinal, and parahippocampal cortices 1 . The entorhinal and perirhinal cortices are the gateway between the hippocampus and the neocortex, and have strong reciprocal connections with the entire neocortex 2,3 . Observations that patients with hippocampal system damage show difficulties forming new long-term memories for facts and events 4,5 led to our understanding that the hippocampal system is essential for establishing long-term memories 1 . However, patients with hippocampal damage can still recall remote memories [6][7][8] , suggesting that these memories can be supported by the neocortex 9,10 . Previously, we established a long-term visuoauditory associative memory in rats critically dependent on the entorhinal cortex by pairing a visual stimulus (VS) with electrical stimulation of the auditory cortex 11 .Many neurons in the entorhinal cortex contain cholecystokinin (CCK) [12][13][14] . They project to and enable long-term potentiation (LTP) in the auditory cortex by responding to a formerly ineffective sound or light stimulus after pairing with a noise-burst stimulus 15 . Infusion of a CCK antagonist into the auditory cortex prevents the formation of this visuoauditory association, similar to inactivation of the entorhinal cortex 11,15 .In this study, we aimed to confirm the formation of visuoauditory associative memory between sound and light stimuli. To test our hypothesis that CCK is a memory-writing chemical for visuoauditory memory in the neocortex, we determined whether the presence of CCK allowed implantation of a memory in the cortex of anesthetized rats retrievable in a behaviorally relevant context. Rats with electrodes implanted in the bilateral auditory cortex were trained to...