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Fifty-kilohertz USVs emitted by Non-Resident and Resident rats during the first 30 min of saline SA for sessions 15–16. a Total number of calls (means ± SEM) when exposed to cocaine-paired versus heroin-paired cues. b Scatterplots of calls emitted when exposed to cocaine-paired versus heroin-paired cues. c Preference score, for cocaine versus heroin, is calculated as the ratio of log-transformed calls emitted when exposed to cocaine-paired versus heroin-paired cues
