Abstract
Ultrasonic vocalizations (USVs) in rats are thought to contain ecological signals reflecting emotional states. These USVs are centered on 50-kHz, and frequency modulation (FM) is hypothesized to indicate positive emotion; however, results from recent studies are inconsistent with this hypothesis. We suspected that such inconsistencies might result from ambiguity in defining frequency modulation, and problems with acoustic analyses and behavioral protocols. We addressed these problems by applying quantitative methods for USV analyses and using a food reward operant paradigm. Our results revealed that frequency modulation varied according to the degree of positive outcomes, but the direction of change was opposite to what had been observed in previous studies. The FM in 50-kHz USVs decreased as animals learned the task and obtained more reinforcement, while USV amplitude increased as learning progressed. To reconcile these results with those from prior studies, we suggest that FM in 50-kHz USVs should be taken as an index of reward prediction errors, and USV amplitude should be considered as an index of positive emotion.
Highlights
Rats (Rattus norvegicus) vocalize in the 2 – 4-kHz audible range, and the ultrasonic range above 20-kHz is beyond the human audible range (Anderson, 1953)
These results suggest that dopaminergic projections from ventral tegmental area (VTA) to nucleus accumbens are related to emission of 50-kHz ultrasonic vocalizations (USVs)
These findings were somewhat unexpected because the number of pellets animals obtained increased as the session progressed, and this change should have yielded more frequency modulated” (FM), which are considered to be associated with positive emotion
Summary
Rats (Rattus norvegicus) vocalize in the 2 – 4-kHz audible range, and the ultrasonic range above 20-kHz is beyond the human audible range (Anderson, 1953). Flat calls are not affected by appetitive or food reward contingencies (Burgdorf et al, 2008) Such observations indicate that 50-kHz USVs, especially FM calls, could reflect positive emotion caused by appetitive states such as reward expectation (Ahrens et al, 2009). These USVs could be a good index of vulnerability to habit-forming drugs (Taracha et al, 2012). Individuals experiencing an experimental paradigm produce more flat calls than naïve individuals would (Wöhr, Houx, Schwarting, & Spruijt, 2008) It is not conclusive whether 50-kHz USVs, especially FM calls, are completely associated with positive affect
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