Rat ultrasonic vocalizations in associative learning tasks
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Date
2025
Authors
Walkey, John Benson
University of Lethbridge. Faculty of Arts and Science
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Publisher
Lethbridge, Alta. : University of Lethbridge, Dept. of Neuroscience
Abstract
This thesis investigated how rat ultrasonic vocalizations (USVs) relate to reinforcement and associative learning under conditions of electrical brain stimulation and natural reward. Adult male Long-Evans rats received either medial forebrain bundle (MFB) stimulation or sucrose pellets while under mild calorie restriction. The goal was to determine whether the number and subtypes of 50-kHz calls reflected dopaminergic activation and learning across repeated sessions.
Analyses demonstrated that MFB stimulation produced a clear and consistent suppression of 50-kHz vocalizations immediately following stimulation, indicating that activation of this pathway temporarily inhibited rather than enhanced vocal behaviour. This effect was observed across animals and sessions, suggesting that stimulation interrupts the emission of USVs rather than eliciting calls associated with positive affect. In contrast, food delivery did not significantly alter the number of ultrasonic calls, and there was no evidence that pellet reward alone generated measurable increases in vocal production.
Across days of testing, there was a gradual increase in the overall number of calls made by the rats. This change may reflect increased anticipation of reward as animals became more familiar with the task structure, but it may also represent a non-associative effect of environmental habituation or reduced novelty stress. Together, these findings suggest that while 50-kHz USVs can serve as sensitive indicators of affective and motivational state, their modulation depends critically on experimental context and timing relative to reinforcement.
These results contribute to a more nuanced understanding of ultrasonic vocalizations as behavioural indices of affect. Specifically, they indicate that MFB stimulation, though reinforcing in other paradigms, suppresses ongoing vocalization behaviour, and that food reward alone does not evoke strong appetitive USVs within the parameters tested.
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Keywords
rat ultrasonic vocalizations , associative learning tasks , electrical brain stimulation , natural reward , dopamine neurons