Insensitivity to reward shifts in zebrafish (Danio rerio) and implications for assessing affective states
Theory and empirical findings predict that individuals in a negative affective state are more sensitive to unexpected reward loss and less sensitive to unexpected reward gain compared to individuals in a neutral or positive affective state. We explore the use of sensitivity to reward shifts measured during successive contrast tasks as an indicator of affect in zebrafish (Danio rerio). In line with the assumption that exposure to rewarding stimuli induces a relatively positive affective state compared to exposure to stimuli that they do not prefer, we confirmed that zebrafish prefer enriched over barren environments, suggesting that the enriched environment is associated with positive affective states. We trained individuals to swim down a channel for food rewards of differing value and then presented them with unexpected increases or decreases in reward value. Contrary to our hypothesis, individuals conditioned to a high-value reward continued swimming at the same speed when reward value was downshifted, thus showing no successive negative contrast effect and appearing insensitive to reward loss. Individuals whose rewards were upshifted gradually increased their speed, but did not display successive positive contrast effects typical of sensitivity to reward gains. In both cases, housing type did not result in differences in swim time. One potential explanation is that goal-directed control of behaviour is necessary for an animal to show a successive contrast response to unexpected reward gain or loss, and the behaviour of zebrafish in this task was under habitual control, perhaps due to over-training. If so, refinements to task design and training procedures will allow further progress with this assay.
KeywordsCognitive bias Reward sensitivity Successive negative contrast Animal affect Fish Environmental enrichment
We would like to thank John Ahern, Tania Long and Darren Cipolla for assistance with animal husbandry. We also thank Sharon Koh for assistance with data collection. This research work was funded by the Margaret Catto Award and K. Handasyde.
This research was funded by the Margaret Catto Award and K. Handasyde.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
Research involving human participants and/or animal rights
This article does not contain any studies with human participants performed by any of the authors.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted (The University of Melbourne, AEC Project 1212695.3).
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