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Abstract

Scientific assessment of affective valence (positivity or negativity) in animals allows us to evaluate animal welfare and the effectiveness of 3Rs Refinements designed to improve wellbeing. Judgement bias tasks measure valence; however, task-training may be lengthy and/or require significant input from researchers. Here we develop an automated and self-initiated judgement bias task for rats which capitalises on their natural investigative behaviour. Rats insert their noses into a food trough recess to start trials. They then hear a tone (2 or 8 kHz) and learn either to ‘stay’ for 2 s to receive a food reward or to ‘leave’ the trough recess promptly to avoid an air-puff. Which contingency applies is signalled by two different tones. Judgement bias is measured by responses to intermediate ambiguous tones. We carried out two experiments to investigate this new task. In Experiment 1, 36 of 40 (90%) rats reached training criterion on the tone-discrimination task in a mean of 23.1 (sem: 1.14) sessions. Half the rats were partially-reinforced during training and they were more likely to ‘stay’ during ambiguous and negative tones than rats that were fully-reinforced (Likelihood-ratio test (LRT) of effect of removing predictor variable from model: Chi-square=17.71, df=1, P=0.001). When exposed to prior short-term positive affect manipulations (15 min gentle handling; enrichment), rats tended to show more ‘stay’ responses (LRT Chi-square=3.28, df=1, P=0.07) than when they were exposed to negative ones (15 min small box; isolation). In Experiment 2, all rats were partially-reinforced during training, and 11 of 12 (92%) rats reached criterion in 17.5 (sem: 0.65) sessions. Rats exposed to a prior short-term positive affect manipulation (16 food rewards in 15 min) tended to make more ‘stay’ responses (LRT Chi-square=3.75, df=1, P=0.053) than those exposed to a relatively negative one (1 food reward in 15 min). This task capitalises on natural investigative behaviour, can be learnt in fewer sessions than other automated variants, generates 4-5 self-initiated trials/min, yields generalised responses across ambiguous tones as expected, and can be tested repeatedly. Affect manipulations generate main effect trends in the predicted directions, albeit not quite significant at P<0.05, and not localised to ambiguous tones perhaps indicating that affect manipulations altered food and/or air-puff valuation which influenced responses to all tones. Further construct validation is thus required. We also find that reinforcement contingencies during training can affect responses to ambiguity. The task is likely to be readily translatable to other species and should facilitate more widespread uptake of judgement bias testing.