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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.
