Testing the emotional value of facial stimuli using attention bias in Rhesus Macaques, Macaca mulatta.
Approximately 2000 rhesus macaques, Macaca mulatta, are used annually forbiomedical research in the UK, the psychological wellbeing and welfare of theseprimates is a primary concern. Attention bias is a measure of cognitive bias usedto assess whether an individual is stressed. Current methods using attention biasto evaluate the welfare of macaques, use images of conspecifics expressing anaggressive face vs. a neutral face with either opened eyes or closed eyes.Nevertheless, it is commonly acknowledged that direct staring, eyes open, is seenas a threat to macaques. The exact emotional value of these conspecific ‘neutral’facial stimuli, however, is not known.In order to refine attention bias testing for its use in primate cognitive research,understanding the exact value of facial stimuli used is important. This study usesattention bias to explore whether ‘eyes open’ vs. ‘eyes closed’ has the samerelative threat value, or whether ‘eyes open’ could actually be perceived asaggressive. Three different facial expressions (neutral eyes closed, neutral eyesopen and aggressive) of unfamiliar monkeys were presented as paired stimuli tomeasure attention bias to facial stimuli. Twenty-eight individuals were presentedwith counterbalanced presentations of the three facial stimuli in order to identifythe emotional value of stimuli. Image pairs were presented simultaneously for ~3s.Video footage was blind-coded, frame by frame, for gaze towards stimuli.Behavioural reactions in-trial were coded to highlight any variation in keybehavioural reactions. Five minute instantaneous scan samples were conductedpost-trial to assess variation in time budget.IIIResults showed that there was a significantly greater attention bias towards thethree facial combinations; the greatest bias was toward the eyes open vs.aggressive facial combination. Other factors including monkey ID, reproductivestatus, trial, stimulus monkey, previous exposure, age (in months), and matrilinewere considered within the results. Mixed models were used to also show that agehad a significant effect on monkeys’ attention bias and previous exposure tostimuli significantly reduced attention bias. In-trial behavioural reactions showed asignificantly greater representation of extreme reactions to the eyes open vs.aggressive facial combination compared to all other facial combinations. Post-trialbehaviours showed a significantly greater representation of fear/avoid behavioursafter the eyes open vs. aggressive compared to the eyes closed vs. aggressivefacial combination.