Page 39 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Emotional attention is modulated by familiarity
bonobos performed a dot-probe task with pictures of black rabbits and goats. Only
after all the apes were able to correctly pass 95% or more of the trials within one
session, we moved on to the experiment. The criterion of a successful trial was to
immediately press the probe while attending to the screen. 2
To start a training or experimental session, we called forth the highest- ranking participating individual of the subgroup that was present in the enclosure. In the experiment, a trial started with the presentation of the start dot in the middle, lower part of the screen (Figure 3). After the bonobo pressed the dot, a neutral and an emotional stimulus appeared on the left and right side of the screen for 300 ms (Kret et al., 2016, 2018; Petrova et al., 2013). Stimuli were always either of bonobos familiar to the participant or of unfamiliar individuals (thus, we never combined an emotional picture of a familiar with a neutral picture of an unfamiliar or vice versa). Stimuli were subsequently followed by another dot (the probe) replacing either the neutral or emotional stimulus. The probe remained on the screen until touched, after which an apple cube was provided through the auto-feeder system. After a delay of 2000 ms the next trial started. Each test session consisted of 25 trials in which the location of the stimuli on the screen (left/right) and the location of the probe (behind the emotional or neutral stimulus) were counterbalanced, and the order of stimulus presentation was randomized based on emotion category and familiarity. In each session, half of the trials consisted of emotional and neutral stimuli of familiar individuals, and half of emotional and neutral stimuli of unfamiliar individuals. If a trial was deemed unsuccessful, it was repeated at the end of the study. In total, each bonobo finished between 21 to 24 sessions and on average a total of 541 trials (SD = 28.76, Table S3).
Non-participating bonobos were distracted by the animal caretaker with a body- part training in which bonobos were instructed to present specific body parts to the animal caretaker, and were rewarded with an apple cube for each correct presentation, just like the participating bonobos when they completed a trial. Importantly, bonobos were never separated from their group members, thus sometimes leading to disruptions during the experiment. From the recorded videos, two experts coded unsuccessful trials by looking at the following events: bonobos were distracted by other bonobos or did not attend to the screen, another individual pressed the probe, hands were switched within a trial, or bonobos performed movements that interfered with the task (self-scratching or nose wiping). The experts showed high agreement in coding (ICC = .95, p < .001).
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