Page 103 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Emotions hold the attention of bonobos and humans
Data preparation
After data collection finished, we realized that in version 3, 6, and 9 of the task, we accidentally showed one stimulus twice. These repetitions were removed from further analyses (31 datapoints). Furthermore, for five participants, there was a technical malfunction with the eye tracker resulting in 60% or more data loss. Thus, the data of these participants were excluded from further analyses. Similar to Experiment 1, we created ROIs in Tobii Studio, and extracted data on Total Fixation Duration per ROI using the Tobii Fixation Filter (see Experiment 1).
Statistical analyses
The analysis procedure for humans was similar to that of the bonobos. We were
interested in the total looking duration to emotional stimuli across trials, thus 4 calculated the proportional looking duration to emotional stimuli (PLDemotion). Within
the three-second trial window, human participants looked on average 2.66 s (SD =
0.38) to the target and distractor combined (raw, unweighted values) when human
stimuli were displayed, and 2.64 s (SD = 0.43) when bonobo stimuli were displayed.
Similar to what we did for the bonobos, we calculated the weight of a trial depending
on how long a participant looked at the stimuli relative to their average looking
duration to the stimuli (M = 1, SD = 0.15, range [0.005 – 1.51]).
For the PLDemotion across trials, we ran Bayesian zero-one-inflated beta regression models, similar to Experiment 1. Model 1 involved only the intercept, model 2 examined effects of Species displayed on the stimulus, and model 3 assessed an interaction effect of Species and Emotional Category. All models included a random intercept for ID (participant), and used weakly informative priors. Each model was checked using the WAMBS checklist (Depaoli & van de Schoot, 2017). We conducted all of our analyses using RStudio (v. 1.4.1106, R Core Team, 2020) and the package brms (Bürkner, 2017, 2018).
Results
In model 1, we found robust evidence for a longer PLDemotion in human participants (Mdn = 0.53, 89%CI [0.52 – 0.54], pd+ = 100%, Table 2), meaning that humans looked relatively longer to emotional stimuli than to neutral stimuli. In the second model with Species included as a factor, we found robust evidence for longer PLDemotion of stimuli depicting humans (Mdn = 0.55, 89% CI [0.54 – 0.56], pd+
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