Page 102 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 4
behind the laptop and the experimenter started the 5-point calibration procedure. After finishing the calibration, participants filled in their age and sex in the task, and then the experimenter started the task. After finishing the task, participants were given the opportunity to ask more questions about the study and were given a debrief form containing the explanation and goal of the study.
To make direct comparisons between bonobos and humans possible, the difference in the task completed by both species was kept to a minimum. Whereas bonobos first completed all trials with bonobo stimuli and then trials with human stimuli, human participants first completed 10 trials of either bonobo or human stimuli, followed by 10 trials of the opposite species, and then followed by yet another 10 trial of the species that they started out with. Human participants thus completed 30 trials in one session. We created 10 versions of the task to control for order effects. In version 1, participants completed 10 trials with human scenes, followed by 10 trials containing bonobo scenes, and then another 10 trials with human scenes. In version 2 of the task, participants started with 10 trials with bonobo scenes, followed by 10 trials with human scenes, and again 10 trials with bonobo scenes. We continued alternating this sequence for the remaining 8 versions. We tested 10 participants per version of the task. This meant that every participant saw stimuli only once, but since we had 100 unique stimuli (50 combinations) and 10 versions of the task, every stimulus combination was repeated three times overall, resulting in 30 datapoints per stimulus combination.
In human participants, the trial sequence was fully automated. Because the bonobos could not be instructed, humans received minimal instructions as well, namely that they should pay attention to the screen and not move their head too much. Similar to the bonobo version of the task, humans started out with a 9-point grid that was shown for 3 seconds. The grid was followed by a black screen for 4 seconds, and then the fixation video for 3 seconds. Next, two stimuli of an emotional and a neutral bonobo or human were shown for 3 seconds, followed by a black screen shown for 4 seconds (Figure 2). Participants could take a short break between every set of 10 trials where they were allowed to move their head, but were requested to remain seated. When ready, participants could continue to the next 10 trials by pressing the space bar, followed by the 4 seconds black screen indicating the start of a new trial. At the end of the last set of 10 trials, participants saw a screen on which they were thanked for their participation.
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