Page 96 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 4
Each test session consisted of 10 trials and started with a 9-point grid to check for calibration accuracy, shown until the experimenter manually continued the experiment. The presentation of the grid was followed with a black screen displayed for 4 seconds, and subsequently followed by a fixation video (a sped-up nature movie) positioned in the middle of the screen. Only when the participant’s fixation was on the video for more than one second, the experimenter moved on to the next trial sequence. Bonobos were then presented with two stimuli on the left and right side of the screen; one emotional and one neutral image (location was counterbalanced). Stimuli were presented for 3 seconds, in accordance with previous eye tracking tasks with great apes (e.g., Kano et al., 2015, and see Hopper et al., 2020, for a review). After 3 seconds, the experiment continued with a black screen shown for 4 seconds, and this concluded a trial. After 10 trials, the task ended automatically. Bonobos first completed all the trials with bonobos before moving on to the human stimulus set.
On average, the bonobos were tested on 33.5 sessions (SD = 3.12), and 355 trials (trials with bonobo stimuli: M = 191.5 , SD = 23.84; trials with human stimuli: M = 163.5, SD = 46.57). Furthermore, trials were repeated in order to compensate for data loss (e.g., due to disruptions by other bonobos). On average, all unique stimulus combinations were repeated 3.59 times (SD = 1.49).
Figure 2. Trial sequence for participants. The test started with a 9-point grid, and each trial started with a black screen (4s), followed by a fixation video. Finally, two stimuli were shown on both sides of the screen (3s).
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