Page 139 - Demo

Absence of cognitive bias to flanges in orang-utans1376category within the session (either flanged or unflanged stimuli). Because there was only one dot on the screen (either green or red), they were “forced” to select this one. After their response, they would be presented with a stimulus from the corresponding category for 4s (820x1134 px) and receive a reward, followed by a 2s inter-trial interval. In total, subjects had to pass 8 forced trials (4 green, 4 red) at the start of each session, in order to probe the association between dot colour and stimulus category within the session. Hereafter, they were presented with 16 choice trials. The start and end of each choice trial were essentially the same as for the forced trials. However, instead of being presented with one coloured dot, subjects could now choose between the red dot and the green dot, thereby controlling the stimulus category on the screen. The dots were presented in a circular way, equidistant from the center of the screen and always located exactly opposite of each other. Note that this differs from the method that Watson et al. (2012) describe, who presented the choice dots always at the same location on the screen. However, we noticed during the familiarisation sessions that the orang-utans would show anticipatory responses because they would know the exact location where the dots would appear. Therefore, we chose to randomize the location of the choice dots in a circular way. Importantly, the coloured dots were always located at the same distance from the center of the screen, where subjects needed to tap to advance to the choice dots. With regard to training, all individuals were already familiar with clicking dots for a reward. Therefore, we mainly had to familiarise them with the specific task (between July and October 2021). To this effect, all participating subjects fulfilled eight sessions. The first six sessions presented them with pictures of animals and flowers. Importantly, in these sessions we had not yet implemented the randomized location of the choice dots. They were presented on fixed locations, as in the original method (Watson et al., 2012). Because we noticed that individuals would sometimes anticipate the appearance of the choice dots by clicking their location repeatedly before onset, we decided to run two final sessions in which we implemented the randomised circular presentation described above. Subjects could only participate in the experimental sessions after participating in all eight of the familiarisation sessions. In total, six subjects fulfilled this criterion: all individuals except for the two flanged males.In total, each subject participated in six experimental sessions between September and December 2021, depending on whether the subject already Tom Roth.indd 137 08-01-2024 10:41