Page 136 - Demo

Chapter 6134StimuliDot-probe taskFor dot-probe task with flange size manipulation, we collected 72 images depicting front-facing Bornean or Sumatran orang-utan males with flanges. The images were collected through image hosting websites and social media groups. We edited the stimuli in GIMP (v2.10.32). First, we cropped the faces. Second, we consecutively selected the flanges on the left and right side of the face, respectively. We defined the width of the flange as the distance between the horizontally most peripheral point of the face and the most peripheral point of either the eye region or beard. Hereafter, we increased the width of the flanges (measured in pixels) with 15 percent to obtain the stimulus with enlarged flanges, and we decreased the width with 15 percent to obtain the stimulus with reduced flanges. We chose 15 percent to make sure that the stimuli would not become abnormal in terms of flange size. In total, this resulted in 72 combinations of enlarged and reduced stimuli.Using the same 72 images, we created the stimulus set for the dot-probe with symmetry manipulation Here, we could only include the images where the faces of the orang-utans appeared to be nearly exactly frontally facing. To determine this, we visually expected whether the eyes and nostrils were at a similar distance from the vertical midline of the face and whether they were of approximately similar size. This was the case for 49 of the images. Next, we created symmetrical versions of the face by mirroring either the left or the right hemisphere at the vertical midline of the face. Thus, from every stimulus, we obtained two symmetrized versions: one based on the left hemisphere and one based on the right hemisphere. Importantly, in some stimuli we employed an extra step to remove squinting that resulted from the mirroring. To this effect, we selected one of the eyes, and mirrored it, resulting in more congruent gaze direction of the eyes. Furthermore, some of the mirrored stimuli were characterised by abnormal facial shape, which is a well-known issue in symmetrized stimuli (Rhodes, 2006). If this was the case, we excluded the stimulus. In total, we obtained 80 stimulus pairs consisting of one symmetrized face and the original face showing natural variation in symmetry.Preference taskFor the preference task, we used 104 stimuli (52 flanged, 52 unflanged) of Bornean orang-utans. The stimuli were collected from the Internet, mainly from release Tom Roth.indd 134 08-01-2024 10:41