Infant Face Preference Tracking Protocol

During their first and second visits infants were administered a face preference task very similar to that reported by [28] . Looking behaviour was recorded with a Tobii eye tracker. The Tobii system has an infrared light source and a camera mounted below a 17 in. flat-screen monitor to record corneal reflection data. The Tobii system measures the gaze direction of each eye separately and from these measurements evaluates where on the screen the individual is looking. During the eye tracker tasks the child is seated on his/her caregivers lap, at 50–55 centimeters from the Tobii screen. The height and distance of the screen are adjusted for each child to obtain good tracking of the eyes. First a five-point calibration sequence is run, with recording only started when at least four points are marked as properly calibrated for each eye. Gaze data were recorded at 50 Hz.
In the present task, 14 different arrays, each with five stimuli, were presented (see Fig. 1 for an example). Each array contained a colour image of one of fourteen different faces with direct gaze used as the target. Different exemplars from each of the following categories: mobile phones, birds, and cars were also included in the array. Another stimulus was a visual ‘noise’ image, generated from the same face presented within the array, by randomizing the phase spectra of the faces whilst keeping the amplitude and colour spectra constant [33] (link). The slides were counterbalanced for gender, ethnicity, and vertical and horizontal location of the face within the array. To verify that faces were similar to other categories in terms of visual saliency, saliency ranks were calculated for each area of interest on all 14 slides using the Saliency Toolbox 2.2 [63] (link).2 Categories had very similar average saliency ranks. When placed at a distance of 55 cm from the child the five individual images on the slide had an eccentricity of 9.3° and covered an approximate area of 5.2° × 7.3°.
Before each slide a small animation was presented in the center of the screen to ensure that the children's gaze was directed to the centre. Each slide presentation lasted 15 s. To assist in maintaining the children's attention, the visual presentation was accompanied by music. If the child stopped looking at the slide one of the experimenters prompted the infant to look at the screen again, without naming or referring to any of the stimuli. When the infant looked away for more than 5 s, the experimenter terminated presentation of the given slide. Rectangular AOIs were defined around each object image and the center of the screen using Tobii Studio software. Gaze data were extracted for each AOI: centre, face, noise, car, bird, phone, and total (the entire slide).