Morphed faces task
Participants completed a morphed faces task (Burkhouse et al., 2014) in which they viewed grayscale faces from a standardized stimulus set of child actors (Egger et al., 2011) displaying a variety of emotions (fear, happy, sad, or neutral). Each face was 26.5 cm tall (16 visual angle) and 16.5 cm wide (10 visual angle). The stimuli consisted of emotional and neutral photographs from each actor morphed to form a continuum of 10% increments between the two photographs. Each emotion was represented by 4 continua (2 male and 2 female actors) for a total of 12 continua. A total of 11 morphed images were used from each continuum, representing 10% increments of the two emotions ranging from 100% neutral to 100% target emotion (e.g., 100% neutral, 0% fear; 90% neutral, 10% fear; 80% neutral, 20% fear). The pictures were presented one at a time in the middle of the screen for 3 s, after which they disappeared and participants were asked to indicate which emotion was being presented using the following four response options for each image: fear, happy, sad, or calm/relaxed. The inter-trial interval varied randomly between 500 and 750 ms. The stimuli were presented in semi-random order with the condition that no two images from the same actor were presented consecutively. Each of the 132 images was presented twice for a total of 264 trials, with a rest after every 55 trials. Consistent with previous research (Burkhouse et al., 2014; Burkhouse et al., 2016; Jenness et al., 2015), and to provide an adequate number of trials within each morph level, images were binned into three separate morph conditions for analyses: low (10%, 20%, and 30%), medium (40%, 50%, 60%, and 70%), and high (80%, 90%, and 100%). During the morphed faces task, continuous electroencephalography (EEG) was recorded using a custom cap and the BioSemi ActiveTwoBio system (Amsterdam, Netherlands). The EEG was digitized at 24-bit resolution with a sampling rate of 512 Hz. Recordings were taken from 34 scalp electrodes based on the 10/20 system. Offline analysis was performed using the MATLAB extension EEGLAB (Delorme & Makeig, 2004) and the EEGLAB plug-in ERPLAB (Lopez-Calderon & Luck, 2014). All data were re-referenced to the average of the left and right mastoid electrodes and bandpass-filtered with cutoffs of 0.1 and 30 Hz. EEG data were processed using both artifact rejection and correction. Large and stereotypical ocular components were identified and removed using independent component analysis (ICA) scalp maps (Jung et al., 2001). Artifact detection and rejection was then conducted on epoched uncorrected data to identify and remove trials containing blinks and large eye movements at the time of stimulus presentation. Epochs with large artifacts (>100 lV) were excluded from analysis. Consistent with previous studies measuring LPP responses in children (Dennis & Hajcak, 2009; Kujawa, Klein, & Hajcak, 2012) the LPP was calculated as the mean activity 400 to 1000 ms following face onset averaged across occipital (O1, O2, and Oz) and parietal (P3, P4, PO3, PO4, and Pz) electrode sites separately for each emotion and morph level.