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.