The ERP recordings were always performed
before the eye-tracking sessions so that the infants would not become familiar with the AV stimuli prior to ERP testing, thus minimising habituation of neural responses. A separate eye-tracking-only control study confirmed that there was no effect of the order of presentation on eye-tracking results (see Control study S1). Twenty-two healthy full-term infants (six boys) aged between 6 and 9 months (mean ± SD age BI 6727 molecular weight 30.7 ± 4.3 weeks) took part in both the eye-tracking (ET) and ERP tasks. The study was approved by the University of East London Ethics Committee and conformed with the Code of Ethics of the World Medical Association (Declaration of Helsinki). Parents gave written informed consent for their child’s participation prior to the study. Video clips were recorded with three female native English speakers articulating /ba/ and /ga/
syllables. Sound onset was adjusted in each clip to 360 ms from stimulus onset, and the auditory syllables lasted for 280 – 320 ms. Video clips were rendered with a digitization rate of 25 frames per s, and the stereo soundtracks were digitized at 44.1 kHz with a 16-bit resolution. selleck screening library The total duration of all AV stimuli was 760 ms. Lips movements started ~ 260–280 ms before the sound onset (for all speakers). Each AV stimulus started with lips fully closed and was followed immediately with the SB-3CT next AV stimulus, the stimulus onset asynchrony being 760 ms, thus giving an impression of a continuous stream of sounds being pronounced. The paradigm was designed as a continuous speech flow specifically to minimize the input of face- and movement-related visual evoked potentials. In order to examine how much of the ERP amplitude is explained by the visual evoked potentials, an additional control study was carried out with auditory stimuli only (see Control study S2, Fig. S1). For each of the three speakers, four categories of AV stimuli were created: congruent visual /ba/ – auditory /ba/ (VbaAba), visual /ga/ – auditory /ga/ (VgaAga), and two incongruent pairs. The incongruent pairs were created from the original
AV stimuli by dubbing the auditory /ba/ onto a visual /ga/ (VgaAba-fusion) and vice versa (VbaAga-combination). Therefore, each auditory and each visual syllable was presented with equal probability and frequency during the task. For more information on the stimuli see Kushnerenko et al. (2008). The syllables were presented in a pseudorandom order, with speakers being changed approximately every 40 s to maintain the infants’ attention. Videos were displayed on a CRT monitor (30 cm diameter, 60 Hz refresh rate) with a black background while the infant, sitting on a parent’s lap, watched them from an 80-cm distance in an acoustically and electrically shielded booth. The faces on the monitor were approximately life-size at that distance.