Most previous studies of cross-modal interactions in infants have focused on the integration of higher-level visual and auditory stimuli, such as faces and voices, or on the spatial and/or temporal constraints on integrating low-level sensory stimuli to create a unified cross-modal percept. Here we investigate whether auditory cues can alter an infant’s ability to detect and attend to near-threshold visual stimuli.
We used forced choice-preferential looking (FPL), to determine contrast detection thresholds for a visual stimulus (a square subtending 11x11 degrees of visual angle, centered 15 degrees to the left or right of monitor center). The visual stimulus fluctuated in luminance at 1 Hz. The auditory stimulus was a white noise fluctuating in loudness at 1Hz, presented binaurally to be perceived as coming from straight ahead. The same visual stimulus was presented under four different auditory conditions: (1) In-Phase (IP): The visual stimulus fluctuated in-phase with an auditory stimulus (2) Out-of-Phase (OP): The visual stimulus fluctuated out-of-phase with the auditory stimulus, or (3) No Sound (NS): The visual stimulus was presented without a concurrent auditory stimulus. On any given trial the visual stimulus was presented at one of five contrasts (3-100%), which were randomized across trials. Contrast threshold was defined as the contrast yielding 75% correct performance in the FPL task (where correct was defined as looking to the side of the monitor containing the visual stimulus). For each subject, visual contrast thresholds were obtained for two of the three conditions (IP, OP, NS). If synchronized auditory information enhances visual detection, we expect lower contrast thresholds for the IP versus the OP or NS condition. Conversely, if synchronized auditory information hinders visual detection, we expect higher contrast thresholds for the IP condition.
We found that visual thresholds tended to be higher for the IP versus the OP condition in both 3 and 6-month old infants. These effects were due to lower thresholds in the OP condition relative to the control, NS, condition and a trend towards higher thresholds in the IP condition relative to control. Thus, anti-correlated auditory information can enhance visual detectability, while correlated auditory information can diminish visual detectability, under certain conditions. Furthermore, these results further suggest that, for the low-level sensory stimuli used here, infants may be limited in their ability to attend to information in more than one sensory modality at a given time.