Objectives: We assessed the typical developmental trajectory of attention to social and nonsocial events that provide intersensory redundancy (audiovisual) versus no redundancy (unimodal visual) across the ages of 2-8 months. We predicted greater attention (more processing time and fewer disengagements) to events with more redundancy: audiovisual social and nonsocial events > unimodal visual social and nonsocial events, and audiovisual social events > than all other event types, given that social events typically amplify redundancy.
Methods: Data from 705 infants at 2, 3, 4-5, or 6-8 months of age (N = 140, 157, 239, and 169, respectively) were analyzed. Infants were habituated to dynamic displays of bimodal audiovisual versus unimodal visual (silent) social events (women speaking in infant-directed speech) or bimodal audiovisual versus unimodal visual nonsocial events (toy hammer tapping a rhythm). Mean looks away per minute (disengagement) and mean length of time to habituation (processing time) were evaluated.
Results: Age (2, 3, 4-5, 6-8 months) x event type (social, nonsocial) x condition (unimodal, bimodal) between subjects ANOVAs indicated main effects of age with decreasing processing time and increasing disengagements across age, ps < .001. Consistent with our predictions, main effects of condition and event type indicated longer processing times and less disengagement for social than nonsocial events (ps < .005) and for bimodal redundant than unimodal nonredundant stimulation (ps < .001). These main effects were each qualified by interactions with age (ps < .01) where differences between conditions were most apparent for older infants. Slope analyses indicated that attention to bimodal, redundant social events was maintained across age (p > .10), whereas attention to all other event types decreased across age (p < .01).
Conclusions: Consistent with predictions of the IRH, these findings demonstrate that attention (processing time and disengagement) to bimodal, redundant social events is maintained across age, from 2-8 months, whereas attention to unimodal and nonsocial events declines across this period. These are the first findings indicating that social orienting develops gradually across infancy, emerging by 3 months, and is a function of intersensory redundancy. Together with findings of impaired intermodal functioning in autism, these findings suggest that intermodal processing disturbance may underlie social orienting impairments in autism.