Developmental changes in visual and auditory contributions to speech perception.

Previous research has demonstrated that listeners make use of both auditory and visual information in bimodal speech perception. Preschool children appear to evaluate and integrate the two sources of information in the same manner as adults, but the children are less influenced by the visual source. The current experiments test a possible explanation of this difference and extend the question to younger children. It is possible that children are poorer lip-readers than adults and thus have less complete information about the visual source. Children and adults were tested with both auditory and visual sources and were also required to identify speech events on the basis of only the visual source. In addition to replicating the previous findings in the bimodal situation, the current experiments found that children are poorer lip-readers than adults. Furthermore, a positive correlation was observed between lip-reading ability and the size of the visual contribution to bimodal speech perception. A fuzzy logical model of speech perception provided a good quantitative description of the results even with the assumption that the visual information was equivalent in both the bimodal and lip-reading conditions. The results also contradict the categorical perception of speech events and any nonindependence in the evaluation of auditory and visual information in speech perception.