Evidence against a maximum response model of exogenous visual orienting during early infancy and support for a dimensional switching model.

Very young infants orient overtly with eye and head movements to salient events in their visual environments, but those events rarely occur in the absence of competing visual stimuli. Two different models of how this kind of orienting is related to number and distribution of elements in the stimulus field were tested with infants across the age range from 2 to 5 months in four experiments. A set size manipulation in Experiments 1-3 produced data that were mostly inconsistent with the Maximum Response model proposed by Dannemiller (1998), especially at ages over 3 months. Experiment 4 produced data from 3.5-month-olds that were consistent with an alternative Dimensional Switching model that assumed that there was switching across trials in the stimulus dimension that drove orienting. This Dimensional Switching model can explain the small to nonexistent set size effects observed in the first three experiments as well as data from previous experiments using this paradigm. Factors that could produce this kind of dimensional switching over time were considered and other implications of this model for understanding the development of overt visual orienting were discussed.