Stimulus dimensionality and infants' perceived movement in depth

In two experiments infants aged 34 to 70 days observed expanding shadows or filmed stimuli that simulated optical changes produced by an approaching object on a collision course. On the basis of theoretical and empirical work with adult Ss, it was predicted that infants would perceive depth changes and hence withdraw from the stimuli when the optical transformations were two-dimensional rather than one-dimensional despite simultaneous form changes in the display. Results of the first experiment with 12 infants revealed that expansion of shadows in both horizontal and vertical dimensions produced head withdrawal from the projection screen more frequently than expansion in the horizontal dimension alone. A second experiment with nine additional babies showed that expansion of a stimulus undergoing continuous changes in form produced more head withdrawal than the same changes without expansion. For young infants as for adults a rule of "primary tridimensionality" may characterize the visual system: Any two-dimensional expansion of a stimulus will tend to be perceived as movement in depth. Possible qualifications of this rule are discussed.