Seven but not 5-month-old infants extract depth from cast shadows

Introduction: The first description of a depth cue, the proximal information for the location of an object in space, was probably Leonardo da Vinci's advice to painters on how to create the impression that an object is some distance from a background by using cast shadows. This study presents the first evidence that human infants can use this information to control spatial behavior. Purpose: To investigate the infant's sensitivity to the location of a cast shadow as a cue for depth. Method: Preferential reaching to the apparently closer of two toys that were presented using computer generated display was used to gauge the perception of depth. The "near" and "far" toys were actually equally distant from the infant, while an illusion of a depth difference was generated by differences in the location of cast shadows. The left right positions of the toys were randomized over trials. Two conditions were used in this study. An experimental condition in which the infant has one eye covered to remove binocular information for depth and a control condition in which displays were viewed with two eyes to observe whether preferential reaching was reduced. Six reaches in both conditions were required for the results of an infant to be included in the study. The performance of infants with an average age of 30 weeks was compared with that of 21-week-old infants under monocular and binocular viewing conditions. Results: The older infants show a significantly higher tendency to reach toward the toy in the apparently closer display, with an extended cast-shadow, when they viewed it with one eye rather than two. In contrast, the younger (5-month-old) infants did not. Conclusion: The failure to find responsiveness to cast shadow information in 5-month olds argues against the hypothesis that different pictorial depth cues develop at different times. It implies that a single mechanism may underlie the development of all pictorial cues and perhaps other static-monocular information for the environment.