Previous research in our lab demonstrated that 4-month-old infants discriminate between 2D depictions of structurally possible and impossible object pairs. In an eye-tracking paradigm, infants showed longer looking times for impossible relative to possible object displays, and engaged in active comparison of critical regions in these cube stimuli, regions in which depth order of junction parts was reversed in impossible relative to possible displays. Sensitivity to the pictorial depth cue of interposition and detection of inconsistencies in global object structure, therefore, are available early and guide young infants' oculomotor behavior.
Here, we asked whether these perceptual skills would also guide reaching behavior in 9-month-olds. We reasoned that the degree to which infants manually explore depictions of possible versus impossible objects might provide an index of their interpretation of such displays. We measured differences in number of manual gestures attempted towards realistic photographic displays of objects, including possible and impossible cube stimuli. Infants directed a greater number of manual gestures (grasping, pinching, scratching, rubbing, patting) towards the impossible relative to the possible cube display (p [[lt]] .001) and to images of real toys and non-pictorial images such as tree bark (p [[lt]] .05). By 9 months, therefore, infants use the pictorial depth cue of interposition to guide manual investigation of 2D depictions of objects, and behave differently in response to images of possible and impossible objects. Specifically, anomalous depth information induces greater exploration of pictures of impossible objects.
These data suggest that the infant's visual system extracts structural information in 2D images in an attempt to analyze the projected 3D configuration, and this information serves to control both oculomotor and manual action systems. Our findings provide important insights into the development of mechanisms for processing pictorial depth cues and extracting 3D structure from pictures of objects.