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Previous research indicated that 4-month-old infants perceive the unity of a center-occluded object when its visible ends share a common lateral translation in space. The present work investigated the class of motion relationships that can specify object unity to infants, specifically, asking whether it includes all rigid translations. 3 experiments tested the informativeness of 2 axes of translation not previously studied: translation in depth and vertical translation. These motions also allowed assessment of certain interpretations of previous results that invoke specific sensory consequences of lateral movement, rather than perceived motion, as underlying perceived unity. Experiment 1 provided evidence that a small extent of translation in depth specified the unity of an object, but only to the subgroup of infants who detected the motion. Experiment 2 used a greater displacement in depth and found clear evidence for perception of object unity. Experiment 3 indicated that vertical translation, in which the 2 visible areas of the partly hidden object undergo dissimilar changes, also specifies object unity to infants. These results suggest that infants' perception of object unity depends on perceived coherence of motion, no matter how specified, and that the class of informative motions includes all rigid translations.

In three experiments with infants and one with adults we explored the generality, limitations, and informational bases of early form perception. In the infant studies we used a habituation-of-looking-time procedure and the method of Kellman (1984), in which responses to three-dimensional (3-D) form were isolated by habituating 16-week-old subjects to a single object in two different axes of rotation in depth, and testing afterward for dishabituation to the same object and to a different object in a novel axis of rotation. In Experiment 1, continuous optical transformations given by moving 16-week-old observers around a stationary 3-D object specified 3-D form to infants. In Experiment 2 we found no evidence of 3-D form perception from multiple, stationary, binocular views of objects by 16- and 24-week-olds. Experiment 3A indicated that perspective transformations of the bounding contours of an object, apart from surface information, can specify form at 16 weeks. Experiment 3B provided a methodological check, showing that adult subjects could neither perceive 3-D forms from the static views of the objects in Experiment 3A nor match views of either object across different rotations by proximal stimulus similarities. The results identify continuous perspective transformations, given by object or observer movement, as the informational bases of early 3-D form perception. Detecting form in stationary views appears to be a later developmental acquisition.

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