Structural Representations of Objects: Invariance over a Shape-Distorting Transformation

Object perception seems robust over changes in properties such as rotation, reflection, and size. Such changes, however, are shape preserving. To investigate the effect of changes that disrupt overall shape yet keep the global relations among the parts intact, a set of objects was scaled by 50% along just the x-axis. Compression or expansion along a single axis markedly affects the overall shape of an object but leaves the global relations among the parts intact. In an initial encoding phase, subjects viewed images of symmetric and asymmetric three-dimensional nonsense objects and determined whether each faced principally to the left or right. We have previously found that such judgments require subjects to process the objects as global three-dimensional forms. In a subsequent test phase, subjects judged whether a set of both studied and novel objects were symmetric or asymmetric. Half of the studied items were either compressed or expanded along the x-axis in the test presentation. The magnitude of priming was equivalent for the transformed and the untransformed objects. In a second experiment, subjects performed the same initial left/right encoding task followed by a recognition test that assessed explicit memory for the previously presented objects. Although subjects were instructed to disregard changes in scale, recognition performance was reliably better for the untransformed objects. These results support the role of a structural representation system that captures the global relations among the parts and the principal axis of an object and an episodic system which also encodes features of shape.