Figure 1.

Stimulus set composed of three groups of three similar object images. The objects were specified by 57 shape parameters. The figure plane represents a plane of the space spanned by the 57 parameters. One point in the feature space indicates an object shape. We first made three objects (1, 5 and 9) and then made the other six by interpolating two of the three. New objects were determined by the positions on the lines connecting the original objects and a quarter of the distance away from one of the original objects. Thus, the objects made three groups of similar objects. Objects in the three groups appeared similar in shape to dogs, monkeys, and mice. All of the stimuli had the same surface color and were illuminated by the same point source of light from the top.