One of the most famous studies of this type was performed by Shepard and Metzler (1971). They showed participants perspective line drawing of three-dimensional objects (Figure 8.2 presents examples.) On each trial, participants would see two drawings. In some cases, the two drawing depicted (ดูไม่ออก) the same object but with one rotated by some degree. In the other cases, the drawing depicted mirror image reversals; in other words, the objects were similar but not identical. The mirror images were also sometimes rotated. The kinds of rotations used were either in the picture plane (that is, as if the object were going toward or away from the viewer). Shepard and Metzler found that the amount of time it took participants to decide if the two (ดูไม่ออก) drawings depicted the same object or a mirror-image reversal was directly proportional to the angle of rotation between the drawings.