To explore mental images more objectively, researchers give subjects tasks that seem to require the use of mental images. As a critical part of the task is varied, some characteristics of mental images can be deduced. Roger Shepard and his colleagues (e.g., Shepard & Metzler, 1971) designed one of the most often used tasks. In this task subjects were shown two novel visual stimuli and were asked to determine whether the stimuli had the same shape or different shapes. The shapes (random block shapes) were rotated either in the plane or in depth. Subjects reported that they mentally rotated an image in their head until the two stimuli were oriented the same way, and then made their judgment.
In an experiment with 8 adult Ss, it was found that the time required to recognize that 2 perspective drawings portray objects of the same 3-dimensional shape is (a) a linearly increasing function of the angular difference in the portrayed orientations of the 2 objects, and (b) no shorter for differences corresponding simply to a rigid rotation of 1 of the 2-dimensional drawings in its own picture plane than for differences corresponding to a rotation of the 3-dimensional object in depth. (PsycINFO Database Record (c) 2005 APA, all rights reserved)
