The different degrees of rotations performed on the materials in Cooper and Shepard (1973) for mirror-imaged letters (on the right) and normal letters (on the left).

rotation show how people can rotate visual images. Second, studies on image scanning give us some idea of how people can "mentally scan" a visual image. Third are studies on re-interpreting the images of ambiguous figures.

Mental rotation

In a series of experiments the mental rotation of a variety of imaged objects has been examined (e.g., Cooper, 1975; Cooper & Podgorny, 1976; Cooper & Shepard, 1973; Shepard, 1978 for a review; Shepard & Metzler, 1971). For example, Cooper and Shepard presented subjects with alphanumeric items in either their normal form or in reversed, mirror-image form (see Figure 9.6). In the experiment subjects were asked to judge whether a test figure was the normal or reversed version of the standard figure. The test figures were presented in a number of different orientations (see Figure 9.6). The main result was that the farther the test figure was rotated from the upright standard figure, the more time subjects took to make their decisions (see Figure 9.7). These experiments have been carried out on a variety of different objects, indicating that there was some generality to the findings; for instance, digits, letters, or block-like forms have been used. (For more recent research on mental rotation see Cohen & Kubovy, 1993; Takano, 1989; Tarr & Pinker, 1989.)

The impression we get from these experiments is that visual images have all the attributes of actual objects in the world. That is, that they take up some form of mental space in the same way that physical objects take up physical space in the world—that these objects are mentally moved or rotated in the same way that objects in the world are manipulated (see later sub-section on re-interpretation). In short, the image seems to be some "quasi-spatial simulacrum of the 3-D object" (see Boden, 1988). This view, however, is not wholly justified as there are conditions under which mental rotation effects differ from physical

The mean time to decide whether a visual stimulus was in the normal or mirror-image version as a function of orientation. Data from Cooper and Shepard (1973).

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