Presence or absence of "rotational invariance" in mirror-image discrimination

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The purpose of the present study was to investigate experimentally the validity of the information type theory proposed by the present researcher (Takano, 1989). Hollard and Delius (1982) made pigeons to discriminate rotated mirror-images from rotated originals, and obtained results indicating that discrimination was made without modifying the orientations by mental rotation (Shepard 6 Metzler, 1971). They interpreted these results to suggest that the pigeons performed the above task by detecting "rotational invariance" in mirror-images, which cannot be detected by humans. According to the information type theory, however, such "rotational invariance" that makes it possible to discriminate mirror-images does not exist physically. Indeed, the above researchers have not identified the nature of their "rotational invariance." The information type theory has identified such methods that make it possible to discriminate rotated mirror-images without performing mental rotation and without depending on "rotational invariance." If humans are forced to employ one of these methods, they are expected to exhibit the same pattern of performance as that of pigeons. The present study was designed to test this prediction. Unfortunately, the first experiment failed to suppress the use of mental rotation. It is planned to improve the experimental procedures in the next experiment.