Perception of rotated forms: a theory of information types.

The present article proposes a theory of form perception in an attempt to understand puzzling problems in mental rotation and in perception of forms rotated in the frontal-parallel plane. According to the theory, it is critical to distinguish four types of information. They result from the orthogonal combination of two binary distinctions: information can be orientation-free or orientation-bound, elementary or conjunctive. The theory provides an explanation as to when and why mental rotation has to be performed. If two forms can be discriminated only on the basis of conjunctive orientation-bound information, mental rotation or some other functionally equivalent strategy is required. Mental rotation is unnecessary if the forms differ in either type of orientation-free information, provided that the difference is actually encoded as such. This explanation along with the proposed distinctions among the four types of information was supported by two mental rotation experiments and three visual search experiments.