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An experiment has been conducted in order to extend to geometrical figures Posner's (1969) findings of various levels of processing for alphabetical material. Pairs of simple figures (triangles and trapezoids) were tachistoscopically presented to subjects asked to judge if they were "same" or "different". Subjects were instructed to respond "same" either if the two figures had the same shape or the same name, e.g., a scalene triangle paired with a right-angled one. The results support the theory of different levels of processing also for geometrical figures.

Two hypotheses of hemispheric specialization are discussed. The first stresses the importance of the kind of processing to which the stimulus is subjected, and the second stresses the importance of the nature of the stimulus. To test these hypotheses, four experiments were carried out. In Experiment 1 verbal material was employed in a same-different classification task, and an overall right visual field superiority was found. Experiment 2, in which verbal stimuli were subjected to visuospatial transformations (i.e. mental rotations), yielded no laterality effect. In Experiment 3 geometrical figures were employed in a classification task similar to that of Experiment 1, and an overall left visual field superiority was found. In Experiment 4 both verbal and geometric stimuli were employed. The results showed a significant interaction between field of presentation and nature of the stimulus and no interaction between field of presentation and level of processing.