Comparison of the geometric and the contrast models of similarity by presentation of visual stimuli to the left and the right visual fields.

In this study we investigated by means of the "same-different" decision task the process of comparing visual stimuli (schematic faces, familiar objects, houseplants, and nonsense figures) when presented for 100-150 msec to the right or to the left visual hemifields. The analysis of incorrect "same" responses showed that the addition of a common component (e.g., glasses, buttons) to a pair of nonidentical stimuli increased the percentage of incorrect same responses whereas the addition of the same component to one stimulus only in the pair decreased the percentage of incorrect "same" responses. This pattern, which is in accordance with Tversky's contrast model of similarity, is incompatible with any geometric model. Second, for schematic faces the results revealed that the left hemisphere is more sensitive to common than to distinctive features, whereas the right hemisphere is more sensitive to distinctive than to common features. No such interaction was obtained for the other type of stimuli. The implications of these results for models of similarity and the difference between the present findings and the findings of Sergent (1984) are discussed.