Facilitation and interference with a separable redundant dimension in stimulus comparison

A comparison experiment was carried out with four letter stimuli such that some of the letter pairs provided hard and others easy discrimination when the pairs were different. A control condition confirmed the differences in difficulty with black letters. Two other conditions used two letters in each of two colors. When letter discrimination was hard, this redundant dimension produced facilitation of reaction time when the colors differed and were thus compatible with the “different” response required by the letters, and interference when the colors were the same, thus being incompatible with the “different” response. With easy letter pairs, only interference was found with incompatible color. Two additional experiments using classification procedures found color to have very little effect. Previous experiments had also shown different results concerning the separability or integrality of stimulus dimensions with the classification and comparison procedures. The interpretation for the difference suggested here is that response compatibility is an inherent aspect of the comparison procedure, since any dimension can have levels that are the same or different. The compatibility relation is available for only a few dimensional pairs with the classification procedure. When compatibility exists with either procedure, both interference and facilitation can occur, depending on whether the response interactions are compatible or incompatible.