Processing of an irrelevant location dimension as a function of the relevant stimulus dimension.

The effect of an irrelevant location dimension on choice reactions to the relevant stimulus dimension was examined. Experiment 1 used variations of the spatial Stroop task and the Simon task that differed in whether the relevant dimension (location name or color) was similar to the irrelevant location dimension. Congruity of the stimulus dimensions and stimulus-response (S-R) mapping had additive effects in the Simon task but overadditive effects in the Stroop task. Experiments 2-4 showed that each pattern could be obtained for both tasks, suggesting that dimensional similarity is not crucial; overadditivity occurred only when stimulus identification was prolonged. The results can be interpreted in terms of the relative timing of activation for the relevant and irrelevant information, if it is assumed that the activation function for irrelevant location varies across different stimulus-response mappings.