Dissociating influences of key and hand separation on the Stroop color-identification effect

Three experiments examined the influence of distance between response keys (and hands) on the Stroop effect obtained for two-choice tasks in which stimulus colors were identified with keypresses. The Stroop effect was larger when the response locations were close together than when they were far apart, replicating a previous finding. Although this result was obtained only in the initial 30 trials, it was evident in a between-subject design as well as a within-subject design. With more practice, the Stroop effect was of similar size for the close and far separation conditions. Also, when the keys were close together, the Stroop effect was of similar size regardless of whether they were actuated by fingers from one or two hands, providing evidence against anatomical discriminability as a critical factor. Finally, the Stroop effect was numerically larger when the close keys were pressed by sticks held at the far separation than when the far keys were pressed by sticks held at the close separation, implicating distance between the keys rather than the hands as the main factor. The initially larger Stroop effect in RT for close keys could be due to lower spatial discriminability or to an accuracy bias in response thresholds, as suggested by the finding that it was accompanied by a numerically smaller effect in percent error.