Abstract
Two experiments were performed to investigate the magnitude of the confound in the standard control condition of the Stroop experiment. The confound resides in the fact that only color changes from one item to the next in the control condition, whereas both color and configuration of the items that represent color change in the usual experimental conditions. The results of both experiments showed small but significant increases in color-naming time when both colors and non-verbal shapes changed from one item to the next. These findings are discussed in the context of the role of factors in selective attention in the color-naming task. While response competition appears to be the more substantial source of interference in Stroop color-word effects, a smaller but more general source of interference due to selective attention appears in whole-list tasks with more than one dimension of item-to-item change.