On the role of stimulus – response and stimulus – stimulus compatibility in the Stroop effect

In the color–word Stroop task (Stroop, 1935), participants see words that refer to colors printed in a corresponding color (e.g., the word blue written in blue ink; congruent trials) or in a noncorresponding color (e.g., the word green written in blue ink; incongruent trials) and are asked to name the ink color while ignoring the meaning of the word. Performance in this task and other Stroop tasks is consistently better on congruent trials than on incongruent trials (see MacLeod, 1991, for a review). Several researchers have pointed out that the difference in performance on congruent and incongruent trials can result either from differences in stimulus–response compatibility (SRC) or from differences in stimulus– stimulus compatibility (SSC; e.g., De Houwer, 2003; Kornblum, Hasbroucq, & Osman, 1990). On congruent trials, the meaning of the irrelevant word (e.g., blue) is compatible with both the ink color (e.g., blue) and the response (e.g., say “blue”), whereas on incongruent trials, word meaning is incompatible with the ink color and the response. The difference in SRC is assumed to have an effect on the response selection stage. That is, presentation of the irrelevant word automatically activates the responses with a similar meaning. This facilitates response selection on congruent trials but interferes with response selection on incongruent trials (e.g., Cohen, Dunbar, & McClelland, 1990). However, Stroop effects could also be due to differences in SSC that might have an impact on the stimulus-encoding stage. For instance, the presentation of the irrelevant word could facilitate the encoding or identification of the relevant ink color on congruent trials and interfere with this encoding on incongruent trials (e.g., Hock & Egeth, 1970). In an important recent article, Zhang and Kornblum (1998) examined the relative contributions of SRC and SSC to the Stroop effect. Two of their findings supported the hypothesis that differences in SSC do contribute to the Stroop effect. First, they asked participants to perform a Stroop task in which SRC effects could supposedly not operate. In one version of the task, three color words were presented on each trial, only the middle one being relevant. The participants were asked to say one of four digit names out loud on the basis of the identity of the middle color word. For instance, some participants were asked to say “two” when blue was the middle word, “four” when green was the middle word, “six” when red was the middle word, and “eight” when yellow was the middle word. Because the stimuli (i.e., color words) and responses (i.e., digit names) were unrelated (i.e., no overlap; see Kornblum et al., 1990), standard SRC effects could not arise. Despite this, performance was better when the three words were identical (e.g., blue–blue–blue; congruent trials) than when they were different (e.g., green–blue–green; incongruent trials). Hence, Zhang and Kornblum concluded that the meaning of the irrelevant words influenced the identification of the middle word and, thus, that SSC effects do contribute to the Stroop effect. Second, other participants were asked to respond with color names on the basis of the identity of the middle color word. However, the color words were assigned to incompatible responses (e.g., if the middle word is blue, say “green”). The results again showed that performance was better on congruent trials (e.g., blue–blue–blue) than on incongruent trials (green–blue–green). This effect