Abstract: The conflict-driven adaptation in performance has been well documented in the color-word naming Stroop task (Stroop, 1935). The Stroop interference on the current trial is significantly reduced when following an incongruent trial in comparison to a congruent one (Gratton, Coles, & Donchin, 1992). This pattern of sequential adaptation effect, which improves performance following conflict, is named as conflict adaptation effect. In previous studies, both repetition priming effect and the proportion of congruent items have been investigated. There were not investigations having obtained significant conflict adaptation effect when the ratio of incongruent to congruent trials was 50/50 without repetition priming. However, those investigations which have obtained significant conflict adaptation usually utilized a high conflict design where incongruent trials are infrequent (incongruent items: congruent items = 20:80 or incongruent items: congruent items = 25:75) compared with congruent trials. Thus, it is still unclear what determines the presence of the conflict adaptation effect. The present study included three experiments using the color-word Stroop tasks to investigate how the response execution and the conflict observation affected the conflict adaptation effect.
In the three experiments, “D”, “F”, “J”, and “K” keys were pressed when the color of the Chinese characters were RED, GREEN, YELLOW, and BLUE, respectively. In Experiment 1, twenty-seven healthy undergraduates were tested with a color-word Stroop task. All trials began with a fixation presented for 500 ms, followed by a blank interval of 300~500 ms (500~800 ms for Experiments 2 and 3) randomly. Next, a colored character was presented until a key was pressed or for 1,500 ms whichever came first. Then, a blank interval of 800~1,200 ms was presented with the interval randomly. Each participant completed one practice block and four experimental blocks. In Experiment 2, twenty-nine healthy undergraduates were tested with choice reaction time (RT) task and color-word Stroop task. The apparatus and procedure were the same as Experiment 1 except that the four white lowercase letters “d”, “f”, “j”, “k” were used as choice RT task. In Experiment 3, twenty-six healthy undergraduates were tested using the similar task with Experiment 1; however participants did not need to execute response in the previous trials which were cued with “*”. A two-way analysis of variance (ANOVA) was used to analyze the RTs and error rates in the three Experiments.
The results showed that the interaction between previous trial types (congruent vs. incongruent) and current trial types (congruent vs. incongruent), which was thought to index the conflict adaptation effect, was not significant in Experiment 1, which was consistent with the extant studies. Similarly, a significant interaction between response transition (change vs. repetition) and current trial types (congruent vs. incongruent) was present in Experiment 2; however the current congruency effect was affected by the preceding response execution context in a reversed way. By comparison with response change, the Stroop interference on the current trial was significantly reduced when repeating the same response as the previous trial. Interestingly, a significant conflict adaptation effect was obtained using the new design in Experiment 3, suggesting that the absence of previous response benefited the presence of conflict adaptation effect.
In contrast to some recent evidences, our new design does develop the traditional ways of investigating conflict adaptation effect. The present results suggest that the conflict adaptation effect was distorted by the response execution of the previous trials. To obtain pure conflict adaptation effect, it is necessary to exclude response execution in the previous trials. Thus, the observation of conflict favors the presence of the conflict adaptation effect.

Key words: conflict observation, response execution, conflict adaptation effect, Stroop task