Brief articleThe influence of reward associations on conflict processing in the Stroop task
Introduction
Reward is known to be an effective motivator of behavior and a driving force for learning (for a review see Wise, 2004). Numerous studies in humans have demonstrated that reward anticipation is associated with performance improvement in diverse behavioral domains, including response speed and accuracy (e.g., Bijleveld et al., 2010, Knutson et al., 2001), visual discrimination and visual search (e.g., Engelmann and Pessoa, 2007, Kristjansson et al., 2010), cognitive control (e.g., Locke & Braver, 2008), negative priming (e.g., Della Libera & Chelazzi, 2006), and memory processes (e.g., Adcock et al., 2006, Krebs et al., 2009, Wittmann et al., 2005).
While reward generally exerts enhancing effects on behavior, the presence of conflicting information is known to disrupt performance, as commonly demonstrated by conflict paradigms such as the Stroop color-naming task (Stroop, 1935). In this task, subjects respond to the ink color of a color word (e.g., “RED”) while ignoring its semantic meaning. Typically, subjects’ performance is facilitated in trials in which the information in the task-relevant (ink color) and task-irrelevant (word meaning) dimensions are congruent, and impeded if they are incongruent (MacLeod, 1991). According to influential parallel distributed processing models of the Stroop effect, information from both input dimensions is conveyed in parallel, and the ultimate response depends on the relative activation of the two pathways (Carter and van Veen, 2007, Cohen et al., 1990). In the color-naming Stroop task, it has been proposed that automatic reading of the irrelevant word meaning strongly co-activates the corresponding pathway in parallel to the processing of the relevant ink color, and, if incongruent, interferes with performance.
More recently, observations that brain regions implicated in human cognitive control are also critically involved in reward-based learning (Miller and Cohen, 2001, Ridderinkhof et al., 2004, Ridderinkhof et al., 2004, Ullsperger and von Cramon, 2003) have given rise to the question of how far processes related to reward and conflict interact (Holroyd and Coles, 2002, Jocham and Ullsperger, 2009). Supporting such an interaction, it has been demonstrated that reward information has the potential to disrupt the behavioral adjustments that are typically observed subsequent to incongruent trials in a flanker task (van Steenbergen, Band, & Hommel, 2009). According to this study, the commonly observed behavioral adjustments (for a review see Egner, 2007) might be counteracted by the receipt of reward, thereby suggesting a shared mechanism (van Steenbergen et al., 2009). However, these observations were limited to sequential effects, and reward was delivered incidentally (i.e., subjects’ responses were not instrumental to obtaining rewards) and thus it remains unknown how conflict processing would be modulated if reward is associated with components of the task itself.
We sought to investigate this question by associating reward with two of the four ink colors in a Stroop task. While subjects responded to the ink color, the irrelevant semantic meaning of the word could be congruent, incongruent, or neutral with regard to the ink color. In addition to these typical Stroop-paradigm categories, the irrelevant word could be semantically linked to a color that was either part of the potential-reward ink-color subset or not. However, the semantic information was entirely task-irrelevant and thus never associated with obtaining reward.
Based on the notion that cognitive control in concert with attention can differentially emphasize the pathways of potential competing inputs we hypothesized that reward associations in the relevant dimension would further promote effective stimulus processing. Specifically, we predicted general response facilitation and reduced interference in potential-reward as compared to no-reward trials. Additionally, we hypothesized that reward associations with an ink color would generalize to its semantic representation (i.e., word meaning). Consequently, incongruent word meanings that are implicitly linked to reward, might cause greater interference by emphasizing the incongruent information.
Section snippets
Participants
Twenty healthy right-handed subjects participated (mean age ± SD: 22.5 ± 3.2, 14 female) and gave written informed consent in accordance with the Duke Medical Center Institutional Review Board for human subjects. Subjects were paid a basic amount of $15 plus an average reward bonus of $15.
Paradigm and procedure
Subjects performed a version of the classic color-naming Stroop task in which they responded to the ink color of words while ignoring their semantic meaning. A small gray fixation square (visual angle 0.3°) was
Participants
Sixteen healthy right-handed subjects (mean age ± SD: 22.6 ± 3.2, 9 female) participated after giving written informed consent in accordance with the Duke Medical Center Institutional Review Board for human subjects. Subjects were paid a basic amount of $15 plus an average reward bonus of $16.
Paradigm and procedure
Experiment 2 consisted of three successive phases: a color-reward training phase, a rewarded Stroop phase, and an extinction phase in which rewards were no longer given. The color-button mappings, as well as
General discussion
In the present study, using a version of the Stroop task with reward associations for a subset of the ink colors, we systematically manipulated reward anticipation in the task-relevant dimension (ink color), thereby also implicitly imparting reward associations in the task-irrelevant dimension (word meaning). In line with previous research demonstrating the performance-enhancing effect of reward (e.g., Bijleveld et al., 2010, Engelmann and Pessoa, 2007), we found that reward anticipation linked
Acknowledgement
This work was funded by NIH Grant R01-MH060415 to M.G.W. and by DFG Grant BO 3345/1-1 to C.N.B.
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