Whose turn is it anyway? The moderating role of response certainty on the joint simon effect


When a two-choice reaction time (RT) task is distributed between two people, performance in the shared task resembles performance in the whole task alone. This finding has been described as the joint Simon effect (JSE). Task sharing within the joint Simon paradigm has been likened to a doubles tennis or table tennis match. However, in tennis, one's turn is relatively unpredictable, whereas in table tennis, players alternate making returns. Our aim was to test how advance knowledge of whose turn it is to respond influences the JSE and, in turn, impacts explanations of the JSE based on the event-representations available to discriminate action events (e.g., Dolk et al., 2014). Twelve pairs of participants performed the typical (unpredictable) joint Simon task on Day 1, followed by a novel turn-taking version on Day 2. One group responded with 100% predictability (alternating turns, Simple RT condition). A second group also alternated turns but catch trials were included with 83% response predictability (Discrimination condition). The JSE was significantly smaller under Simple compared to Typical conditions (4 vs. 26 ms, p = .001), with no difference between Discrimination and Typical conditions (14 vs. 18 ms, p = .23). Thus, only 100% response certainty suppressed co-representation. In line with the referential coding account of the JSE (Dolk et al., 2014), advance knowledge of response necessity appears to provide a distinctive means of encoding responses that avoids the typical emphasis on response location (and corollary JSE) as a means of discriminating self- vs. other-generated action events.

Acknowledgments: This research was supported by NSERC (awarded to the last author).