Stimulus-response mapping shapes unintended response duration modulation in duration judgment tasks

The ideomotor theory, as originally posited by William James in 1890, suggests that the mere mental representation of a movement has the potential to trigger the actual movement. The precise nature of the representational format—often colloquially referred to as 'an idea'—that serves as a direct conduit for initiating movement remains unclear. Shin et al. (2023) addressed the issue by introducing an experimental paradigm, where participants were required to categorize six audiovisual stimuli durations as 'short' or 'long' by pressing a left or right key. Notably, response durations reflected the nature of the decision category, despite the absence of explicit instructions on how long they were to press the key, thereby demonstrating a genuine ideomotor effect. The current study further examined how categorizing stimuli as long or short influenced motor execution, contingent on the task's difficulty level. With stimuli durations set at 85, 100, 115, 135, 150, and 165 ms for the 'difficult' categorization task, and 85, 100, 115, 165, 180, and 195 ms for the 'easy' task, the results showed a robust ideomotor effect, more pronounced during the 'easy' task. Intriguingly, the preassigned mapping of the hand to the decision category modulated the ideomotor effect differently, particularly in the 'difficult' task; the ideomotor effect was negligible when the left hand was assigned to the 'short' decision category, whereas the right hand to short decision assignment resulted in a strong ideomotor response, F(5, 1) = 24.61, p = <.001, ηp² = 0.552. Such an interaction was not found in the ‘easy’ task, F(5, 1) = 0.445, p = 0.816, ηp² = 0.022. Possible mechanisms underlying the observed pattern may include the cognitive mechanism similar to the spatial-numerical association of response codes (SNARC), or an asymmetric hemisphere control within the context of the ideomotor effect.