Do small dual-task costs reflect ideomotor compatibility or the absence of crosstalk?

Dual-task costs can be greatly reduced or even eliminated when both tasks use highly-compatible S-R associations. According to Greenwald (Journal of Experimental Psychology: Human Perception and Performance, 30, 632-636, 2003), this occurs because the appropriate response can be accessed without engaging performance-limiting response selection processes, a proposal consistent with the embodied cognition framework in that it suggests that stimuli can automatically activate motor codes (e.g., Pezzulo et al., New Ideas in Psychology, 31(3), 270-290, 2013). To test this account, we reversed the stimulus-response mappings for one or both tasks so that some participants had to "do the opposite" of what they perceived. In these reversed conditions, stimuli resembled the environmental outcome of the alternative (incorrect) response. Nonetheless, reversed tasks were performed without costs even when paired with an unreversed task. This finding suggests that the separation of the central codes across tasks (e.g., Wickens, 1984) is more critical than the specific S-R relationships; dual-task costs can be avoided when the tasks engage distinct modality-based systems.