Outline of the two models.

<p>(<b>A</b>) Outline of the two-step model for the ideomotor principle (see e.g. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0054094#pone.0054094-Elsner2" target="_blank">[21]</a>). Response-effect (R-E) couplings are established between key press and tone in the acquisition phase. This results in response facilitation in the test phase when at stimulus presentation the matching tone (with respect to this stimulus) is sounded, but not when the other tone is sounded. (<b>B</b>) Outline of the alternative model based on stimulus-response (S-R) couplings established between Stimulus and Response component 1 and between Effect and Response component 2. The difference in response latencies between compatible and incompatible trials for both movements is accounted for by cooperation and competition mechanisms between the two response-selection processes (triggered by the S-R couplings). Importantly, the two response components are to be understood as an integrate part of the same ongoing response action. This schematic merely temporally separates them along the time line of the task.