Trial structure (A) and task conditions (B). (A) Schematic illustration of a trial. Stimuli are not to scale. Subjects were instructed to discriminate the outline of every object and respond to it as fast and accurately as possible by pressing the correct response button. Response buttons are coloured here for demonstration purposes; in reality they were white coloured and participants learned to associate each button with two colors in the practice session. (B) Schematic example of the four conditions. Incongruent stimuli are defined as either stimulus incongruent (SI) or response incongruent (RI) depending on both the canonical color of a stimulus (in this example: red) and the mapping of the colors to the response buttons (in this example: red and green to the left button, gray and yellow to the right button). In the SI condition, the incongruent color was mapped onto the same response button as the object's canonical color. In the RI condition, the object was presented in one of the incongruent colors that were mapped onto the response button opposite to the button associated with the object's canonical color. In the neutral condition abstract shapes were presented in one of the four colors.