However, a new interruption task was used that allowed a manipulation of response-selection demands. For these interruption trials, initially an empty stimulus box (6° side length) appeared on the screen. After 1000 ms, an arrow (4.8° length) appeared that pointed to one of the four corners of the box and was colored red, blue, green, or yellow. Depending on condition, subjects responded

with their right-hand index finger by pressing keys on the numerical keypad that corresponded to the corners of the square (2, 3, 5, or 6). Subjects in the low-control condition were instructed to press the key that was compatible with the arrow high throughput screening direction. The color dimension was not explicitly mentioned to these subjects. For subjects in the high-control condition the correct key was indicated through arbitrary color-key assignments (red = upper left, green = upper right, yellow = lower left, blue = lower right). Arrow direction and the key indicated by the color were in conflict on 50% of interruption trials. Transitions between the primary task and AZD6244 the interruption task occurred with probability p = .2. As in the critical experimental conditions of the preceding

experiments, subjects alternated between pure endogenous and pure exogenous control blocks. The only difference was that we extended the length of blocks to 100 trials per block. Half of the subjects worked exclusively with the low-control interruption task, the other half with the high-control interruption task. We used the same trial exclusion criteria as in the previous experiments. In this experiment relevant error results were obtained and will be reported alongside with RT results. The mean RTs for the low-demand interruption task was 501 ms (SD = 63). Mean RTs for the high-demand interruption task were 714 ms (SD = 114) for compatible and 816 ms (SD = 169) for incompatible trials.

Corresponding Florfenicol error percentages were 0.7% (SD = .64), 1.4% (SD = 2.2) and 5.9% (SD = 3.0%). Thus, with these interruption tasks, we implemented a strong variation in control demands. Fig. 6 presents RT and error results for the primary tasks as a function of task, interruption, and conflict, separately for the low-demand and the high-demand interruption conditions. As apparent, across all conditions the qualitative RT data pattern was largely similar to the one obtained for the corresponding conditions from the previous experiments. For the analysis, we added as additional factor whether or not the last interruption episode was short (i.e., ⩽2 trial) vs. long (>2 trials). With this categorization of interruption episodes, there was an about equal number of observations in each category. The switch-cost asymmetry, that is the Task × Interruption interaction was highly significant, F(1, 38) = 29.33, MSE = 19629.69, p < .001, and this effect was not modulated by the type of interruption, F(1, 38) = .07. Also, the cost-asymmetry was modulated by conflict, F(1, 38) = 5.63, MSE = 13918.91, p < .03.