The speed of free will.

Do voluntary and task-driven shifts of attention have the same time course? In order to measure the time needed to voluntarily shift attention, we devised several novel visual search tasks that elicited multiple sequential attentional shifts. Participants could only respond correctly if they attended to the right place at the right time. In control conditions, search tasks were similar but participants were not required to shift attention in any order. Across five experiments, voluntary shifts of attention required 200-300 ms. Control conditions yielded estimates of 35-100 ms for task-driven shifts. We suggest that the slower speed of voluntary shifts reflects the "clock speed of free will". Wishing to attend to something takes more time than shifting attention in response to sensory input.