Person:

Wolfe, Jeremy

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.

How swiftly can the object of your attention be changed? Consider two ways to deploy attention: it can be commanded from place to place by a deliberate act of will, or it can run freely without specific instruction. Here we use a visual search task to show that deliberate movement of attention is significantly slower because of an internal limit on the speed of volitional commands. Extensive research on visual search tasks such as finding the letter F among other letters has revealed that stimuli of this sort can be searched at a rate equivalent to one letter every 25-50 ms. The order of search, even of eye movements, is influenced by stimulus salience and eccentricity, but is otherwise random through the set of salient loci. Why is there sparse evidence for systematic scanning of search displays? We argue that searching is free-running ('anarchic') because commanded, ordered deployment of attention is so much slower than anarchic deployment that it is faster overall to make many anarchic attentional deployments than fewer orderly ones.