Task frequency influences stimulus-driven effects on task selection during voluntary task switching.

Task selection during voluntary task switching involves both top-down (goal-directed) and bottom-up (stimulus-driven) mechanisms. The factors that shift the balance between these two mechanisms are not well characterized. In the present research, we studied the role that task frequency plays in determining the extent of stimulus-driven task selection. In two experiments, we used the basic paradigm adapted from Arrington (Memory & Cognition, 38, 991-997, 2008), in which the effect of stimulus availability serves as a marker of stimulus-driven task selection. A number and letter appeared on each trial with varying stimulus onset asynchronies, and participants performed either a consonant/vowel or an even/odd judgment. In Experiment 1, participants were instructed as to the relative frequency with which each task was to be performed (i.e., 50/50, 60/40, or 75/25) and were further instructed to make their transitions between tasks unpredictable. In Experiment 2, participants were given no instructions about how to select tasks, resulting in naturally occurring variation in task frequency. With both instructed (Exp. 1) and naturally occurring (Exp. 2) relative task frequencies, the less frequently performed task showed a greater effect of stimulus availability on task selection, suggestive of a larger influence of stimulus-driven mechanisms during task performance for the less frequent task. When goal-directed mechanisms of task choice are engaged less frequently, the relative influence of the stimulus environment increases.