Koevoet, D.; Van Zantwijk, L.; Naber, M.; Mathot, S.; Van der Stigchel, S.; Strauch, C.

What determines where to move the eyes? We recently showed that pupil size, a marker of noradrenaline release, reflects the effort associated with making a saccade ('saccade costs'). Here we demonstrate saccade costs to critically drive saccade selection: when choosing between any two saccade directions, the least costly direction was consistently preferred. Strikingly, this principle even held during search in natural scenes in two additional experiments. When increasing cognitive demand experimentally through an auditory counting task, participants made fewer saccades and especially cut costly directions. This suggests that the eye-movement system and other cognitive operations consume similar resources that are flexibly allocated among each other as cognitive demand changes. Together, we argue that eye-movement behavior is not only determined by physical saliency, goals, and selection history, but also by a qualitatively distinct and equally fundamental factor: effort.