Effort minimization drives saccade selection

What determines where we move our eyes? We here hypothesized saccade costs to determine saccade selection. We first mapped saccade costs across directions by cueing participants to make a saccade towards a specific direction but to withhold the saccade until after the cue had disappeared. During this phase, we measured pupil size - an indicator of noradrenaline release and mental effort - to index cost. Next, we mapped saccade preferences by presenting any two of the previously cost-mapped saccade directions as a two alternative free choice task. For the first time, we here demonstrate that this cost critically underpins saccade selection: When participants chose between the two possible directions, low-effort options were strongly preferred (R2=0.58). Notably, saccades curved away from high-cost directions, suggesting an active weighing of costs and inhibition of costly alternatives. This general principle held when participants searched in natural scenes: cost remained a predictor of saccade direction preferences. Strikingly, effortful saccade directions were disproportionally avoided as soon as overall load was increased by introducing a secondary auditory counting task (R2=0.50). This implies that cognitive resources are flexibly (dis)allocated from and to oculomotor processes as resource demands change. Together, this shows that even the most subtle differences in cost are actively weighed to tune for resource-efficient behavior. Beyond stimulus material and goals, we therefore argue that eye-movement behavior is largely determined by a distinct and equally fundamental factor: effort.