Accommodation and vergence require effort-to-see

Accommodation and vergence both appear to be influenced by multiple nonsensory factors. "Effort-to-see" is one of these factors. This study was designed to assess the extent to which effort-to-see affects accommodation and vergence. Nine volunteers participated in this study. Stimuli were chosen to stimulate selectively either accommodation or vergence. Accommodation and vergence responses were measured while observers viewed each stimulus with the instruction of "concentrate" or "space-out." Both oculomotor adjustments were accurate when observers "concentrated," but regressed toward the resting posture during identical stimulus conditions when observers spaced-out. Interesting, individual differences in oculomotor behavior were apparent. This study demonstrates that higher-level attentional factors play an important role in accommodation and vergence for active exploration of the three-dimensional environment.