Abstract
Everyday actions often require movements to be planned before the exact target is known, yet most research on grasping has examined situations in which individuals reach toward a single, well-defined object. The present study investigated how grasping actions are planned under target uncertainty using a go-before-you-know (GBYK) paradigm, in which participants initiated their movement before knowing which of two potential objects would become the target. Using 3D motion capture, we examined whether peak grip aperture (PGA) was influenced by the size of a competing nontarget object. Participants grasped cylindrical objects that varied in size under two conditions: GBYK, in which target identity was revealed after movement initiation, and know-before-you-go (KBYG), in which the target was known prior to movement onset. Across two experiments, participants appropriately scaled grip aperture according to the target object in both conditions. However, during GBYK trials, PGA was influenced by the presence of a larger nontarget object. Specifically, when grasping a small target paired with a large nontarget, participants produced larger grip apertures, whereas a small nontarget did not influence grasping toward a large target. This asymmetric interference pattern is inconsistent with a motor averaging account, which predicts bidirectional shifts toward an intermediate aperture. Instead, the findings suggest that under conditions of uncertainty, grasp planning incorporates competing object information in a manner that prioritizes successful object acquisition. Together, these findings extend research on action planning under uncertainty beyond reaching and highlight the dynamic nature of grasp planning when multiple potential targets are present.