Contextual factors determine the use of allocentric information for reaching in a naturalistic scene

Numerous studies have demonstrated that humans incorporate allocentric information when reaching toward visual targets. So far, it is unclear how this information is integrated into the movement plan when multiple allocentric cues are available. In this study we investigated whether and how the extent of spatial changes and the task relevance of allocentric cues influence reach behavior. To this end, we conducted two experiments where we presented participants three-dimensional-rendered images of a naturalistic breakfast scene on a computer screen. The breakfast scene included multiple objects (allocentric cues) with a subset of objects functioning as potential reach targets (i.e., they were task-relevant). Participants freely viewed the scene and after a short delay, the scene reappeared with one object missing (target) and other objects being shifted left- or rightwards. Afterwards, participants were asked to reach toward the target position on a gray screen while fixating the screen center. We found systematic deviations of reach endpoints in the direction of object shifts which varied with the number of objects shifted, but only if these objects served as potential reach targets. Our results suggest that the integration of allocentric information into the reach plan is determined by contextual factors, in particular by the extent of spatial cue changes and the task-relevance of allocentric cues.