Abstract
When interacting with our environment we generally make use of egocentric and allocentric object information by coding object positions relative to the observer or relative to the environment, respectively. Bayesian theories suggest that the brain integrates both sources of information optimally for perception and action. However, experimental evidence for egocentric and allocentric integration is sparse and has only been studied using abstract stimuli lacking ecological relevance. Here, we investigated the use of egocentric and allocentric information during memory-guided reaching to images of naturalistic scenes. Participants encoded a breakfast scene containing six objects on a table (local objects) and three objects in the environment (global objects). After a 2 s delay, a visual test scene reappeared for 1 s in which 1 local object was missing (= target) and of the remaining, 1, 3 or 5 local objects or one of the global objects were shifted to the left or to the right. The offset of the test scene prompted participants to reach to the target as precisely as possible. Only local objects served as potential reach targets and thus were task-relevant. When shifting objects we predicted accurate reaching if participants only used egocentric coding of object position and systematic shifts of reach endpoints if allocentric information were used for movement planning. We found that reaching movements were largely affected by allocentric shifts showing an increase in endpoint errors in the direction of object shifts with the number of local objects shifted. No effect occurred when one local or one global object was shifted. Our findings suggest that allocentric cues are indeed used by the brain for memory-guided reaching towards targets in naturalistic visual scenes. Moreover, the integration of egocentric and allocentric object information seems to depend on the extent of changes in the scene.
Highlights
IntroductionWhen reaching to a visual target in a naturalistic environment, the brain can make use of absolute or relative spatial information for reach planning
When reaching to a visual target in a naturalistic environment, the brain can make use of absolute or relative spatial information for reach planning. This can be formalized in terms of two broad classes of reference frames: an egocentric reference frame that represents the absolute position of an object with respect to the observer and an allocentric reference frame coding the position of an object relative to other objects in the environment (Colby, 1998)
We show that memoryguided reaches to images of naturalistic environments are planned using both egocentric and local allocentric information, but not global allocentric cues
Summary
When reaching to a visual target in a naturalistic environment, the brain can make use of absolute or relative spatial information for reach planning. If landmarks are present while participants reach to remembered targets updated in their visual periphery, the influence of gaze-dependent spatial coding has been found to decrease suggesting a combined use of egocentric and allocentric information (Schütz et al, 2013). Such combination of egocentric and allocentric reference frames is supposed to occur after the intervening saccade at the time of action (Byrne et al, 2010) and depends on heuristics for external cue stability as well as the reliability of egocentric and allocentric cues which determines the weighting in memory-guided reaching
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