Mapping visual spatial prototypes: Multiple reference frames shape visual memory.

Categories provide a fundamental source of information used to structure our perception of the world. For example, when people reproduce the remembered location of a dot in a circle, they implicitly impose vertical and horizontal axes onto the circle, and responses are biased towards the center of each of the resulting quadrants. Such results reveal the existence of spatial prototypes, which function as Bayesian priors and which are integrated with actual memory traces. Spatial prototypes have been extensively investigated and described in previous studies, but it remains unclear what type of information is used to create spatial categories. We developed a new approach that allowed to 'image' patterns of spatial bias in detail, and map the internal representational structure of objects and space. Previous studies, using circular shapes suggested that boundaries are established based on a viewer-based frame of reference, therefore using cues extrinsic to the object. Given that a circle has radial symmetry, the axes imposed cannot come from the shape itself. Here we investigated if the same applies for shapes with clearly-defined symmetry axes and thus intrinsic frames of reference. Using rotated shapes (squares and rectangles), where extrinsic and intrinsic cues are dissociated, we observed flexible usage of multiple reference frames. Furthermore, in certain contexts, participants relied mostly on cues intrinsic to the shape itself. These results show that humans divide visual space as a function of multiple reference frames, in a flexible, and context dependent manner.