Visual processing breaks the world into parts and objects, allowing us not only to examine the pieces individually, but also to perceive the relationships among them. There is work exploring how we perceive spatial relationships within structures with existing representations, such as faces, common objects, or prototypical scenes. But strikingly, there is little work on the perceptual mechanisms that allow us to flexibly represent arbitrary spatial relationships, e.g., between objects in a novel room, or the elements within a map, graph or diagram. We describe two classes of mechanism that might allow such judgments. In the simultaneous class, both objects are selected concurrently. In contrast, we propose a sequential class, where objects are selected individually over time. We argue that this latter mechanism is more plausible even though it violates our intuitions. We demonstrate that shifts of selection do occur during spatial relationship judgments that feel simultaneous, by tracking selection with an electrophysiological correlate. We speculate that static structure across space may be encoded as a dynamic sequence across time. Flexible visual spatial relationship processing may serve as a case study of more general visual relation processing beyond space, to other dimensions such as size or numerosity.
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