Perceiving spatial relations via attentional tracking and shifting.

Perceiving which of a scene's objects are adjacent may require selecting them with a limited-capacity attentional process. Previous results support this notion [1-3] but leave open whether the process operates simultaneously on several objects or proceeds one by one. With arrays of colored discs moving together, we first tested the effect of moving the discs faster than the speed limit for following them with attentional selection [4]. At these high speeds, participants could identify which colors were present and determine whether identical arrays were aligned or offset by one disc. They could not, however, apprehend which colors in the arrays were adjacent, indicating that attentional selection is required for this judgment. If selection operates serially to determine which colors are neighbors, then after the color of one disc is identified, attention must shift to the adjacent disc. As a result of the motion, attention might occasionally miss its target and land on the trailing disc. We cued attention to first select one or the other of a pair of discs and found the pattern of errors predicted. Perceiving these spatial relationships evidently requires selecting and processing objects one by one and is only possible at low object speeds.