Gestalt similarity groupings are not constructed in parallel.

Our visual system organizes spatially distinct areas with similar features into perceptual groups. To better understand the underlying mechanism of grouping, one route is to study its capacity and temporal progression. Intuitively, that capacity seems unlimited, and the temporal progression feels immediate. In contrast, here we show that in a visual search task that requires similarity grouping, search performance is consistent with serial processing of those groups. This was true across several experiments, for seeking a single ungrouped pair among grouped pairs, vice versa, and for displays with tiny spacings between the grouped items. In a control condition that ruled out display complexity confounds, when the small inter-object spacing was removed so that that pairs touched, removing the need to group by similarity, search became parallel. Why is similarity grouping so slow to develop? We argue that similarity grouping is 'just' feature selection - seeing a red, bright, or square group is global selection of those features. This account predicts serial processing of one feature group at a time, and makes new falsifiable predictions about how properties of feature-based selection should be reflected in similarity grouping.