No previews are good news: using preview search to probe categorical grouping for orientation.

We used the preview search procedure (Watson, D. G., & Humphreys G. W. (1997). Prioritising selection for new objects by top-down attentional inhibition of old objects. Psychological Review, 104, 90-122.) to examine distractor grouping in visual search for categorically-defined targets in the orientation dimension (Wolfe, J. M., Friedman-Hill, S. R., Stewart, M. I., & O'Connell, K. M. (1992). The role of categorization in visual search for orientation. Journal of Experimental Psychology: Human Perception and Performance, 18, 34-49). Participants searched for a relatively steep target presented amongst distractors of two shallow orientations. In a preview condition, the different distractors were presented in different time steps and search was found to be worse than a full-set baseline (Experiment 1). Further experiments determined this was not due to attentional capture by new distractors that were steeper than old items, nor to participants using different search strategies in the preview and full-set baselines. However, there were costs to performance when the old distractor group differed in orientation from the new distractors. We attribute the results to the preview condition disrupting grouping between distractors, with the different distractor groups then competing for selection with the target. An examination of the time-course of the preview effect suggested that grouping and segmentation was fast-acting, and separate from a process such as visual marking, involving the slow suppression of distractors over time. Under asynchronous presentation conditions, preview and new distractors that differ from the target orientation category, can compete rather than cooperate in grouping.