Conceptual and physical differences in the category effect.

It has been found in a numbr of tasks that the visual processing of an alphanumeric display is more effrcient when some of the items corne from an irrelevant category. For instance, it takes less time to find a digit in an array of lefters than in an array of other digis (e.g., Egeth, Jonides, & Wdl, 1972), and detection of a digit in a brief display is more accurate when the distractors are letters rather than digits (e.g., Schneider & Shiffrin, 1977). This finding can be referred to as a category effect. The category effect is important in theories of visual information processing because it has suggested to some that, at some level, characters are processed in parallel with unlimited capacity (e.g., Durrcan, 1980; Gardner, 1973). However, this conclusion has not been universally accepted, and the mechanism underlying the category effect remains controversial (Deutsch, 1977; Kahneman & Treisman, 1984; Krueger, 1984). In this strrdy, we tested an often proposed counterexplanation of the category ef ct that can be termed the partial analysis account. The unlimited parallel processing view of the category effect assumes that the items in a visual display are identified and categorized in parallel without requiring effort or attention on the part of the observer. However, such subsequent operations as transferring items to short-term memory or comparing items may limit performance. In this view, the category effect occurs because information about the category of items in the visual display can be used to separate relevant from irrelevant items. For instance, if the observers in a visual detection task know