Conceptual and physical differences in the category effect

In these experiments we tested whether physical differences between letters and digits could account for the category effect in visual detection and partial report. In our task, observers decided whether a target item matched any item in a briefly presented array. This yielded a visual detection task when the target preceded the array and a type of partial-report task when the target followed the array. In Experiment 1, the stimulus set consisted of nine digits and nine letters modified to match the digits on the basis of visual similarity. Partial-report performance was better in a mixed-category condition than in a single-category condition, but no such effect occurred in visual detection. However, the similarity of items between categories may have biased observers against using category information to perform the task. When the similarity was controlled both within and between categories in Experiment 2, a category effect emerged in visual detection as well. A third experiment, using a same-different reaction time task, verified that the stimuli were equally similar within and between categories. The results indicate that physical differences alone cannot explain the category effect.