Although the cognitive ability "perceptual speed" is known to influence search performance by end-users, previous research has not established the mechanism by which this influence occurred. Results from educational psychology suggest that learning that occurs during searching is likely to be infhtenced by perceptual speed. An experiment was designed to test how this cognitive ability would interact with a system feature designed to enhance learning of search vocabulary, specifically,

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... subject descriptors as the first element in the display of a reference. Results showed significant interactions between perceptual speed and the order of presentation of data elements in predicting both vocabulary learning and search performance. These results indicate that searchers with higher levels of perceptual speed will learn additional search vocabulary, and use that vocabulary to complete higher quality searches, when they use a system designed to optimize scanning of subject descriptors. This outcome supports the idea that cognitive abilities influence information system usability, and that usability is determined by interactions between characteristics of users and system features. The findings also suggest that system features that enhance the learning of search vocabulary, such as query expansion mechanisms, can have a significant positive effect on the quality of end-user searching. Purpose Perceptual speed is a cognitive ability defined as "Speed in comparing figures or symbols, scanning to find figures or symbols, or carrying out other very simple tasks involving visual perception" [1] . Allen showed that this ability influences performance of untrained users who search information retrieval systems. People who score higher on standard tests of perceptual speed perform higher quality searches (as measured by standard precision and recall ratios) than those who score lower on the tests This finding is of some interest, and it raises a number of additional research questions. It seems a reasonable assumption that the influence of perceptual speed on search performance occurs primarily in a small number of tasks. In doing a search, a user accomplishes a variety of specific tasks: defining the topic of the search, selecting appropriate search vocabulary, issuing commands or selecting menu choices, viewing retrieved information and making judgments about its relevance or usefulness. The first research problem addressed in this research was identifying the task or tasks most likely to be influenced by perceptual speed. System designers and programmers make choices that influence the usability of information systems. At the same time, cognitive abilities such as perceptual speed influence the usability of information systems. Specific design features can combine with specific user characteristics to ensure that information systems are more usable by some people than others. Accordingly, the second problem addressed in this investigation was to identify a specific design feature that might be associated with the influence of perceptual speed on retrieval performance. This design feature could be incorporated into information systems that would take advantage of the high levels of perceptual speed of some users.