The purpose of this paper is to investigate the effect of irrelevant location information on performance of visual choice-reaction tasks. We review empirical findings and theoretical explanations from two domains, those of the Simon effect and the spatial Stroop effect, in which stimulus location has been shown to affect reaction time when irrelevant to the task. Wethen integrate the findings and explanations from the two domains to clarify how and why stimulus location influences performance even when it is uninformative to the correct response. Factors that influence the processing of irrelevant location information include response modality, relative timing with respect to the relevant information, spatial coding, and allocation of attention. The most promising accounts are offered by models in which response selection is a function of (1) strength of association of the irrelevant stimulus information with the response and (2) temporal overlap of the resulting response activation with that produced by the relevant stimulus information.The role played by stimulus location in visual information processing has been a controversial issue. This issue has been investigated primarily by means ofvisual search tasks in which a target stimulus in an array of distractors must be detected or identified. Reaction time (RT) in such tasks is typically an increasing function ofarray size, and accuracy a decreasing function, when the target is defined by a conjunction of features (e.g., Treisman & Gelade, 1980;Van Zandt & Townsend, 1993), leading some authors to propose that attending to location is necessary for feature integration (e.g., Nissen, 1985;Treisman & Gelade, 1980). As expected if location must be attended in such situations, a stimulus for a second task shows a processing benefit when it occurs in a location adjacent to the target stimulus of the search task rather than in a more remote location (e.g., Hoffman & Nelson, 1981), and precuing the location in the array in which the target stimulus will occur facilitates its processing (e.g., Eriksen & Hoffman, 1972). However, facilitation from a location precue is also apparent when the target is presented alone and requires only a detection response (e.g., Bashinski & Bacharach, 1980;Posner, Snyder, & Davidson, 1980), suggesting that it may be necessary to attend to the locaWe would like to thank Bernhard Hommel, Colin MacLeod, Bob Melara, Jim Neely, Richard Schweickert, Richard Simon, and Howard Zelaznik for helpful comments on previous versions of this manuscript. Reprint requests should be sent to Chen-Hui Lu, Department of Psychology, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, or Robert W. Proctor, Department of Psychological Sciences, Purdue University, West Lafayette, IN 47907-1364. tion of any stimulus on which a response is based. Consistent with this possibility, Tsal and Lavie (1993) recently showed that when the relevant feature of a precue stimulus is nonspatial (e.g., color), there is still facilitation in the identification of a s...