in a two-choice task, the finding that the response to a stimulus is facilitated if the location of the stimulus corresponds to the location of the response, even though stimulus location is irrelevant to the task. For example, if a left (rather than a right) keypress is the required response to a blue stimulus, reaction time will be quicker if this stimulus is presented on the left-hand side than if it is presented on the right (and vice versa). [discovered in 1969 by U.S. psychologist J. Richard Simon (1929– )]