J. Richard Simon (1969; Simon & Rudell, 1967; see also Lu & Proctor, 1995; Simon, 1990; Umilta & Nicoletti, 1990) studied the effects of compatibility between a non-spatial stimulus in a location that was irrelevant, and a location-defined response. In a typical Simon task, subjects are shown a letter (or some other stimulus) on either the left or the right side of the display. Subjects are told to press either a left key or a right key based on the identity of the stimulus (e.g. what letter it is). The location of the stimulus is irrelevant to the task, but it can be either consistent with the location of the response (e.g. a letter requiring a right key press appearing on the right side of the screen) or inconsistent with it (e.g. a letter requiring a right key press appearing on the left side of the screen). Responses are faster and more accurate for consistent than for inconsistent trials. The difference in reaction time is called the Simon Effect.
Simon tasks permit a great deal of variation while still yielding the same consistency effect. For example, auditory stimuli are often used, presented in either the left ear or the right ear (e.g. Mewaldt, Connelly & Simon, 1980; Simon & Acosta, 1982; Simon, Craft, & Webster, 1973; Simon & Small, 1969). When visual stimuli are used, the irrelevant stimulus set is usually horizontal position, but can also be vertical position (e.g. Ladavas & Moscovitch, 1984; Nicoletti & Umilta, 1984, 1985; Proctor & Reeve, 1986; Umilta & Nicoletti, 1985), or even non-spatial attributes such as letter or word (e.g. Kornblum & Lee, 1995; Zhang & Kornblum, 1998).
Simon tasks are also occasionally based on semantic overlap between the irrelevant stimulus and the response. For example, subjects may be told to respond to a letter that is presented along with an arrow that is pointing to the left or the right. In this case, the arrow is irrelevant to the response, but can be either consistent (e.g. a left-pointing arrow on a trial requiring a left-side key press) or inconsistent (e.g. a left-pointing arrow on a trial requiring a right-side key press) with the response. This also produces the usual S-R consistency effect.
NOTE: Sometimes you will see researchers use the term “Simon task” to refer to cases where both the stimuli and the responses are characterized by both position and color, for example by asking subjects to press a green key that is on the left side or a red key that is on the right side. In the Dimensional Overlap taxonomy, this is not a Type 3 task: it is a Type 5 task, like the Hedge & Marsh task. Even though it does have overlap between the irrelevant stimulus location and the response location, there is also overlap between the relevant stimulus color and the response color. The dimensional overlap model therefore contends that the cognitive processing in these tasks is fundamentally different from the cognitive processing involved in a standard Simon task, because of the addition of overlap between the relevant stimulus and the response dimension.