SIMON EFFECT. This proposition states that the spatial relations between stimuli and responses influence participants' behavior in reaction-time experiments even when spatial position is not the relevant stimulus dimension. The effect of such "task-irrelevant spatial correspondence" between stimulus and response was first described by J. R. Simon and A. P. Rudell (1967) in the auditory modality and by J. L. Craft and J. R. Simon (1970) in the visual modality. The phenomenon eventually became known as the Simon effect (cf., Lu & Proctor, 1995), and is generally based on the assumption that it arises from a conflict between the "spatial code" of the stimulus and that of the response (cf., the Stroop effect; and for a "computation model" of the Simon effect, see Zorzi & Umilta, 1995). A typical Simon-effect task involves a testing paradigm in which the participant is presented with two stimuli (e.g., two geometrical shapes) and is instructed to press a left-hand key in response to one of them (e.g., a circle), and to press the right-hand key in response to the other (e.g., a square). The stimuli are presented randomly to the left or right side of a fixation point on a screen. In such a situation, the stimulus position is not "task relevant," meaning that the coding of stimulus position is not necessary for selection of the correct response. However, even though participants are instructed to ignore stimulus location, their reaction-times are faster when the position of the stimulus corresponds to that of the response (i.e., left-left, or right-right) and slower when it does not correspond (i.e., left-right, or right-left). The Simon effect has been explained variously by psychologists in terms of a coding hypothesis, an attentional hypothesis, an orienting reaction, and an integrated model of attention-orienting as a basic process in generating the spatial code. See also STROOP EFFECT/INTER-FERENCE EFFECT/STROOP TEST; REACTION-TIME PARADIGMS/MODELS. REFERENCES

Simon, J. R., & Rudell, A. P. (1967). Auditory S-R compatibility: The effect of an irrelevant cue on information processing. Journal of Applied Psychology, 51, 300-304. Craft, J. L., & Simon, J. R. (1970). Effects of an irrelevant auditory stimulus on visual choice reaction time. Journal of Experimental Psychology, 86, 272-274.

Lu, C.-H., & Proctor, R. W. (1995). The influence of irrelevant location information on performance: A review of the Simon and spatial Stroop effects. Psychonomic Bulletin and Review, 2, 174-207.

Zorzi, M., & Umilta, C. (1995). A computational model of the Simon effect. Psychological Research, 58, 193205.

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