In 1969, J. Richard Simon reported a surprising finding: when participants responded to a non-spatial stimulus attribute (e.g., tone pitch) with a left or right keypress, responses were faster when the stimulus happened to occur on the same side as the required response. This stimulus-response (S-R) correspondence effect, now called the Simon effect, has become one of the most studied phenomena in cognitive psychology because it reveals the automatic activation of spatial response codes.
The Basic Phenomenon
In a typical Simon task, participants classify stimuli by a non-spatial attribute (e.g., color: red → left key, blue → right key) while the stimuli appear on the left or right side of the display. Despite position being task-irrelevant, responses are approximately 20–40 ms faster and 2–4% more accurate on corresponding trials (e.g., left stimulus → left response) than on noncorresponding trials (e.g., left stimulus → right response). The effect is remarkably robust, occurring across modalities and response modes.
Dual-Route Models
The dominant explanation is the dual-route model (Kornblum, Hasbroucq, & Osman, 1990; De Jong et al., 1994). Two parallel processing routes contribute to response activation:
Automatic route: Stimulus location → Spatially corresponding response (fast, transient)
On corresponding trials, both routes activate the same response, facilitating performance. On noncorresponding trials, the automatic route activates the wrong response, creating conflict that must be resolved before the correct response is selected. The critical prediction of this model is that the automatic activation is transient — it decays over time, so the Simon effect should diminish or even reverse for slow responses.
The temporal dynamics of the Simon effect are elegantly revealed by delta plots. Unlike most experimental effects (which increase with RT), the Simon effect shows a characteristic negative-going delta plot: the effect is largest for fast responses and diminishes for slow responses. This pattern confirms that the automatic spatial activation is transient, consistent with the dual-route model's prediction that controlled processing eventually overcomes the initial automatic interference.
Computational Models
Several computational models have been developed to account for the Simon effect's time course. Ulrich, Schroter, Leuthold, and Birngruber (2015) developed a diffusion model for conflict tasks (DSTP) that captures both the positive and negative-going delta plot segments. Ridderinkhof (2002) proposed the activation-suppression model, in which an active suppression process inhibits the automatic route, accounting for why the Simon effect shrinks with time and individual differences in the effectiveness of suppression.
The Simon effect has also been examined in the context of sequential modulation (the Gratton effect): the Simon effect is reduced following noncorresponding trials, suggesting trial-by-trial adjustments in cognitive control. This connection to conflict monitoring and adaptation has made the Simon task an important paradigm for studying executive control.