Simon Effect Test — Free 96-Trial Spatial Interference Task

The Simon effect, reported by J. R. Simon in the late 1960s, is the slowing and increase in errors that occur when a stimulus appears on the opposite side from the button you must press. Although location is irrelevant to the task, the body automatically tends to respond toward it, so the task is widely used to measure how well that automatic response is suppressed.

A red or blue circle appears on the left or right of the screen; ignoring its position, you press the left or right button assigned to its color within 1.5 seconds. The color-to-button pairing is randomized for each session and explained before you start. After 16 practice trials come 96 main trials, mixing 48 congruent trials (circle on the same side as its button) and 48 incongruent trials (opposite side).

The reaction-time difference between congruent and incongruent trials shows the cost of filtering out irrelevant location information, making it an index of both interference control and impulsive responding.

The key measure, the Simon effect, is the median reaction time on incongruent trials minus the median on congruent trials, in milliseconds. Medians are used instead of means so that outliers do not skew the result. The app labels 0 ms or below as no effect, under 25 ms as small, under 60 ms as medium and 60 ms or more as large; most adults fall around 20 to 50 ms.

Alongside the reaction-time difference, an error-rate difference (incongruent minus congruent error rate, in percentage points) is shown; a large value means many impulsive errors driven by location. A delta plot dividing reaction times into four speed bins reveals whether the effect is larger for fast responses and fades for slow ones. The recorded headline score is overall accuracy across the 96 trials.

Touch latency adds to both conditions alike and barely affects the effect size, but practice and familiarity with the color-button pairing can shift results from session to session. A trend across several measurements on the same device is more reliable than any single value.

The ability to suppress spatial interference weakens with fatigue. On short-sleep days, late at night or after alcohol, the Simon effect grows and incongruent errors increase. Caffeine speeds responses but can add hasty, location-driven errors. With age, overall slowing tends to come with a slightly larger effect, and how you hold a large device and reach the left and right buttons also matters.

Soundary saves your results and shows the change from your previous test. Repeating every one to two weeks on the same device at a similar time, and following the trend in Simon effect and incongruent error rate, lets you separate fluctuation from steady change. This is a self-check, not a diagnostic tool.

If incongruent errors remain frequent even after you are rested and familiar with the rule, and in daily life your hands often act before you think in ways that cause trouble, consider discussing an attention and impulsivity assessment with a psychiatrist or other professional.