Simon Effect Test: Can You Ignore Where It Appeared?
The Simon effect is one of the cleanest demonstrations in attention research: you answer based on color, and the side a stimulus appears on is completely irrelevant — yet when the two disagree, you get slower and less accurate anyway. Your brain maps location to action whether you want it to or not. This test measures your personal Simon cost: the gap between the trials that lined up and the trials that fought you.
A quiet round of Simon Effect Test — Standard. When you start, everything else fades — just you and the task.
No sign-up needed. Press Esc to exit anytime.
Tip
Keep your eyes in the middle. The side the circle appears on is precisely the thing you are trying to ignore, and it pulls hardest when your gaze goes there first. Fixating the center lets the color arrive without your eyes having already voted for a side.
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Simon Effect Test: what it is and where it comes from
What is the Simon effect? The Simon effect test measures response conflict, after Simon and Rudell (1967): you answer by a stimulus's color while ignoring which side of the screen it appears on. Answers slow down when the stimulus sits on the side opposite the correct key. Focani's free browser version runs 20 trials per session and needs no sign-up.
| Specification | Simon Effect Test |
|---|---|
| Primary cognitive domain | Interference control — in the “Holding a rule under pressure” group |
| Original citation | 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(3), 300–304.doi:10.1037/h0020586 |
| Core dependent metrics | Mean reaction time (ms) · Reaction-time variability (SD, ms) · Congruent vs. incongruent reaction time (ms) · Shortest accurate response window (adaptive tier, ms) |
| Standard stimulus / trial parameters | Compatible and incompatible trials are balanced across a few hundred trials so the compatibility effect can be estimated per participant. |
| This version | Standard — 20 trials · Extended — 32 trials · Adaptive — 40 trials |
| Browser adaptation notes | Visual rather than auditory, and far fewer trials per condition. The direction of the effect is reliable at this length; its size in any one session is not. |
Full sources for every test, and what each version changes, are on the instructors page.
How to play
- A colored circle appears on the left or the right of the field.
- Answer by COLOR — teal is left (← key), amber is right (→ key). Tapping works too.
- Where the circle appeared is irrelevant. It will still pull at you.
- Finish all trials to see your accuracy and your Simon cost — the milliseconds the mismatch costs you.
Simon Effect Test tips
Ways to play it better — each one comes from how this version actually scores, not from general advice about concentration.
Keep your eyes in the middle
The side the circle appears on is precisely the thing you are trying to ignore, and it pulls hardest when your gaze goes there first. Fixating the center lets the color arrive without your eyes having already voted for a side.
Rehearse the mapping before you start
Teal-left and amber-right have to be automatic before the first trial. If any part of you is still working out which button is which, the location code — which needs no working out at all — wins that race every time.
You cannot slow your way out of the cost
The Simon cost is the gap between matched and mismatched trials, not your overall time, so an across-the-board slowdown moves both sides together and leaves the gap about where it was. It just costs you score on the way.
One caveat worth knowing: a change of strategy moves your score for reasons that have nothing to do with attention. Your dashboard reads each session against your own recent ones, so if you adopt something from this list, give it a few sessions before reading anything into the trend.
Frequently asked questions
What is the Simon effect?+
It's the finding that people respond faster when a stimulus appears on the same side as the required response, even when position carries no information at all. The slowdown on mismatched trials is the Simon effect, and it shows that location is processed automatically.
How is the Simon effect different from the Stroop test?+
Both are conflict tasks, but the conflict lives in different places. In Stroop, one feature of the stimulus (the word) fights another feature (the ink). In the Simon effect, the stimulus itself is unambiguous — the conflict is between where it is and which hand you need to move. Stroop is stimulus-stimulus conflict; Simon is stimulus-response conflict.
What does a large Simon cost mean?+
It means the irrelevant spatial information leaked further into your response than average on this run. It's a measure of one specific thing on one specific day, not a verdict about you — fatigue and practice both move it noticeably.
Is this the Simon memory game with the colored buttons?+
No — completely different thing. That's a 1978 electronic toy where you repeat a growing sequence of lights. This is the Simon effect from cognitive psychology, named after J. R. Simon, and it has nothing to do with memory.
Where this fits
- Attention Span Test — The same conflict from a different direction: the stimulus is unambiguous, but where it appears fights the hand you need to move. Your Simon cost is the size of that leak.
Related attention paradigms
Holding a rule under pressure — closest to this one
- Stroop Test — practices and measures selective attention and mental control.
- Flanker Test — practices and measures selective attention and interference control.
- Go/No-Go Task — practices and measures inhibitory control and impulse braking.