This experiment demonstrates a real phenomenon in time perception: your brain adapts to a delay between an action and its sensory consequence, and then misjudges what happened first.

What you'll do

Press SPACE a few times to get into a rhythm.
Baseline measurement. On each trial, press as fast as you can when told, then judge whether the flash or your press came first.
Adaptation. Keep pressing while every flash is secretly delayed by 200 ms.
Measurement again, with the delay maintained.
The delay is removed, and you see what that feels like.

Takes about 6 minutes. Use a physical keyboard on a desktop or laptop, and avoid other activity while it runs.

First, get into a rhythm.

Press SPACE repeatedly. Every press produces a flash immediately.

Don't try to synchronize anything. Just press at a comfortable, steady pace.

Press SPACE to begin.

Now we'll measure your sense of timing.

Get ready…

Press SPACE the instant you see the cue. The flash is scheduled from a running estimate of your reaction time, so it can genuinely occur before your press. Whatever actually happened is measured and used in the analysis — guessing is fine, just answer with your gut.

Now keep pressing.

Something about the timing has changed. Don't try to work out what it is. Just keep pressing SPACE at a steady pace.

Last block. Keep pressing.

Something has changed again. Pay attention to the flash.

Your results.

Baseline simultaneity point—

After 200 ms adaptation—

Recalibration shift—

Judgments used—

Trials dropped—

Display refresh—

Your temporal-order curves

Each point is the proportion of trials in a 60 ms bin where you said the flash came first; dot size shows how many trials landed in that bin. The lines are fitted psychometric curves, and each vertical marker is that block's point of subjective simultaneity — the flash timing you were equally likely to call either way.

What's going on?

Your brain continually predicts the sensory consequences of your actions. When a delay is inserted between your keypress and the flash, it recalibrates: it starts treating the delayed flash as the normal consequence of pressing.

The measurement is the point of subjective simultaneity (PSS): the physical flash timing at which you call it either way half the time. Recalibration pushes that point later — a flash has to arrive further after your press to feel simultaneous with it. Once that has happened, removing the delay can make the flash feel like it preceded the press that caused it.

Perception is not a timestamp recorder. Your brain combines sensory signals with predictions about when those signals ought to arrive.