Flash-Lag Effect

from Michael’s Visual Phenomena & Optical Illusions


What to do

Observe the moving blue line, and the second blue line flashing once in a while. Are they aligned? Try while following the line or while fixating the center.

What to observe

You may see two things:

  1. Where the two lines nearly meet, the flashing line appears to lag a little behind the other line
  2. The flashing line seem to have a different angle, where they meet there’s a kink.

As you may have guessed, in reality the two lines align perfectly. If you slow down rotation speed (preset to 15 rpm = revolutions per minute) to 2 or 1 rpm, this becomes obvious.

What else to do

You can change the flash frequency, which is preset to 100 fpm = flashes per minute.

You can change the duration of the flashed line (∆t), preset to 20 ms.
[The actual duration can only occur in increments of 16.6 ms (assuming you have a frame rate of 60 Hz), so the target duration is only a rough indicator of the acutal duration, as indicated in grey below. If your computer’s workload is heavy, these values may dance around.]

You can switch to a different “flash lag” demonstration: use the pop-up menu to select Disk rather than Lines. Now fixate on the central cross, but watch with your inner eye the moving ring. In other words: dissociate gaze direction and attention; this takes some practice.
By now you will have noticed that the blue content of the ring is occasionally replaced by a yellow shape. Is it a full yellow disk or a yellow crescent? If you fixate on the cross, you should only see a crescent. If you follow the ring, you see the full disk. The effect is somewhat subtle.


This is one of the many demonstrations of the “flash-lag” effect. I programmed the first version of this inspired by a fascinating talk by Romi Nijhawan in Freiburg. The explanation, in a nutshell: Our mental perception and planning mechanisms need to take into account the delays in afference, computation & in the efference. Thus moving objects are “predated” / “perceived” a bit ahead of their assumed trajectory; the flash (being essentially stationary) is not. Consequently, one perceives a positional disparity between briefly flashed stationary and moving objects.

I used to think that the flash-lag effect is “the same as” MacKay’s stroboscopic illusion, but since it does not need large luminance differences, it is not completely the same. Both belong, however, to the general scope of problems to integrate sensory information constrained by sensory delays depending on, e.g. ambient luminance, and intermodal latency differences in the order of decades of milliseconds.