Please, look at the cross. Where is the upper moving bar when the flashed bar appears?

We usually perceive the upper moving bar to the right of the flashed bar but actually both bars are perfectly aligned (you can check it by using the movie controller).

What is causing the illusion?

The light that reaches our eyes should be transformed into neural signal and then travel to different areas of the brain for further processing. This takes time and this time is estimated to be around 0.1 seconds or even more. So, when something happens in the world, we are not instantaneously aware of it. We need some time to see.

What happens for a moving object? Do we see the object in a location that lags 0.1 seconds the real location in the environment? Consider that this is about 10 cm if the object is moving at walking speed (4 km/h).

Flash-lag illusion

According to some theories, the brain makes predictions to compensate neural delays for moving objects. It is proposed that the previous trajectory of the object is used to shift/extrapolate the delayed representation of position.

According to these compensation theories, the moving bar is perceived without a delay. It is perceived in its real location. The flashed bar, however, has not previous trajectory that the brain can use to predict and then it is perceived with a delay of 0.1 seconds. This is what is causing the flash-lag illusion: by the time the flashed bar is perceived the moving bar is already ahead in the trajectory. Although appealing, the existence of these compensation mechanisms is controversial.

You can experience another related illusion: Instead of looking at the cross, follow the moving bar with the eyes. You will see that the flashed bar is not perceived aligned with the cross anymore. Rather, it looks misaligned to the right. This flash- mislocalization illusion might be related to compensation mechanisms during pursuit eye movements.

Visuomotor timing compensates for changes in perceptual latency. White, A. L., Linares, D., Holcombe, A. O. Current Biology,18(20), pR951-pR953.

Motion signal and the perceived positions of moving objects. Linares, D., López-Moliner, J., Johnston, A. Journal of Vision, 7(7):1, 1-7.

Absence of flash-lag when judging global shape from local positions. Linares, D., López-Moliner. Vision Research, 47, 357-362.

The flash-lag is reduced when the flash is perceived as a sensory consequence of our action. López-Moliner, J., Linares, D. Vision Research, 46, 2122-2129.