The illusion of time: Perceiving the effect before the cause

A novel temporal illusion, in which the cause of an event is perceived to occur after the event itself, provides some insight into the brain mechanisms underlying conscious perception. The illusion, described in the journal Current Biology by a team of researchers from France, suggests that the unconscious representation of a visual object is processed for around one tenth of a second before it enters conscious awareness.

Chien-Te Wu and his colleagues at the Brain and Cognition Research Centre in Toulouse used a visual phenomenon called motion-induced blindness, in which a constantly rotating background causes prominent and motionless visual stimuli to disappear and reappear, as demonstrated in the video below. Fixate on the flashing green spot in the centre, and you’ll notice that the surrounding yellow spots begin to disappear and reappear after about ten seconds. Then replay the clip and focus on any of the yellow spots; you’ll see that it is a visual disappearance illusion. Exactly how it works is unclear; according to one hypothesis it is due to the properties of neurons in area V1 of the visual cortex.

The researchers first used a variation of these stimuli to test the occurence and duration of the motion-induced blindness effect. In these pre-test trials, seven participants were presented with a static yellow ring on a rotating background, and asked to report when the ring disappeared from and reappeared to conscious awareness, by respectively pressing and releasing a button. This was repeated 200 times for each participant, and the reported durations in all trials – between a few hundred milliseconds and several seconds – were plotted onto a graph. The data from each participant were then divided into four equal sets, and the average of the lowest 25% was calculated, to give a value called PreQ25.

In the test trials, a dot was flashed for 50 milliseconds in the location of the ring after it was perceived to disappear. For each participant, this was timed so that the dot appeared at the exact time delay given by the PreQ25 value in the previous trials. In 75% of the trials, the dot was perceived to appear before the reappearance of the ring. As expected, it hastened the perceived reappearance of the ring, as revealed by second graph plotting motion-induced blindness duration. In the remaining trials, the ring should have been perceived to reappear before the dot was flashed. However, the participants reported seeing the ring before the dot in around 90% of trials. Evidently, the perceived time sequence of events had been reversed – the participants reported seeing the cause (the dot) after the effect (the reappearance of the ring).

How might this be explained? One possibility is that the duration of motion-induced blindness was shorter in the test than in the pre-test trials. But further measurements of the duration, carried out during  post-test trials, showed that it had not changed significantly across the trials. Another possible explanation is that the flashed dots were also rendered invisible by the illusion, as it was presented in the same location as the ring, but a second experiment ruled this out. The participants were presented with two rings on opposite sides of the roating background. A dot was flashed in the centre of each, half a second after the onset of the illusion, and they were asked to state the order in which they perceived the dots to appear. This was then repeated on a static background. The reported order was the same in both conditions, showing that the illusion had no effect on the perception of the dots. 

Wu and his colleagues therefore conclude that the unconscious representation of the ring is perceived with a shorter time delay than the flashed dot. That is, because the visual system contained a representation of the ring before it was rendered invisible, it could reactivated quickly and fast-tracked into conscious awareness when it was perceived to reappear. The flashed dot, on the other hand, was a completely novel stimulus, so took longer to enter the stream of consciousness. 

The researchers then carried out another experiment designed to measure the time difference between processing in the conscious and unconsious streams. They repeated the first experiment, but this time introduced a subtle change in the colour of the ring, which occurred at various times relative to the flashed dot. After each trial, the participants reported whether they perceived the ring or the dot first, and were also asked what colour the ring was when it first reappeared. By plotting colour choice against the time of the colour change, the researchers could calculate exactly when the unconscious representation of the ring entered into conscious awareness.

This revealed that the colour change needed to occur about one tenth of a second before the flashing dot in order for the ring and the dot to be perceived as appearing simultaneously. In other words, after the flashed dot induced perception of the ring, the first ring colour that the participants tended to perceive was the colour that the ring had been about 100 milliseconds before the dot appeared, rather than the colour it was at the time the dot was flashed.

Finally, the researchers investigated whether the time illusion could be induced by a time mismatch between the conscious and unconscious representations of the same object, rather than by differences in the time at which two different objects appear. To do so, they modified the last experiment, and introduced a prominent change in the colour of the ring. Here, the dot was not flashed – instead, the colour change occured while the ring was perceptually suppressed, and triggered the ring’s early reappearance. The participants were required to state which the colour the ring was when it first reappeared into their awareness. Remarkably, the colour most often reported was the old one, even though it was the change to the new colour that triggered the reappearance of the ring. So although the new colour was registered unconsciously by the visual system, it was the old colour that was perceived when the ring reappeared. 


Wu, C.-T., et al (2009). The Temporal Interplay between Conscious and Unconscious Perceptual Streams. Curr. Biol. DOI: 10.1016/j.cub.2009.10.017.

10 thoughts on “The illusion of time: Perceiving the effect before the cause

  1. Crazy! I find that when the yellow dots disappear, and I notice it, that my eyes seem to re-scan (I can feel it!) and they pop back up. The harder I try to concentrate on the blinking green dot, the longer the yellow disappear for. But it’s hard!

  2. Sounds like perceptual frame-snapping.
    I have seen wheel spokes appear to reverse rotation when they get aroune 10 rpm – in broad daylight – not stroboscopic.

  3. Fascinating.
    (This may be a naive idea of what’s going on, or maybe it’s just rephrasing what you’ve said in other terms. If so, then apologies.) But surely the illusion in the first video must involve a story featuring at least two things. a) We know that there are fewer color receptors at the periphery of the eye (which would explain why the illusion is more effective when you’re closer to the picture and the picture is enlarged). b) The receptors are color-sensitive, so perhaps the perception of yellow is “crowded out” by the blue background.
    (Granted, that wouldn’t be enough of a story, because we also need to explain why movement matters. I guess then memory must play a role, perhaps in the way you so clearly explained later in the post.)
    Leading up to my question: would the same effect happen if the background matrix were a different color?

  4. Here’s a very interesting 44 min. talk by Prof. Meister from Harvard that he gave at the place where I work a few days back on the subject of visual perception and some amazing stuff that the neurons in the retina do.
    Unfortunately the demos are not visible due to the way the talk was recorded.

  5. “In the remaining trials, the ring should have been perceived to reappear before the dot was flashed. However, the participants reported seeing the ring before the dot in around 90% of trials.”
    Doesn’t this mean they saw what was predicted 90% of the time, in contradiction to your conclusion:
    “Evidently, the perceived time sequence of events had been reversed – the participants reported seeing the cause (the dot) after the effect (the reappearance of the ring).”

  6. I’ve been aware of this for some years now, and this article gave me a name for it. I believe this phenomenon can account for some motor vehicle accidents, where a driver fails to see a significant object along his way due to distraction of other moving objects in his visual field, or due to flashing lights along the road. The giant LED movie screens now being erected along major highways may thus have some unfortunate consequences for the traveling public.

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