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.
- Igniting the flame of consciousness
- Reading the contents of working memory
- Visual images reconstructed from brain activity
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.