Until now, neuroscientists assumed that the processes leading to conscious perception were rather rigid and that the timing did not vary.
However, scientists at the Max Planck Institute for Brain Research in Frankfurt have now demonstrated that the timing of this process, far from being rigid, is in fact variable.
When the brain possesses some prior information - that is, when it already knows what it is about to see - conscious recognition occurs faster.
The researchers found that participants perceived stimuli more efficiently and faster if they knew what to expect.
On their way from the eye, visual stimuli are analysed in manifold ways by different processing stages in the brain.
It is not until they have passed several processing steps that the stimuli reach conscious perception. This unconscious processing prior to perception usually takes approximately 300 milliseconds.
To investigate this, the scientists showed the participants images with a background of randomly distributed dots on a monitor.
During an image sequence, the distribution of the dots systematically changed such that a symbol gradually appeared.
Following each image, the participants indicated if they could see the symbol by pressing a button.
As soon as the symbol had appeared fully and was clearly recognisable, the scientists presented the same image sequence in reverse order, such that the symbol gradually faded again.
During the entire experiment, electroencephalographic (EEG) activity of the participants was measured.
Whereas the participants took relatively long to recognise the symbol in the first sequence of images with increasing visibility, the threshold of awareness in the second, reverse presentation of images was much lower.
The participants were able to recognise the letters even at very poor resolution.
"Expectations based on previously acquired information apparently help to perceive the object consciously," said Lucia Melloni, first author of the study.
Once the participants knew which symbol was hiding in the random field of noise, they were able to perceive it better.
The scientists have thus confirmed previous studies, according to which people perceive moving objects better if they already know in which direction the objects will move.
Moreover, the measurements of EEG activity produced astonishing results.
"We found that the timing of EEG activity for conscious perception changed depending on the person's expectations," said Melloni.
If the participants could predict what they were going to see, the characteristic EEG pattern for conscious perception took place 100 milliseconds earlier than without prior expectations.
The scientists might thus have found a conclusive explanation for the contradictory results of other neuroscientific research groups.
Depending on the study, they had sometimes found very early and sometimes very late EEG activity correlating with conscious perception.
"Our research explains this variability in timing. Apparently, the brain does not process the stimuli rigidly and at the same speed; rather, it is flexible," said Wolf Singer, Director of the Department for Neurophysiology at the Max Planck Institute for Brain Research.