Expectations speed up conscious perception

The human brain works incredibly fast. However, visual impressions are so complex that their processing takes several hundred milliseconds before they enter our consciousness.

Scientists at the Max Planck Institute for Brain Research in Frankfurt am Main have now shown that this delay may vary in length. When the brain possesses some prior information - that is, when it already knows what it is about to see - conscious recognition occurs faster. Until now, neuroscientists assumed that the processes leading up to conscious perception were rather rigid and that their timing did not vary.

This diagram shows the experimental set-up used by Max Planck researchers to examine how the human brain processes visual information. Initially, the subjects participating in the trial had to fix a cross on a screen. Then, they were presented with an image with randomly distributed pixels (random field of noise) for a duration of 500 ms.

After that, a response screen displayed a question mark and subjects could indicate by pressing a button if they were able to recognize a symbol within the random field of noise. The same was repeated with screens, in which the hidden symbol became gradually more and more recognisable. As soon as the symbol had appeared fully and was clearly recognisable, the scientists presented the same screens in reverse order, showing each symbol fading gradually.
 

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. The Max Planck scientists were now able to demonstrate that the timing of this process, far from being rigid, is in fact variable. In an experiment, participants perceived stimuli more efficiently and faster if they knew what to expect.

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.