Although the human brain is incredibly fast, processing of visual impressions is a complex endeavor taking the brain several hundred milliseconds before they enter our consciousness.
New research finds that recognition may be expedited if the brain possesses some prior information — that is, when it already knows what it is about to see. The research overturns what neuroscientists assumed — that the processes leading up to conscious perception were rather rigid and that their timing did not vary.
On their way from the eye, visual stimuli are analyzed in several 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.
Researchers at the Max Planck Institute for Brain Research in Frankfurt, Germany, were 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 recognizable, 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.
The participants took relatively long to recognize the symbol in the first sequence of images with increasing visibility, but the threshold of awareness in the second, reverse presentation of images was much lower. The participants were able to recognize the letters even at very poor resolution.
“Expectations based on previously acquired information apparently help to perceive the object consciously,” said Dr. 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 Dr. Wolf Singer, director of the Department for Neurophysiology at Max Planck.
Processing is thus faster if the brain only has to compare the incoming visual information with a previously established expectation. As a result, conscious perception occurs earlier. In contrast, if the brain has to assess a stimulus from scratch due to a lack of prior information, the processing takes longer.
These results may show that previous EEG studies have been interpreted incorrectly.
“Since the interpretation depends heavily on the sequence of events, EEG activity may have been incorrectly allocated to consciousness processes,” Singer said. “In light of these results, it appears necessary to reinvestigate the neuronal correlates of consciousness.”
Source: Max Plank Institute