Expectations speed up conscious perception

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Expectations speed up conscious perception

February 3, 2011

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 furt am Main have now shown that this delay may vary in length.

When the brain possesses some prior information & #8722; that is, when it already

knows what it is about to see & #8722; 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. (Journal of

Neuroscience, January 26 2011)

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.

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”, says 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”, says Lucia 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”, explains Wolf Singer. 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”, surmises Wolf Singer, the Director of the Department for

Neurophysiology at the Max Planck Institute for Brain Research in furt. “In

light of these results, it appears necessary to reinvestigate the neuronal

correlates of consciousness.”

Original publication:

Lucia Melloni, Caspar M. Schwiedrzik, Notger Müller, Eugenio , Wolf

Singer

Expectations change the signatures and timing of electrophysiological correlates

of perceptual awareness

Journal of Neuroscience, January 26, 2011, 31(4):1386-1396