When the brain switches to conscious perception
This study, conducted by researchers at the CEA and Inserm, is based on experiments involving electroencephalograph (EEG) recording of brain activities associated with visual stimuli that were sometimes perceived and sometimes not perceived consciously in the same person. The stimuli were a series of images in which two words were displayed less than half a second apart. Volunteers were asked to say whether they saw the words. In this type of experiment the second word is not systematically perceived, offering the researchers ideal conditions for analysing the chains of neuronal processes involved in conscious and unconscious perception and the differences between these two types of processing. The high temporal accuracy of the electroencephalographic technique enabled them to determine the exact moment when the two types of perception were differentiated.
It was found that the neural routes taken were the same during the first moments of information processing. However, 270 milliseconds after the second word was displayed the researchers observed a difference in activation between the perceived and non-perceived words. From 300 milliseconds they found a series of activations that were present only for the perceived words. The conscious and unconscious chains thus seem to diverge rapidly after 270 milliseconds. The researchers also found that the conscious perception of the second word depended on the speed at which the first one was processed: the more slowly the first word was processed, the more likely it was that the second word was not perceived. In other words, the absence of perception of the second word was due to the fact that the neuronal resources necessary for conscious perception were busy processing the preceding information, and so were not available for the second word.
Conscious perception thus seems to involve an optional late activation (after 270 ms) that is propagated rapidly in a network of brain areas located in the frontal and parietal lobes. In the long term the characterisation of these neuronal mechanisms of conscious perception may give us a better understanding of a certain number of health disorders that appear to be associated with defective conscious processes, such as schizophrenia.
Press contact:
Célie Simeray – tél : 01 40 56 14 88 – email : celie.simeray@cea.fr