Tuesday September 25 2007
CEA
Brain imaging reveals the mechanisms involved in the transition from unconscious to conscious
Plos Biology, (2007) EOP Sept.25.
CEA
A joint CEA-Inserm team from NeuroSpin , working in tandem with the CNRS, has focused research towards the transition between unconscious and conscious perception. The team has published a study in the 25th September issue of PLoS Biology showing that conscious perception, which culminates in visual conscience, is achieved at around 270 milliseconds after a flashed stimulus, and that it is preceded by several unconscious neuronal stages characterized in detail by the research team.
What is the brain’s role in conscious perception? How does it react when the threshold to consciousness is crossed? CEA-led techniques in neuroimaging have been coupled with an experimental psychology study to extend our knowledge of these processes.
The research team obtained these results by presenting healthy volunteers with two stimuli, where the first stimulus (a number) served as a ‘target’, while the second stimulus (a series of letters) played the role of a visual mask designed to camouflage perception of the first stimulus. Perception of the first stimulus turned out to be dependent on the target-mask interval. When the stimuli are presented at virtually the same time, there is no perception of the first stimulus, as if it remains ‘invisible’. It therefore follows that longer target-mask intervals give subjects a greater chance of perceiving the first stimulus. The researchers went on to evaluate the threshold for access to conscious perception (‘visual consciousness') and study the changes in brain activity occurring a the crossover of this threshold.
Employing high-density electroencephalography, which measures event-related potentials close to the scalp, the team observed two distinct phases in the time course of cortical activity. In the first phase, which occurs before 270 ms, it is the posterior cortices associated with recognizing visual stimuli that were activated. This activation was able to occur in the absence of any conscious perception (‘subliminal’ processing). In the second phase, after the 270-mss threshold, when the perception of stimuli becomes conscious, it is the more anterior frontal and parietal lobes that are mobilized to propagate and amplify this cortical activity.
This research may well improve our understanding of the neuronal mechanisms at the root of certain pathologies believed to be associated with impaired conscious processing, as recently demonstrated by this same team for schizophrenia[1].
[1] Del Cul, A., Dehaene, S. and Leboyer, M. Preserved subliminal processing and impaired conscious access in Schizophrenia.Archives of General Psychiatry, (2006) Dec; 63(12):1313-23.
Legend: the figure highlights the forntal cortex to show the two steps involved in visual perception:
1) a first step taking place in the posterior brain regions (the curves farthest right) where rthere is a progressive increase in activity with less masking.
2) a second, lter step, characteristic of the frontal regions, showing a dramatic (non-linear) increase in activity when the stimulus becomes visible - a step also characterized by amplified posterior region activity related to the first step.
Référence :
A. Del Cul, Baillet, S. Dehaene, Brain dynamics underlying the nonlinear threshold for access to consciousness, Plos Biology, (2007) Sept.25.
Press contacts:
Antoine Del Cul
Inserm U.562 - Cognitive Neuroimaging
CEA/SAC/DSV/DRM/NeuroSpin
Bât 145, Point Courrier 156
F-91191 GIF/YVETTE, FRANCE
http://www.unicog.org/
Tel: 33 (0)1 69 08 81 87
Fax: 33 (0)1 69 08 79 73
E-mail: antoine.delcul@cea.fr