Doubts are emerging about one of our leading models of consciousness. It seems that brain signals thought to reflect consciousness are also generated during unconscious activity.
A decade of studies have lent credence to the global neuronal workspace theory of consciousness, which states that when something is perceived unconsciously, or subliminally, that information is processed locally in the brain.
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In contrast, conscious perception occurs when the information is broadcast to a 鈥済lobal workspace鈥�, or assemblies of neurons distributed across various brain regions, leading to activity over the entire network.
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Proponents of this idea, at France鈥檚 national institute for health in Gif-sur-Yvette, and his colleagues, discovered that when volunteers view stimuli that either enter conscious awareness or don鈥檛, their brains show identical EEG activity for the first 270 milliseconds. Then, if perception of the stimuli is subliminal, the brain activity peters out. However, when volunteers become conscious of the stimuli, there is a sudden burst of widespread brain activity 300 ms after the stimulus.
This activity is characterised by an EEG signal called P3b, and has been called a neural correlate of consciousness.
Brian Silverstein and Michael Snodgrass at the University of Michigan in Ann Arbor, and colleagues wondered if P3b could be detected during unconscious processing of stimuli.
The study involved the oddball paradigm, in which one stimulus is presented frequently, interspersed with an oddball or rare stimulus. In other research the oddball stimulus prompted a stronger brain response.
The twist in the new experiment was to ensure that all stimuli were entirely subliminal. Subjects were frequently shown the word 鈥淟EFT鈥� for 7 ms, immediately followed by a pattern that masked the word, preventing it from entering conscious awareness. Just 7 ms was long enough for the brain to unconsciously register the word, but too brief for the volunteer to say whether they saw anything. Occasionally, volunteers were shown the word 鈥淩IGHT鈥�, followed by a mask.
The rare stimulus produced a strong P3b signal, widely spread across the brain. 鈥淓ven though they don鈥檛 know [what] the stimuli are, the brain is still able to recognise that there is something unexpected that occurs,鈥� says Silverstein.
The team interprets the P3b signal as evidence for complex, sustained, unconscious brain activity, suggesting that P3b is not a neural correlate of consciousness, thus contradicting the global neuronal workspace theory.
Neuroscientist of the University of Sussex in the UK is impressed by the rigorous methods used in the study, but cautions against interpreting the P3b signal as indicative of complex cognition. 鈥淭he subject isn鈥檛 doing anything that need involve complex, sustained cognitive activity,鈥� says Seth.
Still, he agrees that the study raises questions about neural correlates of consciousness. 鈥淭he neural signatures of conscious processing are likely to be more complex and interesting than just a P3b,鈥� says Seth. 鈥淸The study] is pushing us towards more refined explanations that actually connect neural dynamics to what it is like to be conscious, beyond relying on overly simple signatures like the P3b.鈥�
Journal reference: Cortex,
(Image: Deco/Alamy Stock Photo)
