Why are we conscious? Why do we experience our perceptions instead of detecting stimuli mechanically, automatically, thoughtlessly? The deep answer to that question may elude our grasp for some time yet; indeed, many scientists and philosophers believe that consciousness is a fundamental property that cannot be derived from anything else in nature.
But neuroscientists have been making progress towards answering a more accessible question: What brain activities are associated with consciousness? Four years ago, European researchers reported finding a neural “signature” of conscious awareness in human adults; now a team including some of the same scientists report finding a weaker version in infants.
“It seems likely that we’ll find this response in primates too, and probably in some other animals such as dogs,” says Sid Kouider, a neuroscientist at the National Center for Scientific Research in Paris, France, and lead author of the new report in Science.
Consciousness and the “global workspace”
In the study reported in 2009, principal investigator Lionel Naccache and colleagues described a pattern of brain activity that corresponds to conscious visual perception. Using electrodes planted in the brains of ten epilepsy patients—electrodes that are needed to help physicians mark functional areas prior to epilepsy surgery—the researchers measured voltage changes during conscious and non-conscious perception of visual stimuli, in this case images of words. The researchers could render a word-image (e.g., “cousin”) inaccessible to conscious perception by keeping the exposure shorter than about 50 milliseconds and following it with a longer-duration presentation of a masking image (e.g., “&&&&&&”).
In those experiments, mere non-conscious, subliminal perception was associated with neural signals that surged up through the visual cortex to a basic associational region called the parietal cortex, but did not extend much beyond there, and decayed swiftly after the stimulus ceased. By contrast, conscious perception—verified by the subjects—occurred only when the stimulus was long and strong enough to ignite a much wider and longer-lived network of cortical activity, apparently driven by the prefrontal cortex at the front of the brain and feeding back to the visual cortex in a reverberative loop.
The study’s findings largely agreed with the “global workspace” (GW) model of consciousness, first advanced by cognitive scientist Bernard Baars in the late 1980s, and since then developed further by Baars, Naccache, Stanislas Dehaene and others. According to GW the principal biological function of this reverberative, brain-wide activity is to keep information in mind and available for use by a broad set of brain regions—a kind of working memory.
"The GW proposes that at any given time many modular cerebral networks are active in parallel and process information in an unconscious manner. Information becomes conscious, however, if the corresponding neural population is mobilized by top-down attentional amplification into a self-sustained brain-scale state of coherent activity that involves many neurons distributed throughout the brain."[link]
“Probably throughout evolution there has been a mechanism like this to allow you to work on information even when it has disappeared from sight,” Kouider says.
An objective marker of consciousness?
In the new study, Kouider and his colleagues looked for the same neural sign of conscious perception in infants—who otherwise cannot verify that they are aware and experiencing.
This time the researchers used scalp-mounted rather than intracranial electrodes, and faces rather than words as test stimuli. The test subjects were infants in three age groups: five, twelve, and fifteen months.
Despite the underdevelopment of their prefrontal cortices and associated nerve fibers, Kouider’s infant subjects showed evidence of the same neural signature of conscious perception. Their brains required a longer exposure to a stimulus image to ignite the “conscious perception” circuit. Moreover that ignition was relatively weak and delayed—taking on average more than a second to start up in the five-month olds, and nearly a second in the 12- and 15-month-olds, compared to about 0.3 seconds in adults. Even so, this basic neural correlate of consciousness appeared to be on the way to full development. “It appears from our study that the neural mechanisms of attentional amplification and perceptual maintenance are already present very early on,” Kouider says. “They’re just not yet fully functional and they’re very slow.”
Kouider expects to find evidence for the same kind of brain-spanning “workspace” circuit in non-human primates and other relatively intelligent animals—“as long you have some regions [similar to the human prefrontal cortex] that are dealing with executive functions and others dealing with sensory information, and those regions are connected,” says Kouider.
A correlate, not a deep explanation
The global workspace theory links consciousness to a functional advantage—better problem-solving capability—and thus hints at the reason for its evolution. The GW theory is also somewhat testable, says Kouider: “You can make clear predictions, and see in the brain whether it matches your predictions.”
What the GW theory lacks is an explanation for how a global neural workspace actually gives rise to conscious awareness. “Most neuroscientists refer to [the global workspace] as a ‘correlate’ of consciousness because it's not really clear what the relationship is,” says James A. Reggia, a computer scientist at the University of Maryland and the author of a recent review of consciousness research.
Theories for how consciousness arises are few and necessarily speculative. The ultimate test would be to use such a theory to build an artificial conscious entity—but how would one ever know that it is really conscious and not merely simulating consciousness?
Perhaps the most popular fundamental theory of consciousness these days is the Information Integration Theory (IIT) developed by University of Wisconsin neuroscientist Giulio Tononi. IIT holds that consciousness is a property of any system that contains and functionally combines information (see “The Fledgling Science of Consciousness”). Thus, while a human brain may be capable of a high degree of consciousness, even the simplest information-processing system—a thermostat, for example—would be at least minimally conscious.
“IIT is a much more elegant theory because it says that there is space and time and energy and mass in our universe, and at that same fundamental level there is consciousness—so that anything that is complex will have a certain amount of awareness: it will feel like something to be that complex entity,” says Christof Koch, a neuroscientist and consciousness researcher who is currently chief scientist at the Allen Institute for Brain Science, and favors Tononi’s theory.
While IIT may seem not just speculative but also outlandish compared to GW, the two don’t necessarily clash. Conceivably, the latter is only a special—less elaborate and more biologically-centered—case of the former. “In principle IIT and GW are fully compatible,” says Koch. “But I think we’re still far away from a final theory.”