Consciousness and the Brain, written by French neuroscientist Stanislas Dehaene, is a fascinating overview of the mechanisms, boundaries, and possibilities of consciousness, from the point of view of an applied researcher. The work of great Dutch primatologist Frans de Waal led me to Dehaene; time permitting I will summarize some of de Waal’s insights in future posts.
The primary claim of Consciousness and the Brain is that genuine consciousness is indicated by conscious access: the ability for attended information to enter awareness and become reportable to others. It is the capacity to bring to mind accessible perceptions and thoughts. Vigilance, the state of wakefulness, and attention, the focusing of mental resources onto specific information, are not sufficient to constitute consciousness in Dehaene’s view.
“Consciousness is global information broadcasting within the cortex: it arises from a neuronal network whose reason to be is the massive sharing of pertinent information throughout the brain.” This network is referred to as a global neuronal workspace or simply “global workspace”. It enables a number of capabilities including:
- purely mental operations;
- the ability to keep data in mind indefinitely;
- performing arbitrary mental operations;
- reporting to others;
- the enablement of autonomy (augmenting the spontaneous activity that also occurs in the brain).
Experiments can teach us much about consciousness and the theory of the global workspace. Dehaene focuses on experiments with “minimal contrast”: “a pair of experimental situations that are minimally different but only one of which is consciously perceived”. Riding the line in this way helps determine the mechanisms that govern conscious perception.
Unconscious mechanisms play a massive role in our lives and being. While much of our mental activity, and much of what drives our lives, are unconscious mechanisms, virtually all of the brain’s regions can participate in both conscious and unconscious thought. Unconscious mechanisms can be of a higher order than we may assume; for example conscious attention is not required to bind the elements of a scene together. The unconscious binding together of systems occurs in vision, language, attention, and even certain mathematics. (Notably, several of these examples match areas of recent progress in machine learning using neural networks.) These unconscious operations can be detected and measured through careful experiment.
That said, consciousness provides unique capabilities that even sophisticated unconscious processing cannot. This is because “conscious perception transforms incoming information into an internal code that allows it to be processed in unique ways.” For example:
- the stable perception of objects moving through our visual space, even as we ourselves move;
- the ability to store and retain lasting thoughts, which permit us to learn over time;
- the ability to sequentially process information according to rules, similar to the functioning of a digital computer;
- the ability to route information to arbitrary systems in our brain;
- the ability to share our thoughts and perceptions through the use of language.
Through brain imaging and experimentation we are now able to identify the signatures of conscious thought. These signatures include:
- The ignition of our parietal and prefrontal circuits.
- A late slow wave in our brains, referred to as a “P3 wave”.
- A late and sudden burst of high-frequency oscillations.
- The synchronization of information exchanges across distant brain regions.
These signatures are detectable through various means including FMRI (Functional Magnetic Resonance Imaging), MEG (magnetoencephalography) and EEG (electroencephalography).
Because consciousness appears to emerge from the building, looping, and coordination of signals from different brain subsystems (see above), consciousness lags the real world. Consciousness is formed from loops in the brain. It is these loops that permit construction of mental images given incomplete sensory data; for example there are massive differences between our the raw, imperfect visual data that enters our eyes (e.g. the blind spot in our eyes directly behind our optic nerve, or our limited color range perception outside the center of our attention) and our conscious perception.
The ability to read the traces of conscious thought allow us to theorize about consciousness. Global neuronal workspace theory claims that the human brain has developed efficient long-distance networks to select relevant information and disseminate it throughout the brain. Consciousness is an evolved device that allows to attend to a piece of information and keep it active. Conscious information can then be routed to other areas based on goals.
Within our brain there are collections of neurons that send reinforcing signals to each other under certain conditions, for example when a particular person, event, or sensation is perceived or remembered. During conscious perception, a small subset of workspace neurons become active, while most others are inhibited. The panoply of signaling related to inhibited clusters, for example, all clusters that do not pertain to the 2016 Chicago Cubs, form a recognizable signal which is referred to as the P3 wave. In other words, a primary signifier of conscious thought is the signal resulting from the repression of neurons. The P3 wave is in a sense a “negative thought signal”.
Not all unconscious thought is the same. Several types can be defined. Preconscious thought is information already encoded by an active assembly. It can become conscious at anytime. Subliminal thought is input given or processed so weakly that we lack the capability of attending to it. Disconnected patterns are mental activities which have no relation to conscious thought, for example our regular breathing. Diluted mental activity is neural information that has been “downsampled” for use by other systems in the brain, and therefore cannot be brought to consciousness. For example a visual pattern that flickers so fast that you cannot see it. Early levels in our visual system may register this flickering but it is transformed by later levels.
Detecting and theorizing about consciousness allows us to ask, and potentially answer, deep questions. For example, a series of recent experiments seems to indicate the presence of consciousness within severely injured patients who cannot express their consciousness. An example is the case of Jean-Dominique Bauby, the author and subject of The Diving Bell and the Butterfly. A patient without the ability to communicate or move can be asked to imagine, for example, riding a bicycle for 30 seconds. The patient’s brain can be scanned via MRI during this period, and compared to a control group to establish that the neuronal clusters responding to bike riding, along with the signatures of consciousness, are present.
These questions can be asked of non-humans as well. Dehaene says “I would not be surprised if we discovered that all mammals, and probably many species of birds and fish, show evidence of a convergent evolution to the same sort of conscious workspace” as found in humans. (Here is the primary connection to Frans de Waal’s work.) Experiments show that monkeys, dolphins, and even rats and pigeons possess at least the rudiments of metacognition: thinking about thinking. “Animal behavior bears the hallmark of a conscious and reflexive mind.”
It is then worth asking what is uniquely human about human consciousness. Dehaene provides informed speculation. Perhaps it is our ability to combine our core brain systems using a “language of thought”: the inner voice that is nearly always present inside of us. Perhaps also it is our capacity to compose our thoughts using nested or recursive structures of symbols. This implies that language evolved as an internal representational device, not just as a communication system with other hairless apes. The ability to compose and nest may underlie many of our unique human abilities, such as the ability to craft complex tools, perform higher mathematics, and our self-consciousness. An examination of brain areas that are particularly well-developed among humans (as opposed to other primates) seems to support these theories at a high level.
Dehaene closes by theorizing what it would take to build artificial consciousness using computers. This topic, while fascinating, is not a primary subject of Dehaene’s book, and is best saved for another time.