The origins of consciousness have plagued the minds of philosophers and scientists for centuries. In the past decade, neuroscientists have begun to map its neural basis, that is, how the brain, through its intricate connections, transforms electrical signals between neurons into consciousness.
Yet the field is fragmented, an international team of neuroscientists recently wrote in a new article Neuron. Many theories of consciousness contradict each other, with different ideas about where and how consciousness arises in the brain.
Some theories even try it out in a mano-a-mano test by imaging volunteers’ brains as they perform various tasks in clinical testing centers around the world.
But unlocking the neural basis of consciousness does not have to be confrontational. Instead, theories can be integrated, wrote the authors, who were part of the Human Brain Project – a large-scale European initiative to map and understand the brain – and specialize in decoding brain signals related to consciousness.
Not all authors agree on the specific brain mechanisms that allow us to perceive the external world and construct an inner world of ‘self’. But by working together, they merged their ideas, showing that different theories are not necessarily mutually incompatible; in fact, they could be consolidated into a general framework of consciousness and even inspire new ideas that help unravel one of the brain’s greatest mysteries.
If this succeeds, the joint mission can extend beyond our own brains. Brain organoids, or “mini-brains,” which roughly mimic early human development, are becoming increasingly sophisticated, raising ethical concerns about their potential for developing self-awareness (to be clear, there are no signs). Meanwhile, similar questions have been raised about AI. A general theory of consciousness, based on the human mind, could potentially help us evaluate these artificial constructs.
“Is it realistic to reconcile theories, or even strive for a unified theory of consciousness?” the authors asked. “We take the position that the existence of multiple theories is a sign of the health of this emerging field… so that multiple theories can contribute to our understanding simultaneously.”
Lost in translation
I am conscious. You too. We see, smell, hear and feel. We have an internal world that tells us what we experience. But the boundaries become blurry for people in various stages of coma or for those who are confined: they can still perceive their surroundings, but cannot physically respond. Every night and during anesthesia we lose consciousness during sleep. Yet somehow we regain consciousness. How?
With extensive brain imaging, neuroscientists today agree that consciousness arises from the brain’s wiring and activity. But there are several theories that discuss this How electrical signals in the brain produce rich and intimate experiences of our lives.
Part of the problem, the authors wrote, is that there is no clear definition of “consciousness.” In this article they have divided the term into two experiences: an outer and an inner. The outer experience, called phenomenal consciousness, is when we immediately realize what we are experiencing, for example seeing a total solar eclipse or the Northern Lights.
The inner experience is a bit like a ‘gut feeling’ in that it helps form expectations and types of memories so that by drawing on it we can plan behavior and actions.
Both are aspects of consciousness, but the difference has been barely delineated in previous work. It makes comparing theories difficult, the authors wrote, but that’s what they wanted to do.
Meet the contenders
Using their ‘two experiences’ framework, they examined five prominent theories of consciousness.
The first, the global neuronal workspace theory, depicts the brain as a kind of city. Each local ‘hub’ of the brain region dynamically interacts with a ‘global workspace’, which integrates and transmits information to other hubs for further processing, allowing information to reach the level of consciousness. In other words, we only perceive something when all the bits of sensory information – sight, hearing, touch, taste – are woven into a temporary neural sketchpad. According to this theory, the seat of consciousness is located in the front parts of the brain.
The second, integrated information theory, takes a more globalist view. The idea is that consciousness arises from a series of cause-and-effect reactions from the brain’s networks. With the right neural architecture, connections and network complexity, consciousness arises naturally. The theory suggests that the back of the brain stimulates consciousness.
Then there’s dendritic integration theory, the coolest new kid in town. Unlike previous ideas, this theory said goodbye to the front or back of the brain and instead zoomed in on individual neurons in the cortex, the outer part of the brain and a hub for higher cognitive functions such as reasoning and planning.
The cortex has extensive connections with other parts of the brain, for example the parts that encode memories and emotions. One type of neuron, deep in the cortex, is particularly noticeable. Physically, these neurons resemble trees with extensive ‘roots’ and ‘branches’. The roots are connected to other parts of the brain, while the upper branches help calculate errors in the neuron’s computing. In turn, these higher branches generate an error signal that corrects errors through multiple rounds of learning.
Although the two compartments are physically connected, they do their own business: turning a single neuron into multiple computers. Here’s the crux: there is a theoretical “gate” between the upper and lower neural “offices” for each neuron. During consciousness, the gate opens, allowing information to flow between the cortex and other brain areas. In dreamless sleep and other unconscious states the gate closes.
Like a light switch, this theory suggests that consciousness is supported by turning individual neuron gates on or off on a large scale.
The last two theories state that recurrent processing in the brain – that is, learning from previous experiences – is essential for consciousness. Instead of “experiencing” the world, the brain builds an internal simulation that continually predicts the “here and now” to determine what we perceive.
A unified theory?
All theories have extensive experiments to support their claims. Who is right? For the authors, the key is to think of consciousness not as a singular concept, but as a kind of ‘ladder’. The brain functions at multiple levels: cells, local networks, brain areas and finally the entire brain.
When examining theories of consciousness, it is also useful to distinguish between different levels. For example, dendritic integration theory – which takes into account neurons and their connections – is at the level of individual cells and how they contribute to consciousness. It makes the theory “neutral,” in the sense that it can be easily integrated into larger-scale ideas—ideas that rely largely on neural network connections or across larger brain areas.
Although it is seemingly difficult to reconcile different ideas about consciousness, two principles tie them together, the team wrote. One of these is that consciousness requires feedback, within local neural circuits and in the brain. The other is integration, in that any feedback signals must be easily incorporated back into neural circuits so that they can change their output. Finally, all authors agree that local, short connections are essential, but not sufficient. Consciousness requires long-distance connections from the cortex to deeper brain areas.
Is an integrated theory of consciousness possible? The authors are optimistic. Defining multiple aspects of consciousness (immediate reactions versus internal thoughts) makes it clearer how to explore and compare the results of different experiments. For now, global neuronal workspace theory focuses primarily on the “inner experience” that leads to consciousness, while others attempt to address the “outer experience” – what we experience immediately.
To merge the theories, the latter groups will need to explain how consciousness is used for attention and planning, which are hallmarks of immediate responses. But fundamentally, the authors wrote, they are all based on different aspects of neuronal connections near and far. With more empirical experiments and as increasingly sophisticated brain atlases come online, they will move the field forward.
Hopefully, the authors write, “an integrated theory of consciousness… may be within reach within the coming years or decades.”
Image credits: SIMON LEE / Unsplash