The origin of consciousness has puzzled philosophers and scientists for centuries. Over the last decade, neuroscientists have begun to understand the neural fundamentals—that is, how the brain, through its intricate connections, converts electrical signals between neurons into consciousness.
But the field is fragmented, an international team of neuroscientists recently wrote in a new article Neuron. Many theories of consciousness contradict each other and there are different ideas about where and how consciousness arises in the brain.
Some theories are even pitted against each other in a man-on-man test. It involves imaging the brains of volunteers performing various tasks in clinical testing centers around the world.
But unlocking the neural basis of consciousness does not have to be confrontational. Rather, theories can be integrated, wrote the authors, who were part of the Human Brain Project – a large European project 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 internal world of “self”. But by working together, they brought their ideas together and showed that different theories are not necessarily incompatible with each other – in fact, they could be consolidated into a general framework of consciousness and even inspire new ideas that help unlock one of the brain’s greatest mysteries.
If successful, the shared mission could extend beyond our own borders. Brain organoids, or “mini-brains,” that roughly mimic early human development are becoming increasingly sophisticated, raising ethical concerns about their potential to develop self-awareness (to be clear, there is no evidence of this). Similar questions have now 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 argue that the existence of multiple theories is a sign of the health of this emerging field… such that multiple theories can simultaneously contribute to our understanding.”
Lost in translation
I am conscious. You are too. We see, smell, hear and feel. We have an inner world that tells us what we experience. However, for people in various stages of coma or for those who are imprisoned, the boundaries become blurred – they can still perceive their surroundings, but cannot react physically. We lose consciousness every night in our sleep and during anesthesia. But somehow we regain consciousness. How?
Given extensive brain imaging, neuroscientists now agree that consciousness arises from the brain’s wiring and activity. But several theories dispute this How Electrical signals in the brain create rich and intimate experiences of our lives.
Part of the problem, the authors say, is that there is no clear definition of “consciousness.” In this work they have divided the term into two experiences: an external and an internal one. The external experience, called phenomenal consciousness, occurs when we immediately recognize what we are experiencing – for example, seeing a total solar eclipse or the Northern Lights.
Internal experience is a bit like a “gut feeling” in that it helps form expectations and types of memories so that we can plan behaviors and actions by using this feeling.
Both are aspects of consciousness, but the difference is rarely spelled out in previous work. It makes it difficult to compare theories, the authors wrote, but that’s exactly what they set out to do.
Meet the competitors
Using their “two-experiences” framework, they examined five prominent theories of consciousness.
The first, the global neural workspace theory, presents the brain as a kind of city. Each local brain region “hub” interacts dynamically with a “global workspace” that integrates information and sends it to other hubs for further processing – allowing information to reach the level of consciousness. In other words, we only perceive something when all sensory information – sight, hearing, touch, taste – is woven into a temporary neural sketchpad. According to this theory, the seat of consciousness is in the frontal 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 of the brain’s networks. With the right neural architecture, connections, and network complexity, consciousness arises naturally. The theory is that the back of the brain stimulates consciousness.
And then there’s Dendritic Integration Theory, the coolest new trend in the world. Unlike previous ideas, this theory abandoned the front and back of the brain and instead focused on individual neurons in the cerebral cortex, the outermost part of the brain and hub for higher cognitive functions such as logical thinking and planning.
The cortex has extensive connections to other parts of the brain – such as those that encode memories and emotions. Of particular note is a type of neuron deep in the cortex. Physically, these neurons resemble trees with extensive “roots” and “branches.” The roots connect to other parts of the brain, while the upper branches help calculate errors in the neuron’s processing power. These upper branches in turn generate an error signal that corrects errors through multiple rounds of learning.
Although the two compartments are physically connected, they carry out their own tasks: turning a single neuron into multiple computers. Here’s the kicker: There is a theoretical “gate” between the upper and lower neuronal “offices” for each neuron. During consciousness, the gate opens, allowing information to flow between the cerebral cortex and other brain regions. In dreamless sleep and other unconscious states the gate closes.
Like a light switch, this theory suggests that consciousness is supported by turning individual neuronal gates on or off on a large scale.
The last two theories propose that recurrent processing in the brain – that is, it learns from previous experiences – is essential for consciousness. Instead of “experiencing” the world, the brain builds an internal simulation that constantly predicts the “here and now” to control what we perceive.
A unified theory?
All theories are supported by extensive experiments. So who is right? For the authors, the key is to view consciousness not as a single concept, but as a kind of “ladder.” The brain functions at multiple levels: cells, local networks, brain regions and finally the entire brain.
When examining theories of consciousness, it also makes sense to differentiate between different levels. For example, dendritic integration theory, which takes neurons and their connections into account, looks at the level of individual cells and their contribution to consciousness. It makes the theory “neutral” in that it fits easily into larger-scale ideas—those based primarily on neural network connections or across larger brain regions.
Although it seems difficult to reconcile different ideas about consciousness, two principles connect them, the team wrote. One is that consciousness requires feedback within local neural circuits and throughout the brain. The other is integration, in which all feedback signals must be easily integrated back into neural circuits so that they can change their outputs. Finally, all authors agree that local, short connections are important but not sufficient. Long-distance connections from the cortex to deeper brain regions are required for consciousness.
So is an integrated theory of consciousness possible? The authors are optimistic. By defining multiple aspects of consciousness – immediate reactions versus internal thoughts – it becomes clearer how the results of different experiments can be examined and compared. Currently, global neural workspace theory focuses primarily on the “inner experience” that leads to consciousness, while others attempt to tackle “outer experience” – what we experience directly.
For the theories to be brought together, the latter groups must explain how consciousness is used for attention and planning, which are hallmarks of immediate responses. But fundamentally, the authors write, they all rely on different aspects of neural connections near and far. With more empirical experiments and the online publication of increasingly sophisticated brain atlases, they will advance the field.
Hopefully, the authors write, “an integrated theory of consciousness…might be within reach in the next few years or decades.”
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