Scientists and philosophers have long struggled to explain how the brain generates conscious experiences. Some doubt whether the objective tools of science can ever get to grips with a phenomenon that is so subjective. Even so, researchers have begun to identify the changes in brain activity that accompany awareness, and they also have some fascinating ideas about why consciousness evolved.
How the brain conjures conscious awareness from the electrical activity of billions of individual nerve cells remains one of the great unanswered questions of life.
Each of us knows that we are conscious, in terms of having thoughts, perceptions, and feelings, but we are unable to prove it to anyone else. Only we have access to the mysterious essence that allows us to experience those thoughts, perceptions, and feelings.
In the 1990s, the philosopher David Chalmers described this inaccessibility to external, objective scrutiny as the “hard problem” of consciousness.
He proposed that an easier task for scientists to tackle would be its “neural correlates” — where and how brain activity changes when people have conscious experiences.
Apart from curiosity, scientists are most likely motivated to discover the neural correlates of consciousness in order to help diagnose and treat disorders of consciousness, such as persistent vegetative states and some psychiatric disorders.
Consciousness has several distinct dimensions that they can measure. Three of the most important ones are:
- wakefulness or physiological arousal
- awareness or the ability to have conscious mental experiences, including thoughts, feelings, and perceptions
- sensory organization, or how different perceptions and more abstract concepts become woven together to create a seamless conscious experience.
These three dimensions interact to produce our overall state of consciousness from moment to moment. For example, when wide awake, we are in a state of high awareness, but as we drift off to sleep at night, both wakefulness and awareness subside.
Awareness and physiological arousal return during REM (rapid eye movement) sleep, which is when vivid dreams are mostly likely to occur. But these sensory experiences are mostly disconnected from external stimuli and detached from the concepts that anchor us to reality while we are awake.
In a similar way, altered states of consciousness, such as those induced by psychedelic drugs or low oxygen levels, involve normal levels of arousal but disorganized sensory experiences.
These can include hallucinations of sounds, smells, or sights, but also synesthesia, when there is cross-talk between usually discrete senses, such as sounds that evoke visual experiences.
People in a coma, or under anesthesia, can have levels of wakefulness and awareness that are even lower than during non-REM sleep.
Meanwhile, in a strange hybrid state of consciousness known as unresponsive wakefulness syndrome, or a vegetative state, patients undergo daily cycles of sleep and wakefulness, but without showing any sign of awareness.
Despite spending long periods with their eyes open, they do not exhibit behavioral responses to external stimuli.
Some of these patients will recover limited signs of awareness, known as a “
Patients in different states of consciousness have provided vital clues about the neural correlates of consciousness.
Techniques such as functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) have revealed the neural activity that accompanies these states.
Scientists detect patterns of activity through measures of functional connectivity, using statistical techniques to reveal correlations between the timing of neural events in different parts of the brain.
This reveals the networks of brain regions that are involved in working memory, attention, and mind-wandering, for example.
When they combine data from fMRI and EEG, researchers discover that activity deep inside the brain, in the
The thalamus operates like a sensory relay station, while the default mode network is a collection of regions within the
By contrast, during non-REM sleep and under anesthetic, functional connectivity breaks down between the thalamus and the default mode network and cortical networks that are involved in attention.
In disorders of consciousness, researchers can see reduced functional connectivity and physical damage that affects the connections between the cortex and deep brain structures.
This demonstrates how important these connections are for maintaining wakefulness and information exchange across the brain.
Despite advances in our understanding of the neural correlates of consciousness, however, doctors still have trouble diagnosing patients who are unable to respond to questions or commands.
They cannot tell whether such a patient is completely unconscious, conscious but disconnected from external stimuli, or conscious and aware of their environment, but unable to respond.
A completely new approach, reported recently in
Rather than recording the activity in particular brain regions or networks of regions, the new technique measures gradients of activity across the brain.
This is analogous to recording the steepness of terrain on a map, and how that steepness might change over time, rather than just the locations of roads, towns, and villages.
This innovative way of investigating brain activity, which considers how functional geometry shapes temporal dynamics, takes into account the fact that each region and network has multiple functions and connections.
“Consciousness is complex and studying it is like solving a scrambled Rubik’s cube,” says first author Dr. Zirui Huang, research assistant professor at the University of Michigan Medical School Department of Anesthesiology.
“If you look at just a single surface, you may be confused by the way it is organized. You need to work on the puzzle looking at all dimensions,” he adds.
Dr. Huang and his colleagues looked at how gradients of neural activity — as measured by fMRI — changed with the three main dimensions of consciousness:
- arousability or wakefulness
- sensory integration.
The researchers drew on existing fMRI data recorded from the brains of people who were awake, under anesthetic, in a so-called vegetative state — known as “
They discovered an activity gradient that corresponded to changes in arousal levels that stretched from the visual and default mode areas to networks involved in attention.
Another gradient changed in step with awareness and stretched from regions involved in perception and action to regions responsible for the integration of information and formation of abstract concepts.
A third gradient, linked to sensory organization, spanned the visual system and the somatomotor cortex, which helps to control movement.
“We demonstrated that disruptions of human consciousness — due to pharmacological, neuropathological, or psychiatric causes — are associated with a degradation of one or more of the major cortical gradients depending on the state,” notes Dr. Huang.
He told Medical News Today that some patients who are unable to respond in any way may still be conscious. “It is therefore important to develop behavior-independent and task-independent approaches to assessment,” he said.
“According to our work, cortical gradient measurements have the potential to reduce the uncertainty of clinical assessment of consciousness in those patients,” he added.
Scientists like Dr. Huang have discovered a lot about its neural correlates, but their findings do not shed much light on how the brain generates consciousness — the “hard problem”.
Other scientists have argued that to learn more about the causes of consciousness, you have to manipulate brain activity and see what happens.
Transcranial magnetic stimulation, or TMS, is a noninvasive technique that changes brain activity in a tightly controlled way.
The technique involves passing a strong electrical pulse through a conductive coil held close to the participant’s head, creating a brief magnetic pulse that induces electrical activity in the part of the brain closest to the coil.
This boosts the electrical activity of millions of neurons within a region just a few centimeters in diameter.
In an awake participant, a TMS pulse triggers a complex chain of activity across multiple cortical areas, whereas in non-REM sleep, the same pulse has a much briefer, more localized effect.
During REM sleep, however, the pulse evokes a pattern of activity similar to that seen in the brain of a person who is wide awake.
Intriguingly, in patients who are awake but in a vegetative state, TMS induces short-lived, local effects that are similar to those seen during the non-REM sleep of a healthy person.
But in the brain of awake, minimally conscious patients, TMS evokes a similar response to that seen in healthy, awake people.
Researchers have therefore concluded that the complexity and spatial extent of communication between brain regions subsides as levels of wakefulness and awareness fall.
Despite advances in our understanding of the neural correlates of consciousness, the question remains: How does consciousness arise from brain activity?
Scientists have proposed several theories. Two prominent ideas are global neuronal workspace theory (GNWT) and integrated information theory (IIT).
Global neuronal workspace
In brief, GNWT proposes that there is a network of long-range connections that span the brain, called the global workspace.
Neural information becomes conscious when it gains access to this workspace, which allows it to be broadcast throughout the brain, including to specialized hubs for memory, perception, motor output, and attention.
Crucially, it is a case of “winner takes all” for a particular interpretation of sensory data when it gains access to the global workspace.
According to Prof. Michael Graziano, professor of psychology and neuroscience at the Princeton Neuroscience Institute, NJ, and Dr. AaronSchurger, assistant professor in the Crean College of Health and Behavioral Sciences at Chapman University in Irvine, CA, this explains, for example, “bistable” optical illusions.
Two examples of these are the Necker cube and face-vase illusion, in which conscious awareness switches between two alternative interpretations of the same sensory data.
The communication and integration of information between brain regions is at the heart of many theories of consciousness.
One of these, IIT, uses a complex equation to calculate an entity’s degree of consciousness from how well it integrates information.
According to the equation, even inanimate objects such as rocks and teapots have a glimmer of consciousness.
This is in keeping with the philosophical theory of panpsychism, which proposes that consciousness is a fundamental property of all physical systems.
How it feels to control attention
Prof. Graziano has a problem with all these theories of consciousness. He argues that they cannot even be termed theories because they do not actually explain consciousness, they only describe it.
He likens this to the difference between Newton’s law of gravitation, which is an equation that calculates gravitational force, and Einstein’s theory of general relativity, which explains what gravity actually is.
Prof. Graziano’s own theory of consciousness, the attention schema theory, or AST, does not claim to have cracked the hard problem of consciousness, but instead seeks to explain why we believe we are conscious.
According to AST, animal brains evolved a model — or schema — of their current state of attention that they could use to predict and control their ongoing focus of attention.
In order to control our bodies, the brain uses an internal model of our limbs, their location in space, and how they work. Prof. Graziano believes the control of attention is no different.
What is more, he argues that the attention schema leads people to believe that they have an internal essence —consciousness — that allows them to focus their attention.
Amazingly, expert meditators may be able to achieve a state of pure awareness, free from any thoughts, perceptions, feelings, or even a sense of selfhood, time, or space.
This special state of consciousness has provided more clues about the neural correlates of consciousness.
Researchers at the University of Freiburg in Germany used EEG to study the brain of a former Buddhist monk who described this as a “clear, aware openness.”
They found that content-free awareness was associated with a sharp fall in alpha waves and an increase in theta waves.
There was also a decoupling of the activity of the sensory cortex and dorsal attention network, but strong activity within the attention network itself. Meanwhile, there was a decline in the activity of the default mode network.
Prof. Graziano said content-free awareness in meditation may correspond to the predictive model of attention described in his theory.
He told MNT:
“I think the model of attention, representing the property of attention itself, is possible to construct in isolation, unassociated with a thing to which you are attending. It may indeed be the content-free awareness that meditators talk about. I do think that meditation is essentially practice in using your attention schema.”
Even without thoughts, sensations, or feelings, it seems the mysterious essence of conscious awareness remains.