How does the human brain create consciousness, and why?
ISLAMABAD: 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.
Share on PinterestHow much do we really know about human consciousness? Image credit: Oxana Pervomay/Stocksy.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.Three dimensions of consciousness
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 “minimally conscious stateTrusted Source,” such as the ability to respond to instructions or follow a moving object with their eyes.
The terrain of brain activity
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 Nature CommunicationsTrusted Source, may provide a way to assess such a patient’s wakefulness, awareness, and sensory organization.
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.
Three gradients of consciousness
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 “unresponsive wakefulness syndromeTrusted Source” — or who had a psychiatric diagnosis such as schizophrenia.
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.