Scientists still cannot fully explain how physical activity in the brain produces subjective experience, but research is beginning to clarify how consciousness can be measured, how it may persist in apparently unresponsive patients, and which features of neural activity are most important. Work spanning neuroscience, artificial intelligence, plant science and astrobiology is also challenging the idea that consciousness is a simple, uniquely human state that requires a conventional brain.
Imaging can be used to measure the degree of variation and complexity across brain regions that correspond to states of consciousness. Credit: AJ Photo/Hop Americain/SPL
Our own consciousness can seem impossible to doubt. We experience the world through vivid sensations, such as a sad song or a childhood memory that transports us to another time and place.
Yet scientists and philosophers have struggled for centuries to explain how physical processes in the brain produce the subjective, first-person experience of being aware of one’s own existence. This challenge, known as the “hard problem” of consciousness, remains a central question in research.
Scientists have not yet explained how brain activity generates the whole of our inner lives, but the field is making progress. Researchers can increasingly measure patterns of brain activity and connectivity, and they are searching for hidden signs of consciousness in people who appear unresponsive.
A developing view, informed by neuroscience and fields including plant science, artificial intelligence and astrobiology, treats consciousness not as a single state that is either switched on or off, but as a potentially broader and more coordinated process. The rapid development of AI has intensified debate over whether consciousness is exclusive to humans and whether a brain is necessary for it at all.
“People are often troubled by the fact that their understanding remains vague,” says Matthias Mischel, a cognitive science philosopher at the Massachusetts Institute of Technology in Cambridge. “But it’s very important to note that definitions in the history of science come at the end, not the beginning, of the scientific process. We claim to be in the midst of it now, and it’s an exciting place to be.”
Finding the neural foundations of consciousness
There is no direct test for human consciousness, but researchers have made progress in identifying what are known as neural correlates of consciousness (NCCs). These are individual signals, or combinations of events, that indicate the minimum neural mechanisms needed to initiate a conscious experience. Certain neurons might need to fire, for example, or particular patterns of brain activity might signal an internal experience.
Below such thresholds, a person might perform an action without being consciously aware of it—for example, briefly zoning out while driving before quickly regaining attention.
Researchers have identified some NCCs by studying people in states not normally associated with consciousness, including sleep, anaesthesia and coma. Melanie Bolly, a neurologist at the University of Wisconsin–Madison, says this work is important for distinguishing genuine NCCs from brain activity that simply reflects wakefulness or responses to the environment.
“This is a major advance, separating out the true features of consciousness, rather than simply measuring what is associated with conscious experience and its consequences,” Boley says.
Research has suggested that about a quarter of people who are medically unconscious might actually experience a form of hidden consciousness, remaining aware of the world around them. In a landmark 2024 study, a subset of 241 people who had become apparently unresponsive after brain injuries were asked to imagine activities such as playing tennis or opening and closing their hands. Brain-imaging scans showed activity associated with these tasks in dozens of patients, raising questions about whether consciousness can persist despite a lack of outward response.1
These findings are encouraging researchers to move away from treating consciousness as a binary state and towards viewing it as a dynamic process that exists along a continuum. This approach has helped produce measures designed to test fundamental ideas about consciousness, including the perturbation complexity index (PCI).2
PCI estimates a person’s level of consciousness by measuring how the brain responds to brief magnetic pulses. Electroencephalography (EEG), which uses electrodes to record electrical activity, measures the response.
In a conscious brain, neural responses generally spread across different regions in complex and distinctive patterns. These properties are known as integration and differentiation, respectively. During sleep or anaesthesia, responses tend to become simpler and more repetitive.
“PCI is a very accurate correlate of consciousness and is thought to be useful in many research settings,” says neurophysiologist Johan Frederick Strom of the University of Oslo. However, widespread clinical adoption has been limited because collecting the metric is laborious, he adds, “and yet it is gaining traction.”
Testing theories of consciousness
Improved measurement has encouraged researchers to develop hypotheses that aim not merely to detect consciousness, but to explain what it is. More than 450 such theories exist. Many differ in their scale or emphasis, but they share some characteristics. Two of the most prominent are Global Neuronal Workspace Theory (GNWT) and Integrated Information Theory (IIT).
GNWT proposes that most brain activity is restricted to specialized regions: visual areas process shapes and colours, for instance, while language areas process words. When information becomes sufficiently important to be broadcast across multiple regions, it enters a global workspace and becomes conscious.
A face whose representation remains within the visual system might trigger simple unconscious responses. A face that activates regions throughout the brain’s global workspace could instead contribute to a conscious experience that supports activity across multiple systems.
IIT addresses a different question: what is the structure of experience? When people look at the world or recall a memory, they experience a unified scene rather than separate colours, sounds and textures. IIT proposes that consciousness depends on how strongly the parts of a system influence one another while remaining distinct. Both integration and differentiation are therefore needed to produce a range of conscious states and experiences.
Under IIT, a conscious system must be more than a collection of independent parts. It must form a whole whose states cannot be fully explained by examining each component separately.
Testing these theories is difficult because experiments have not always been neutral or rigorous. An analysis of hundreds of experiments published in 2022 found that consciousness research had often been conducted in ways that biased results in advance towards particular theories.3 This makes it difficult to compare studies directly, says Christoph Koch, a consciousness researcher at the Allen Institute for Brain Science in Seattle, Washington, who has studied the subject for decades.
“People get so attached to their theories that their confirmation bias can get in the way of honest, collaborative efforts,” he says.
Lucia Meloni, a cognitive neuroscientist at Germany’s Ruhr University Bochum, recently led the first effort to force a direct comparison between the two schools of thought. She brought together supporters of both theories and neutral participants, asking them to agree in advance on a series of experiments and predictions. The group tested the theories’ underlying assumptions.4
The researchers used three measurement techniques: EEG; functional magnetic resonance imaging (fMRI), which tracks brain activity by detecting subtle changes in blood flow; and magnetoencephalography (MEG), which measures the magnetic fields produced by electrical currents in the brain.
Plants don’t have brains, but they can make decisions and solve problems. Credit: Jason Milligan/Courtesy of Understory
A key difference between the theories concerns which parts of the brain are most important for conscious experience. GNWT predicts that a broad range of brain activity will suddenly “fire” when a stimulus produces a conscious experience. It also expects the prefrontal cortex to play an important role as it uses information to generate complex behaviour.
IIT instead emphasizes a “hot zone” at the back of the brain, including parts of the systems responsible for vision, hearing and touch. These areas are thought to be important because related stimuli converge there to create a unified experience.
The research team found that conscious information was represented in both areas. The results also challenged important predictions from both theories: the prolonged activity that IIT predicts in the posterior cortex was not observed, while the widespread firing predicted by GNWT was limited. The study did not resolve the debate, but suggested that both theories might capture part of the explanation.
Meloni does not view this outcome as a failure. “We’re happy with what we found because both theories remained untouched,” she says. “There is no room for growth if your predictions are correct, but you can do better science when you question your thinking.”
How far can consciousness extend?
Scientists are also broadening their investigations through collaborations across disciplines. Evidence from plant and non-human animal biology, robotics and the search for extraterrestrial life could help identify the conditions under which consciousness emerges.
It remains unclear whether consciousness requires a brain—or even neurons. Astrobiologists have long searched for intelligent life, but the vastness of the known universe means there is no reason to assume that any such life would resemble humans.
Source: www.nature.com
