The scientific investigation into how the brain creates conscious experience focuses on subjective experience, often called qualia. Qualia refers to the personal, felt quality of an experience, such as “what it is like” to see the color red or feel pain. Explaining how electrochemical signals translate into this inner, first-person perspective remains one of the most significant unsolved problems in science. Researchers approach this challenge by identifying the physical brain processes that reliably correspond to a conscious state.
Finding the Physical Markers of Consciousness
The empirical search for consciousness centers on identifying the Neural Correlates of Consciousness (NCCs). NCCs are defined as the minimum set of neural events sufficient for a specific conscious experience. This research attempts to pinpoint the where and when of consciousness by isolating brain activity that occurs only when a person is aware of a stimulus. A common experimental design uses binocular rivalry, where a visual stimulus is sometimes consciously perceived and sometimes not, even though the physical input remains the same.
In binocular rivalry experiments, the brain alternates between consciously perceiving only one image at a time, even when different images are presented to each eye. Techniques like electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) contrast brain activity during the conscious perception phase with the unperceived phase. These studies consistently highlight the “posterior hot zone,” which encompasses the temporo-parietal-occipital cortices in the back of the brain. Activity in this area appears to be directly correlated with the content of visual awareness.
Initial processing of sensory information occurs in the cortex regardless of awareness. However, the emergence of a conscious percept is marked by a sudden, intense burst of activity. This activity is characterized by synchronized neuronal firing across widespread brain regions, particularly at high frequencies. This large-scale network engagement involves reentrant signaling, where signals travel from sensory areas back to the same areas, creating a recurrent loop of information processing. This pattern of widespread, synchronized activity, rather than activity in any single brain region, appears to be the physical signature distinguishing conscious from unconscious processing.
Leading Scientific Theories of Consciousness
Empirical findings from NCC research have led to theoretical models explaining how these physical markers generate subjective experience. Two prominent frameworks are Global Workspace Theory (GWT) and Integrated Information Theory (IIT). These theories offer distinct perspectives on the necessary functional and structural requirements for consciousness to arise.
Global Workspace Theory (GWT)
Global Workspace Theory (GWT) posits that consciousness acts like a central “global workspace” for the brain, akin to a theater stage where information is broadcast to specialized, unconscious processors. The brain is composed of many specialized modules that operate in parallel, handling tasks like vision, memory, and motor control. When a piece of information becomes relevant, it gains access to this global workspace.
This mechanism involves “ignition,” where a stimulus triggers a sudden, widespread propagation of neural activity. Once information ignites the workspace, it is made globally available to all other specialized brain systems. This allows for coordinated action, planning, and reporting. The prefrontal and parietal cortices are thought to be the primary hubs responsible for this “broadcasting” function. GWT suggests that this wide accessibility of information across the brain constitutes the conscious experience.
Integrated Information Theory (IIT)
Integrated Information Theory (IIT) argues that consciousness is a fundamental property of a physical system capable of integrating information. The theory is grounded on two core properties of conscious experience: it is highly differentiated (allowing for a vast number of different experiences) and highly integrated (it is a single, unified experience). IIT proposes a mathematical measure, called Phi, to quantify the degree of irreducible cause-effect power within a system.
A system must be both complex and unified to have a high Phi value, indicating a high degree of consciousness. For example, a digital camera sensor is highly differentiated but not integrated; cutting it in half results in two separate systems. In contrast, the human brain is highly differentiated, but its intricate, reciprocal connections ensure it operates as a unified whole. The parts causally influence the whole and each other. IIT claims that consciousness is identical to the maximal amount of integrated information (Phi) generated by a physical system.
Testing and Altering Conscious States
The theoretical understanding of consciousness has direct application in clinical settings, particularly for diagnosing patients with disorders of consciousness. Researchers must distinguish between patients who are awake but unaware, such as those in a Vegetative State (VS), and those who show inconsistent signs of awareness, categorized as a Minimally Conscious State (MCS). Arousal, indicated by an open-eye sleep-wake cycle, is separate from awareness, which requires evidence of subjective experience or intentional behavior.
A sophisticated method for assessing awareness combines Transcranial Magnetic Stimulation (TMS) with high-density EEG. A TMS pulse briefly stimulates the cortex, and the resulting electrical reverberation across the brain is recorded. In a fully conscious brain, the pulse triggers a complex, widespread, and sustained pattern of activity, reflecting high differentiation and integration. In contrast, in an unconscious state, such as deep sleep or a vegetative state, the signal is either localized and simple or it quickly decays.
This TMS-EEG measure of cortical complexity, called the Perturbational Complexity Index, provides a quantitative metric aligning with Integrated Information Theory. General anesthesia provides a reversible model of lost consciousness. It works by temporarily disrupting the functional connectivity between distant brain areas. Under anesthesia, the measurable information integration and complexity of the brain’s electrical activity significantly decreases. This confirms that the capacity for large-scale communication is a necessary requirement for conscious awareness.