For decades, scientists have sought the origin of consciousness in the outer cortex—the most evolved and complex layer of the brain. Leading theories, such as Integrated Information Theory and Global Neuronal Workspace Theory, view the neocortex as the central hub for processing, integrating information, and generating subjective human experience. However, an extensive review of over a century of neuroscience research is now forcing scientists to reconsider. Evidence from children born without a cortex (anencephaly) and animal experiments suggests that more ancient brain structures, particularly the subcortex and the brainstem, may be sufficient for basic forms of consciousness. This finding not only challenges the foundation of current theories but also opens up a new discussion on the prevalence of consciousness, consequently impacting patient care and ethical considerations for animals.
The Traditional View Centering on the Cortex
Most efforts to explain consciousness in modern neuroscience have focused on the cortex, specifically the neocortex. This is the phylogenetically newest part of the brain, responsible for high-level cognitive functions such as language, logical reasoning, and self-awareness. Scientists have argued that the sheer complexity and massive connectivity of the approximately 15 billion neurons in the cortex are the only factors capable of generating the conscious experience of “I.”
The two most dominant theories, Integrated Information Theory (IIT) and Global Neuronal Workspace Theory (GNWT), both position the cortex as essential. IIT proposes that consciousness depends on the level of integrated information within a system, and the cortex possesses the highest integrated complexity. GNWT, meanwhile, sees consciousness as information broadcast and widely accessible within a global “workspace,” primarily located in the prefrontal and parietal cortical areas.
In this conventional view, the brain regions beneath the cortex, known as the subcortex, are often relegated to the role of simply maintaining wakefulness. They are compared to electrical power or a general activation system, necessary to “turn on” the cortex, but not the actual mechanism that creates the content of consciousness itself. The cerebellum, another ancient structure, has historically been considered entirely irrelevant to subjective experience.
Clinical Evidence Challenges the Necessity Claim
The assumption of the absolute necessity of the cortex is being undermined by clinical and empirical evidence. One of the clearest counter-arguments comes from rare cases of children born with hydranencephaly, a condition where most or all of the cortex fails to develop. According to most cortex-centric theories, these children should be in a permanent vegetative state, lacking any subjective experience.
However, reports describe remarkable behaviors in these individuals. They can exhibit clear emotions, such as smiling when happy or frowning when distressed, and even recognize the voices and faces of their caregivers, as well as show enjoyment of music. While this is not adult-level consciousness, these behaviors indicate the presence of some form of basic experience, a fundamental “sense of existence.” This suggests that some core functions of consciousness do not require the cortex.
Animal experiments yield similar conclusions. Decortication studies (surgical removal of the cortex) in various mammals, including rats, cats, and monkeys, show that they retain the ability to express several complex behaviors: they can still play, show basic emotions, groom themselves, and even learn new tasks. This evidence strongly challenges the notion that the cortex is the minimal and mandatory structure for generating any form of consciousness.
The Brainstem as the Causal Core of Basic Consciousness
With the cortex removed from its role as the sole center, attention shifts to subcortical structures, particularly the brainstem. This is the phylogenetically oldest part of the brain, controlling vital functions like heart rate, respiration, and the sleep-wake cycle. The brainstem houses the Ascending Reticular Activating System (ARAS), a crucial neural network that maintains wakefulness.
In the emerging model, the brainstem is not just the “power switch” but the source of core consciousness, or basic qualia—the raw feeling of existence and the body’s internal states. Neuroscientist Mark Solms is a strong proponent of this hypothesis, arguing that subcortical regions, particularly the brainstem and structures related to emotion and bodily needs (homeostasis), are where the initial feeling of “being a subject” arises.
Clinical reality supports this view: focal and severe damage to the brainstem can directly lead to coma or permanent loss of consciousness, whereas widespread cortical damage often results only in specific awareness deficits (like visual loss) but does not extinguish the general capacity for arousal. This confirms the brainstem as the most critical structure for maintaining the state of being awake, which is foundational to all conscious experience.
The Cerebellum: An Unexpected Role in Refining Experience
The cerebellum, a structure containing about 80% of all neurons in the brain, has traditionally been assigned only the role of motor coordination. However, recent studies have uncovered its unexpected link to consciousness and cognition. Neural networks between the cerebellum and the cortex are now understood to be intricately connected, extending beyond motor circuits.
Electromagnetic stimulation experiments and pathology studies have indicated that the cerebellum can directly influence the content of consciousness. Cerebellar stimulation can alter how a person perceives sensory information, while damage to the cerebellum (not congenital) can lead to complex psychiatric disorders, including hallucinations and sudden emotional changes. This suggests the cerebellum may function as a modulator or refiner of consciousness, helping to enrich, expand, and regulate the complexity of subjective experience, rather than generating it from scratch.
New Paradigm: A Graded and Widespread Consciousness
The overall conclusion from this re-evaluation is a hierarchical model of consciousness. Rather than an “all-or-nothing” phenomenon, consciousness is viewed as a spectrum: ancient structures (brainstem, subcortex) are responsible for core consciousness or the rudimentary feeling of existence; while newer structures (cortex, cerebellum) are responsible for extended consciousness or complex, sophisticated cognitive content.
This shift in perspective has profound implications for both medicine and philosophy. In patient care, it suggests that assessing consciousness should not solely rely on cortical activity signs but place greater emphasis on brainstem structures to determine the possibility of basic experience. This could improve the diagnosis and prognosis for patients with disorders of consciousness.
Ethically, if consciousness does not require a highly developed cortex, it may be a much more widespread phenomenon in the animal kingdom, existing in many less complex-brained species. This will force us to reconsider current ethical frameworks regarding animal rights and suffering, opening a broader discussion about how far consciousness has evolved and where it truly resides.
