Consciousness is at once the most familiar and the most elusive of phenomena. It is the backdrop of every sensation and thought, yet capturing its essence in clear scientific terms has challenged philosophers and neuroscientists for centuries. When we talk about consciousness, we usually mean the capacity to have subjective experience — for example, the taste of a crisp apple or the sound of a favourite song. Neuroscientists define consciousness as the ability to have such experiences . Recent work has suggested that the brain areas responsible for those experiences might be more ancient and widespread than previously thought . To understand why that conclusion matters, it’s useful to start with a historical tour of how researchers have thought about consciousness and to examine how new findings may reshape those ideas.
From Philosophy to Early Neuroscience
The quest to understand consciousness predates modern science. Ancient philosophers like Aristotle and Ibn Sīnā (Avicenna) debated the nature of the mind and its relationship to the body . In the seventeenth century René Descartes introduced a linguistic shift; he used the term consciousness to describe introspective psychological experience . Descartes was a dualist, arguing that mind and body were distinct but intimately connected, and he believed the brain provided that link . Later thinkers such as Baruch Spinoza and Gottfried Leibniz proposed alternative views, ranging from monism to pre-established harmony, signalling that consciousness had become a philosophical battleground .
Scientific attempts to link consciousness with the physical brain began in earnest in the nineteenth century. Neuroscientists like Gustav Fechner and Hermann Helmholtz demonstrated that electrical activity in the brain correlates with sensory stimulation . David Ferrier and John Hughlings Jackson used direct brain stimulation in animals and patients to map sensory experiences to specific cortical areas . These early experiments showed that the brain and subjective experience are causally related, establishing the foundation for a neuroscientific approach to consciousness .
The early twentieth century brought a backlash. Sigmund Freud’s focus on the unconscious and speculative introspection prompted behaviourist John Watson to argue that psychologists should restrict themselves to observable behaviour . His behaviourist manifesto discouraged the scientific study of internal experiences . Despite this, clinicians such as Wilder Penfield showed that electrical stimulation during neurosurgery could elicit vivid conscious sensations, while studies of split‑brain patients revealed that the two cerebral hemispheres can support separate streams of perception . These findings hinted that consciousness might depend on specific brain structures, but there was little consensus.
By the late twentieth century the pendulum swung back. Francis Crick and Christof Koch famously argued that consciousness could and should be explained neurobiologically, focusing on neural correlates like awareness and short‑term memory . Around the same time, sophisticated neuroimaging techniques began to deliver detailed views of the brain in action . Consciousness science blossomed into an interdisciplinary field, drawing on philosophy, psychology and computer science . Today, the study of consciousness is no longer a fringe pursuit: there are societies, journals and research centres devoted to it .
Major Scientific Theories
Several theories have emerged to explain how subjective experience arises from neural activity. While none has achieved universal acceptance, they provide useful frameworks for designing experiments and interpreting data.
Global Neuronal Workspace Theory
Global Neuronal Workspace Theory (GNWT) proposes that information becomes conscious when it is broadcast across a network of widely distributed brain regions. As an example, think about the placement of your tongue. Sensory information about your tongue’s position is always available, yet you only notice it when the brain’s global workspace amplifies and distributes that information. GNWT posits that consciousness depends on ignition — a sudden burst of neuronal spiking that makes selected information available for verbal report and decision‑making. The theory emphasises access consciousness: the ability to use information rather than the raw feel of experience. GNWT predicts that prefrontal and parietal cortices are critical hubs in this workspace.
Integrated Information Theory
Integrated Information Theory (IIT) shifts the focus from information broadcasting to information integration. IIT suggests that conscious experience is irreducible: all components of an experience are bound together such that altering any part changes the whole. The theory attempts to quantify consciousness with a mathematical measure called phi (Φ), representing the amount of integrated information within a network. IIT predicts that the so‑called ‘hot zone’ in the temporo‑parietal–occipital cortex contains networks with high Φ . According to IIT, those posterior regions rather than the prefrontal cortex are the seat of experience.
Higher‑Order and Attention Schema Theories
Higher‑Order Theories (HOTs) propose that a mental state becomes conscious when we have a thought about that state. For instance, listening to a song involves processing auditory signals, but according to HOTs it becomes a conscious experience when a higher‑level representation acknowledges the music. HOTs thus locate consciousness in regions responsible for meta‑cognition and self‑reflection.
Attention Schema Theory, a more recent idea, distinguishes between attention (focusing resources on certain stimuli) and awareness (the brain’s model of its own attention). Experiments showing that people can pay attention to stimuli without being aware of them support this separation. Attention Schema Theory suggests that consciousness is the brain’s simplified model of its attentional processes, constructed to monitor and control them..
Challenging the Theories: Recent Experiments
Adversarial Test of GNWT and IIT
One of the most ambitious recent efforts to compare theories directly was an adversarial collaboration published in Nature in April 2025. A consortium of researchers brought together proponents of GNWT and IIT to design a large‑scale experiment and preregister predictions . Human participants viewed images for different durations while their brain activity was measured with functional MRI, magnetoencephalography and intracranial recordings . The results were a mixed bag. Information about conscious content was found in visual, ventrotemporal and inferior frontal cortices . Sustained activity reflecting how long a stimulus was presented appeared in occipital and temporal areas, while synchronisation between frontal and early visual areas tracked specific content . These findings partly supported both theories but challenged their core claims. The absence of sustained synchronisation within posterior cortex contradicted IIT’s prediction that network connectivity specifies consciousness . At the same time, the limited role of prefrontal cortex undermined GNWT’s emphasis on frontal ignition . The study highlighted the need for more rigorous, theory‑neutral testing and suggested that no single existing theory yet captures the full complexity of conscious experience .
Evidence for Subcortical Contributions
While GNWT and IIT focus on cortical regions, new research is calling attention to the brain’s older structures. In 2025 a comprehensive review of over 100 years of data argued that neuroscientists may have undervalued subcortical regions and the cerebellum . The review compiled findings from neuroimaging, stimulation, patient studies and animal experiments. Neuroimaging shows that the cortex, subcortex and cerebellum all display activity correlated with consciousness . Brain stimulation studies reveal that altering activity in any of these regions can change experience: stimulating the neocortex can produce hallucinations or distort the sense of self , whereas stimulating deep subcortical structures can induce depression, wake a monkey from anaesthesia or knock a mouse unconscious . Even the cerebellum — long considered irrelevant to consciousness — affects sensory perception when stimulated .
Importantly, the review found that only damage to the oldest brain regions reliably abolishes consciousness . People born without a cerebellum or parts of their cortex can still appear conscious and lead relatively normal lives . Rare cases of children born without most of the neocortex show signs of emotional responses, playfulness and recognition . In animals, removing the neocortex does not eliminate behaviours like play, grooming, parenting and learning . The review concluded that the body, subcortex and environment may be sufficient for basic experiences, with the cortex refining rather than creating consciousness . These arguments challenge cortico‑centric theories and suggest that consciousness might be more widespread across species than previously believed .
Single‑Neuron Recordings in the Subcortex
Direct evidence for subcortical involvement comes from a 2025 study that recorded single neurons in patients undergoing deep brain stimulation. Researchers asked patients to detect faint vibrations on their hand while measuring activity in the subthalamic nucleus and thalamus . About 23% of the recorded neurons fired differently when participants detected a stimulus compared with when they did not . This study provides neurophysiological evidence that subcortical structures contribute to perceptual consciousness . The authors argue that these findings call for a less cortico‑centric view of the neural correlates of consciousness .
Primitive Feelings and the “Lizard Brain”
Some researchers have described the subcortex as the brain’s “lizard brain” because it includes ancient structures like the brainstem, hypothalamus and limbic nuclei. These regions regulate hunger, thirst, pain, pleasure and fear — what neuroscientists call primary affects. In a synthesis of evidence, Peter Coppola notes that fundamental subcortical responses mediate reinforcement learning and can support surprisingly complex behaviours across species . He argues that the cortex elaborates on this basic platform, adding language, planning, memory and abstraction . Such a perspective suggests that consciousness may begin with simple feelings and that higher cognitive functions build upon them . Recognising the potential sufficiency of subcortical circuits has ethical implications: animals and patients with limited cortical function may still have rudimentary awareness .
Implications for Medicine and Ethics
The shift towards recognising subcortical contributions carries important consequences for clinical practice. Physicians often assess consciousness by measuring cortical activity and behavioural responses, for example using EEG or functional MRI during coma evaluations. If subcortical integrity is critical, clinicians may need to develop new tests that probe the function of deep brain structures . Diagnoses of persistent vegetative state might change if patients retain subcortical function and thus some level of awareness . The review raises difficult questions about end‑of‑life decisions and highlights the need for careful interpretation of brain‑injury cases.
Animal research is also affected. If basic consciousness arises from ancient brain circuits, then a wider range of animals — perhaps including reptiles, fish and invertebrates — could have subjective experiences. This possibility invites a broader ethical consideration of how we treat other species. It also intersects with debates about artificial intelligence: if consciousness depends on embodied, evolutionarily ancient mechanisms, replicating it in silicon may require more than just sophisticated algorithms.
Unanswered Questions and Personal Reflections
Despite impressive strides, consciousness remains a frontier. Different theories emphasise different neural processes — broadcasting, integration, meta‑representation or attention. Experiments have begun to test specific predictions, but results often challenge existing frameworks . The growing evidence for subcortical involvement does not eliminate cortical theories; rather, it suggests that the neural basis of consciousness is layered. Basic feelings and arousal states might arise from brainstem and limbic circuits, while the cortex enriches those states with memory, reasoning and language . That picture resonates with everyday experience: the raw feeling of hunger or pain is distinct from the narrative we construct about our lives.
As someone fascinated by these topics, I see beauty in the diversity of perspectives. The adversarial collaboration between IIT and GNWT proponents is an encouraging model for science: instead of clinging to pet theories, researchers work together to design fair tests and accept when their predictions fail . The increasing openness to subcortical contributions reminds us that evolutionary history matters; consciousness did not appear suddenly in humans but likely evolved gradually from primitive affective states . There is humility in recognising that even simple organisms may have rudimentary experiences and that our own awareness may rest upon ancient mechanisms we share with them.
Conclusion
The study of consciousness has travelled a long road, from philosophical speculation to modern neuroscience. Early philosophers debated mind–body relationships, nineteenth‑century scientists first linked brain activity to sensation, mid‑century behaviourism sidelined subjective experience, and late‑twentieth‑century neuroscientists resurrected it with fresh tools . Major theories like GNWT and IIT have provided frameworks, yet recent adversarial tests show that no single model captures the full richness of conscious experience . Emerging evidence suggests that the brain’s most ancient regions may support basic forms of consciousness , with the cortex adding nuance and complexity . Recognising this layered architecture not only advances our understanding of the mind but also prompts ethical reflection on how we treat patients and animals. As research continues, embracing open collaboration and a willingness to revise our theories will be essential for approaching the enduring mystery of consciousness.