Neuroscientists propose a surprising new home for consciousness - PsyPost

A new theoretical paper proposes that the most basic feeling of being a conscious self originates in the brainstem rather than the outer layers of the brain. By examining how the brain processes pain, anesthesia, and brain injuries, the authors suggest that raw sentience is computed in an ancient subcortical structure, located beneath the cortex, that acts as a gatekeeper for attention. The hypothesis, published in The Neuroscientist, challenges dominant models of the mind that place conscious awareness within the cerebral cortex.

The brainstem is a stalk-like structure at the base of the brain that connects the upper brain to the spinal cord. Leading theories of consciousness locate it in the cerebral cortex, the wrinkled outer layer of the brain responsible for complex thought and language. In the authors' view, these theories acknowledge the brainstem's importance but treat it mainly as a power supply that keeps the cortex running. Yet a 2025 adversarial study of human brain activity failed to confirm key predictions of the two leading cortex-centered theories of consciousness, leaving open the question of how and where the brain generates subjective experience.

To address this puzzle, the paper draws on a distinction from philosophy between two types of awareness: access consciousness and phenomenal consciousness. Access consciousness refers to the specific things a person is aware of at any moment, such as sights, sounds, or memories. Phenomenal consciousness, which the authors call raw sentience or the "me inside," is the underlying feeling of existing as a conscious observer, a presence that remains completely intact even if a person closes their eyes and blocks their ears.

Clinical and animal observations have repeatedly challenged the idea that the cortex creates this raw sentience. For instance, the author of a 2007 review on brain organization argued that children born without a cerebral cortex still display conscious awareness and emotional responses. Similarly, a 2015 clinical case study of a neurological patient found that a person with extensive damage to the brain areas widely believed to process pain could still fully feel and react to painful stimuli.

Tiny disruptions in a specific part of the brainstem can have the opposite effect. In a 2001 rat experiment led by the new paper's first author, delivering microscopic doses of an anesthetic drug directly into a specific pocket of the brainstem rapidly and reversibly switched off pain responses and other signs of consciousness. The new research, led by Marshall Devor of The Hebrew University of Jerusalem alongside colleague Mark Baron, brings these lines of evidence together into an evolutionary framework proposing that raw conscious feeling originated, and still resides, in the brainstem.

Rather than conducting a single new laboratory experiment, the authors developed a hypothesis-generating theoretical synthesis drawing on clinical neurology, evolutionary biology, and their own laboratory's prior animal experiments. They first evaluated clinical evidence from human patients undergoing awake brain surgery, patients experiencing epileptic seizures, and individuals with strokes or other brain lesions.

In reviewing surgical data, the authors point out that applying mild electrical currents directly to the visual, auditory, or touch areas of an awake patient's cerebral cortex reliably causes the patient to see flashes of light, hear sounds, or feel tingling. Yet stimulating the cortex almost never produces a feeling of pain. Even in the insula, a folded region tucked deep within the cortex whose rear portion is considered the strongest candidate for a cortical pain center, one study evoked pain at only 8 of 313 tested sites. Pain is also rarely reported as a warning sensation, or aura, at the start of epileptic seizures, even though seizures can involve almost any part of the cortex and often produce flashes of light, sounds, or smells.

By contrast, direct electrical stimulation of subcortical pain pathways, including in the brainstem, reliably triggers feelings of pain in patients and animals. The authors also note that while extensive damage to the cerebral cortex can erase specific abilities, such as causing blindness or eliminating awareness of one side of the body, it does not cause a loss of raw consciousness or prevent a person from feeling pain. Small strokes or injuries in the upper brainstem, however, often plunge human patients into a coma.

To pinpoint the exact location of this consciousness switch, Devor and Baron examined data from their laboratory's rodent studies focusing on the mesopontine tegmentum, a region in the upper brainstem. The 2001 mapping study involved 828 microinjections in 182 rats. Only injections into one tiny node, now called the mesopontine tegmental anesthesia area, caused the animals to lose muscle tone, pain responses, and other signs of consciousness, and injections placed even slightly off target had no such effect. In later work, the team found that volumes of 10 nanoliters or less (a nanoliter is a billionth of a liter) could produce anesthesia within seconds.

The researchers also examined what happened when they permanently destroyed cells in this area with a chemical toxin called ibotenic acid. When the damage was limited to a small patch, the rats did not fall into a coma. Instead, they woke up from surgery, resumed sleep-wake cycles with some insomnia, and became less sensitive to anesthetic drugs given through the bloodstream. From this, the authors conclude that the anesthesia area itself is not essential for generating raw consciousness.

When they used larger amounts of the toxin to damage a wider area surrounding that spot, the outcome changed. Six rats that received these larger injections on target fell into an irreversible coma, whereas six rats whose injections landed nearby but off target woke up normally from surgery.