A new theoretical paper proposes that RAW sentience—the basic feeling of being a conscious observer—may originate in the brainstem rather than the cerebral cortex. Its authors argue that evidence from pain responses, anesthesia and neurological injuries challenges the view that conscious experience is generated primarily by the brain’s outer layers.
The paper, published in The Neuroscientist, was written by Marshall Devor of The Hebrew University of Jerusalem and colleague Mark Baron. It brings together findings from clinical neurology, evolutionary biology and earlier animal experiments; it does not report a new laboratory study.
The authors distinguish between access consciousness, or awareness of particular sights, sounds and memories, and phenomenal consciousness: the underlying sense of being present. They argue that the cortex may process specific sensory information while a brainstem circuit supports this more basic feeling and helps determine which signals receive attention.
One of their central arguments concerns pain. Electrical stimulation of areas of the cortex associated with vision, hearing and touch can produce flashes of light, sounds or tingling in awake patients, but rarely causes pain. In one study cited by the authors, stimulation produced pain at just 8 of 313 tested sites in the insula, a region sometimes proposed as a cortical pain center. By contrast, stimulation of subcortical pain pathways, including those in the brainstem, can trigger pain.
The paper also reviews cases in which extensive cortical damage has impaired specific abilities without eliminating raw consciousness or the capacity to feel pain. Injuries to the upper brainstem, on the other hand, can cause coma. These observations do not establish where subjective experience arises, but the authors see them as consistent with a brainstem role that goes beyond simply keeping the cortex awake.
Devor and Baron also revisit rodent experiments led by Devor in 2001. In a mapping study involving 828 microinjections in 182 rats, only injections into a small area of the upper brainstem rapidly switched off pain responses, muscle tone and other signs associated with consciousness. Later experiments found that very small injections could produce anesthesia within seconds.
The effects of permanent damage to the area were different. When researchers destroyed a limited patch of cells, the rats woke after surgery and retained sleep-wake cycles, though they became less sensitive to anesthetic drugs. Larger damage to the surrounding region left six rats in an irreversible coma; six animals with damage nearby but outside the target area recovered. The authors therefore propose that the relevant circuitry lies within a roughly 3-by-4-millimeter zone of the mesopontine tegmentum, overlapping with the parabrachial complex, which receives pain signals.
Their evolutionary model suggests that the brainstem may have developed the capacity for sentience in early vertebrates, translating tissue-damage signals into the experience of pain. The authors acknowledge that it is not possible to know with certainty whether fish or reptiles consciously feel pain. They propose that the mammalian cortex later evolved to analyze complex sights and sounds, with its assessments sent to an existing brainstem system that weighs their survival importance and directs attention.
The theory has significant limitations. Brainstem damage and anesthesia are also compatible with conventional accounts in which the brainstem keeps the cortex awake; neither result alone shows that subjective experience is generated in the brainstem. The argument’s most distinctive evidence concerns pain, but researchers disagree about whether parts of the cortex, including the posterior insula, can themselves generate pain.
Much of the evidence also requires interpreting animal behavior or responses from patients who cannot report their experience. The coma findings from the toxin experiments involved only 12 rats, and the paper gives limited detail about those results. The authors have not identified the specific brainstem signals that might make cortical activity conscious, and their proposal does not resolve how biological processes produce first-person experience. They call for experiments to test the proposed mechanisms directly.
Sources: PsyPost.