Brainstem Theory of Consciousness

The brainstem theory of consciousness asserts that raw, subjective experience, the “me inside,” derives from computations implemented in the brainstem, not the cortex. The argument rests on philosophy’s prototypical example of sentience, pain. Feeling pain does not require cortical processing. Nowhere in the cortex does electrical stimulation reliably evoke pain. Subcortical stimulation does. Painful epileptic seizures are rare. Cortical lesions do not eliminate pain or cause loss of consciousness (LOC). Indeed, some individuals with near-complete cortical loss are sentient. Focal brainstem lesions, however, often cause LOC/coma, as do anesthetics delivered to these same foci (general anesthesia). Clearly, peripheral nociceptor input can be translated into conscious pain experience by the “primitive” brainstem. Fish and reptiles respond to threats using vision, sound, and olfaction, but this capacity is rudimentary. Except for pain, advanced sensory feature extraction requires the supercomputer-like capacity of the mammalian cerebral cortex, available evolutionarily only eons after reptiles. Rather than reinventing sentience circuitry with cortical architecture, evolution exploited the sentience-of-pain capability long functioning in the brainstem with the aid of dedicated axonal pathways. This explains why large cortical lesions disrupt “consciousness of” specific things, without causing unconsciousness, while focal brainstem lesions erase all sentience, including pain, sending the individual into oblivion.