Where in the brain does the feeling of "I" live? Two groups of theories have spent decades looking for the answer in the cerebral cortex - the wrinkled outer layer responsible for complex thinking and speech. But a new paper published in the journal The Neuroscientist proposes moving the search deeper - to the brainstem, an ancient structure at the base of the skull that connects the upper parts of the brain to the spinal cord.
The authors of the hypothesis are Marshall Devor and Mark Baron of the Hebrew University of Jerusalem. They did not conduct a new laboratory experiment but instead brought together data from clinical neurology, evolutionary biology and their own animal experiments. Their conclusion: the elementary sense of existence, which the researchers call "raw sentience" or the "I within," may arise precisely in the brainstem.
In philosophy, two kinds of awareness are distinguished. Access consciousness is the specific things a person perceives at a given moment: images, sounds, memories. Phenomenal consciousness is that very basic feeling of presence that does not disappear even if one closes one's eyes and plugs one's ears. It is this that the authors consider primary.
Clinical observations, according to the researchers, sit poorly with the idea that this primary awareness is created by the cortex. Children born without a cerebral cortex, according to a 2007 review, still show signs of conscious perception and emotions. And a patient with extensive damage to the areas usually associated with pain processing still fully felt painful stimuli - such a case was described in 2015.
The opposite picture is seen with pinpoint intervention in the brainstem. In a 2001 experiment on rats, microscopic doses of anesthetic injected into a specific area of the brainstem quickly and reversibly switched off pain responses and other signs of consciousness. A total of 828 microinjections were made into 182 rats. Only one tiny node produced the effect - it was named the mesopontine anesthesia area. Doses of 10 nanoliters (a nanoliter is one billionth of a liter) induced anesthesia within seconds.
When this area was destroyed with a chemical toxin, the rats did not fall into a coma but woke after surgery and became less sensitive to anesthesia. Damage to a broader zone around it, however, led to irreversible coma in six rats, while six animals that were missed woke up safely. On this basis, the authors place the presumed "consciousness generator" in rodents within an area measuring 3 by 4 millimeters.
A separate argument is pain. Stimulation of the visual, auditory or tactile zones of the cortex in an awake patient causes flashes of light, sounds or tingling, but almost never pain. Even in the insular lobe, buried deep in the cortex, pain could be evoked at only 8 of 313 tested points. Direct stimulation of subcortical pain pathways, including the brainstem, by contrast, reliably causes pain.
In their evolutionary model, the authors suggest that hundreds of millions of years ago the brainstem learned to turn signals of tissue damage into the subjective sensation of pain. The cortex, in their view, works as an unconscious supercomputer: it analyzes visual and auditory signals and sends the brainstem a summary of their importance for survival. The brainstem acts as arbiter and, if a signal is significant enough, "tags" it with consciousness.
However, the theory has weak points. Much of the data on the brainstem is also consistent with traditional views, in which the brainstem merely supports cortical wakefulness. Coma after brainstem damage does not in itself prove that subjective experience is born there. In addition, the conclusions about the consciousness zone rest on only a dozen rats, and transferring results from animals to humans always requires caution. The authors emphasize: this is a hypothesis, not a proven fact, and they propose experiments to test it.