Imagine a child born without most of their cerebral cortex—the brain’s thinking powerhouse—yet still awake, smiling, and responding to the world. This is hydranencephaly, a rare condition where a major in-utero stroke destroys the telencephalic structures (cerebral cortex, thalamus, and basal ganglia), leaving the skull filled with cerebrospinal fluid. Unlike anencephaly, which affects other brain parts, hydranencephaly spares the intact brainstem. These children can survive for years, even into adolescence, though they’re often mislabeled as “vegetative” and institutionalized.
Far from vegetative, they actively engage. Limited by low spinal muscle tone and spastic limbs, they still move their heads and eyes freely. Facial expressions reveal emotions: smiles at toys or sounds, laughter from tickling, frowns from pain. They crawl toward sunlight to bask in its warmth, looking visibly pleased. They orient to familiar voices or touches, prefer their primary caregiver over strangers, and show clear likes—like specific music pieces, instruments, voices, tempos, or styles—that light up their faces with joy.
Subcortical Senses and Emotions at Work
Without cerebral cortex, their perceptions rely on brainstem structures. Vision and hearing process through intact superior colliculi in the tectum. Touch and feelings route via the nucleus tractus solitarius and parabrachial nucleus. Emotions trigger from periaqueductal gray nuclei, expressed through cranial nerve nuclei controlling facial muscles. Life-sustaining functions draw from the hypothalamus, endocrine system, and vagus nerve—hydranencephalic girls even menstruate at puberty.
These responses aren’t reflexive; they’re sustained, stimulus-matched expressions of real feelings. They represent an “elementary experience”—feelings tied to a basic “protoself” (body state representation), modified by simple object interactions. This challenges the idea that sentience, feelings, and emotions require the cortex. Hydranencephalics differ sharply from true vegetative states or akinetic mutism, where world interaction vanishes. They’re more like newborns: minds at work with functional brainstems but immature cortices (unmyelinated at birth), explaining delayed diagnosis via scans after failure to thrive.
The Superior Colliculus: Brainstem’s Hidden Integrator
The superior colliculi offer a glimpse into subcortical mind-making. This tectum structure interlinks with periaqueductal gray, nucleus tractus solitarius, and parabrachial nucleus. It features seven layers: superficial ones (I-III) build visual maps from retina and visual cortex signals, charting the contralateral field. Deep layers (IV-VII) overlay visual, auditory, and somatic (body/spinal/hypothalamic) maps in precise spatial register—the brain’s only spot superposing these senses for efficient integration, feeding motor systems.
In humans, collicular damage is rare; one bilateral trauma case left a patient akinetic mute for months, hinting at disrupted mentation. Without visual cortices (as in hydranencephaly), colliculi enable basic vision—like detecting motion in a visual quadrant—vague but useful, akin to blindsight. Both superior and inferior colliculi likely contribute more substantially from birth.
Compelling evidence elevates colliculi: they generate gamma-range oscillations, linked to neuron synchrony, coherent perception, and possibly consciousness (per Wolf Singer). Uniquely, outside the cortex, this positions them as mind contributors, echoing work by Bernard Strehler, Jaak Panksepp, and Bjorn Merker.
Limits and Lessons
Hydranencephaly reveals brainstem limits: basic sentience without cortical “wider mind.” It’s troubling yet profound, underscoring cortex-independent feelings while affirming the cortex’s irreplaceable role.
Source : Self Comes to Mind: Constructing the Conscious Brain by António Damásio
Goodreads : https://www.goodreads.com/book/show/7766914-self-comes-to-mind
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Thinking Beyond Science 
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