We think of the brain as a map. This region for movement, that region for speech, this fold for recognizing faces. Lose a piece, lose the function. It is a tidy, mechanical model. It is also, in the face of the most extreme neurosurgical interventions, profoundly incomplete.
Consider the hemispherectomy. The surgical removal, disconnection, or functional isolation of an entire cerebral hemisphere. Half the brain. Gone.
It is performed on children with intractable epilepsy arising from catastrophic unilateral conditions: Rasmussen’s encephalitis, hemimegalencephaly, Sturge-Weber syndrome, perinatal strokes . The goal is seizure control. The outcome, however, reveals something far more astonishing about the nature of the organ we call the self.
These children do not merely survive. They thrive. They walk, speak, think, feel, and build lives. The remaining hemisphere—the one that was supposed to control only the opposite side of the body—somehow learns to reach across the void and take the reins of the abandoned side .
This is not healing. This is reinvention.
1. The Surgery: What It Means to Remove Half
The term “hemispherectomy” is slightly misleading. In modern practice, it is often a functional disconnection rather than a complete anatomical removal. The diseased hemisphere is surgically isolated, its connections to the rest of the brain severed, leaving it physically present but functionally inert .
The remaining hemisphere must now assume responsibility for functions that were previously bilateral: motor control, sensory processing, perhaps even aspects of language and cognition that were once distributed.
The brain does not regrow what is lost. The central nervous system lacks the regenerative capacity of peripheral nerves. There are no Schwann cells to clear debris and guide regrowth. There is only scar tissue, inhibitory molecules, and the daunting challenge of doing more with less .
Yet, somehow, it adapts.
2. The Mechanisms: How the Brain Rewires Itself
The plasticity observed after hemispherectomy operates through several overlapping mechanisms, each more remarkable than the last.
The Ipsilateral Corticospinal Tract (iCST). In a normally developed brain, each hemisphere primarily controls the opposite side of the body. The right hemisphere moves the left arm; the left hemisphere moves the right. But there exists, in a latent form, an ipsilateral pathway—uncrossed fibers that connect each hemisphere to the same side of the body . In healthy adults, these pathways are weak, requiring higher thresholds to activate, producing slower responses. After hemispherectomy, however, they are reinforced. A new functional demand transforms a dormant backup system into a primary motor highway .
Axonal Sprouting and New Collaterals. Surviving neurons in the intact hemisphere do not merely work harder. They grow. They send out new axonal branches to denervated targets—the spinal cord, the brainstem, the deep cerebellar nuclei . In animal models, neurons from the intact corticospinal tract have been observed to send collaterals across the midline, reaching the side of the body that lost its cortical input .
Subcortical and Brainstem Remapping. The intact hemisphere also strengthens its connections to subcortical structures: the red nucleus, the pontine nuclei, the superior colliculus, the basal ganglia . These ancient, evolutionarily conserved centers become waystations, relaying cortical commands to the spinal cord via pathways that were always present but never fully utilized.
The Release from Maladaptive Inhibition. Before surgery, the diseased hemisphere may have been doing more than simply failing. It may have been actively inhibiting the healthy hemisphere through transcallosal connections, suppressing its potential . The damaged tissue, still present, still firing aberrantly, may have exerted a kind of neurological tyranny over its healthy counterpart. Removal severs that inhibitory influence. The healthy hemisphere is disinhibited, freed to reorganize without the constant, pathological interference of its neighbor .
3. The Age Factor: Why Children Recover and Adults Do Not
Here the story darkens, and the lesson becomes brutal.
The remarkable recoveries documented after hemispherectomy occur almost exclusively in children, particularly those operated on before the age of five or six . In adults who undergo the procedure—rare, but documented—the outcomes are far grimmer: aphasia, hemiparesis, visual field cuts, memory impairment, and limited functional gain .
This is not because adult brains lack plasticity entirely. They retain it, in diminished form. Stroke rehabilitation in adults demonstrates that the mature brain can reorganize, shifting function to perilesional cortex or recruiting contralesional regions . But the scale is different. An adult brain, faced with the loss of an entire hemisphere, cannot mount the wholesale reorganization observed in children.
The critical period for this kind of radical plasticity closes. Not because the adult brain is rigid, but because it has spent decades pruning. The developing brain, in its initial exuberance, overproduces synapses, axons, and pathways. Experience then sculpts this excess, retaining the useful, discarding the rest. The child who loses a hemisphere still possesses latent pathways that can be strengthened. The adult who loses a hemisphere has already decommissioned those pathways, and the machinery for rebuilding them is slower, less forgiving .
The lesson is stark: you must be shaped early, or the shaping becomes permanent.
4. The Deeper Implication: Identity Is Not Located
We return to the original question. If half the brain can be removed and the remaining half can assume control of the entire body, what does that say about the location of the self?
The folk theory of consciousness holds that the “I” resides somewhere specific—perhaps in the prefrontal cortex, perhaps in some integrated network that constitutes the seat of awareness. Hemispherectomy challenges this model at its foundation. It demonstrates that identity is not a location but a process. It is not housed in a specific region; it is an emergent property of a system that, when deprived of half its hardware, can reconfigure itself on the remaining hardware and continue.
The children who undergo hemispherectomy do not become half-persons. They do not lose half their memories, half their personality, half their capacity for love and language. They remain themselves, because “themselves” was never confined to one hemisphere. It was distributed across the whole, and when the whole was halved, the remaining half learned to host the entirety.
This is not to say there are no deficits. Contralateral hemiparesis is universal. Fine motor control of the hand may never fully return. Visual field cuts persist. Sensation is impaired . But the core of the person—the capacity for relationship, for thought, for experience—survives.
The Unspoken Conclusion: The Brain Is Not a Machine, It Is an Ecosystem
We speak of rewiring, of pathways, of tracts and projections. The language is mechanistic, borrowed from electronics and engineering. It is useful, but it misses the deeper truth.
The brain is not a machine. A machine, when half its components are removed, ceases to function. An ecosystem, when half its territory is destroyed, may reorganize. Species relocate. Food webs reconfigure. Energy flows find new channels. The whole becomes something different, but it remains alive.
The hemispherectomized brain is an ecosystem that lost half its land and learned to farm the remaining half more intensively. It is not a damaged version of the original. It is a new equilibrium, a reorganization of life around absence.
The children who live this reality do not spend their lives mourning the missing half. They simply live. They run, they laugh, they learn, they love. Their brains, against all expectation, found a way.
This is the final, brutal, hopeful truth: the self is not so fragile as we imagine. It can lose half its neural substrate and still persist. It can rewire, reroute, reinvent. It can, when faced with the impossible, simply do more with less.
And that, perhaps, is the most radical lesson of all.
Discover more from THAT Elephant
Subscribe to get the latest posts sent to your email.