The human brain is one of the most complex organs in the body. Weighing around 3 pounds, the brain controls vital functions like breathing, heartbeat, movement, emotions, and thoughts. While the brain may seem indispensable, research has shown that humans can survive and live meaningful lives even after damaging or losing large parts of their brains. Understanding brain resilience and neuroplasticity reveals that the brain has a remarkable ability to reorganize itself and adapt to change. This article will explore how much of the brain humans can live without, from case studies of people with brain damage to theories on essential brain regions.
Can humans survive without a brain?
No, humans cannot survive without a brain. The brain stem, which connects to the spinal cord, controls involuntary functions critical for life like breathing, heart rate, and blood pressure. Damage to the brain stem is fatal. However, some brain stem reflexes can persist in brain dead patients kept on life support.
Case study: Jahi McMath
In 2013, Jahi McMath, a 13-year-old girl, was declared brain dead after suffering complications from tonsil surgery. Her family refused to take her off life support. Five years later, Jahi’s heart was still beating, but she required a ventilator to breathe and a feeding tube for nourishment. Without a functioning brain, she could not think, feel, or be aware. This case illustrates that the brain stem can continue to regulate some basic bodily processes even after higher brain function is lost. However, brain death is generally accepted as the end of conscious existence and legal death.
Can humans survive without cortex?
The cortex, or outer layer of the brain, controls higher functions like reasoning, language, sensory perception, and motor skills. Damage to localized cortical areas impairs specific abilities, but does not make survival impossible.
Case study: civil war wounded
Phineas Gage was a railroad worker who survived a horrific accident in 1848. An explosion shot an iron rod straight through his left cheek and cortex. Shockingly, Gage was still able to speak, walk, and function after losing a large chunk of brain matter. However, his personality changed profoundly. This historic case was the first indication that localized brain lesions could have specific effects without being fatal.
Essential brain regions
Brain stem
As mentioned, the brain stem controls vital autonomic processes necessary for life. This includes:
- Breathing regulation
- Heart rate
- Blood pressure
Damage to the brain stem disrupts the body’s homeostasis, usually resulting in death or permanent vegetative state.
Cerebellum
The cerebellum, located at the base of the brain, modulates balance, posture, movement coordination, and motor learning. People can live without a cerebellum, but lose fine motor control and the ability to play sports, dance, or perform coordinated sequences.
Basal ganglia
The basal ganglia regulate motivation, voluntary movement, reward cognition, and eye movements. Damage impairs initiation and learning of motor plans. People without basal ganglia can survive but may suffer paralysis or an inability to speak or swallow.
Hypothalamus
This tiny structure controls fundamental homeostatic functions:
- Body temperature
- Hunger
- Thirst
- Sexual function
- Sleep cycles
- Hormone release
The hypothalamus cannot be removed without fatal results. However, smaller lesions can disrupt specific functions like sleep, appetite, or temperature regulation.
Hippocampus
The hippocampus is crucial for forming new memories and spatial navigation. When damaged, people lose the ability to create and recall episodic memories. However, they can still remember general knowledge, language skills, habits, and motor memories.
How much cortex can humans live without?
Humans can survive and adapt after losing surprisingly large regions of the cortex:
- One man lived with only 10% of his cortex intact
- Some children develop almost normally after hemispherectomy, surgical removal of half the brain, to treat epilepsy
- People have regained speech and motor function after removal of cortical areas thought to be vital
This cortical plasticity results from:
- Redundancies – brain regions with overlapping functions
- Neuroplasticity – ability of undamaged cortex to reorganize and take over lost functions
- Neuroadaptivity – use of alternative brain pathways to compensate
- Behavioral compensation strategies
However, extensive cortical damage almost always impairs abilities, even if it is not life-threatening. Loss of huge cortical territories impacts:
- Language and communication
- Visual processing
- Motor skills
- Spatial orientation
- Cognition
- Emotional regulation
- Personality
The amount of cortex humans can lose and still live varies based on size, location, and laterality of the lesion. Damage on one side is better tolerated than on both sides. Some focal deficits are more disabling than generalized deficits. But in most cases, remaining cortical tissue can take over if the lesion occurs early in life before functions are fully developed.
Brain deficits by region
Different parts of the brain have distinct roles. Here is an overview of common deficits when specific cortical areas are damaged:
Frontal lobes
Located behind the forehead, the frontal lobes control cognition, behavior, emotions, language, and movement. Frontal lobe damage causes:
- Impaired judgment
- Reduced problem solving
- Difficulty regulating behavior
- Changes in sexual behavior
- Apathy and reduced initiative
- Loss of speech (Broca’s area)
Parietal lobes
The parietal lobes process touch, temperature, pain, and body position. Parietal lobe injury can cause:
- Difficulty writing, buttoning clothes, or using tools
- Impaired arm or leg movement
- Neglecting or being unaware of one side of space
- Number processing deficits (Gerstmann’s syndrome)
Temporal lobes
Important for memory, hearing, and language, the temporal lobes when damaged lead to:
- Memory deficits
- Difficulty comprehending language
- Impaired ability to categorize objects
- Loss of speech (Wernicke’s area)
- Lack of auditory recognition
Occipital lobes
As the visual processing center, the occipital lobes when injured cause:
- Loss of vision or visual field deficits
- Difficulty recognizing colors
- Inability to identify objects or written words
- Impaired coordination between vision and movement
Neuroplasticity: How the brain recovers and remaps
The brain is far from static. Neurons and neural pathways can reorganize and adapt in response to lesions. Neuroplasticity allows redistribution and remapping of functions in new cortical territories after injury.
Mechanisms of neuroplasticity
The brain remaps lost functions through:
- Axonal sprouting – growth of new connections
- Dendritic branching – increased synapse density
- Recruiting existing redundant pathways
- Taking over lost functional maps
- Increased hemispheric connectivity
This plastic reorganization is driven by:
- Neuronal growth factors
- Synaptogenesis – formation of new synapses
- Use-dependent strengthening of circuits
Plasticity declines with age but persists through life. Recovery is most robust in children before ages 7-10.
Functional remapping
After a cortical lesion, residual areas reorganize to take over lost functions, including:
- Motor maps enlarging into nearby damaged territories
- Intact auditory cortex taking over visual processing in the blind
- Contralesional cortex assuming language functions after left hemispherectomy
This allows impressive recovery but is limited by extent of damage and available cortical real estate.
Rehabilitation facilitates neuroplasticity
Targeted rehab promotes neuroplastic remapping through exercise, repetition, and stimulating cortical input. For example:
- Physical therapy helps motor reorganization
- Speech therapy engages language circuits
- Sensory mapping exercises retrain cortical regions
Recovery and adaptation to brain injuries
Despite potentially devastating brain injuries, good recovery is possible in many cases through compensation strategies and neuroplasticity.
Early brain injuries have better outcomes
Younger age at injury leads to better functional recovery thanks to increased neuroplastic potential in the developing brain.
Localized deficits easier to overcome
When injury is confined to a small cortical area, specific impairments can often be managed well through adaptation.
Time aids reorganization
Gradual recovery can continue for months or years after injury as cortical remapping evolves.
Effective rehabilitation is key
Therapeutic activities promote neuroplasticity and teach compensatory strategies. This facilitates recovery.
Intact hemispheres can assume lost functions
After a unilateral hemispherectomy, the remaining hemisphere can remap to process language, vision, and movement.
Neuroprosthetics augment lost abilities
Devices like cochlear implants can provide sensory input to help strengthen compensatory pathways.
Assistive devices aid adaptation
Tools like wheelchairs, reminder apps, and audiobooks enable independence despite cognitive or physical deficits.
Conclusion
The human brain has remarkable resilience. While no one can survive without critical structures like the brain stem, people can adapt and live full lives despite substantial cortical damage, especially if it occurs in childhood. Rehabilitation, assistive technology, and neuroplasticity help the brain compensate for deficits by remapping functions into spared areas. However, extensive injury still poses profound challenges. Understanding the brain’s capacity to reorganize provides hope that people can reclaim meaningful lives after devastating neurological impairment. With rehabilitation and support, most can regain some degree of independence and enjoyment despite disabilities or altered abilities.