Neuroplasticity

"Human beings are genetically programmed, programmed to learn" (F. Jacob, 1981).

Brain vs Computer :

The functioning of the nervous system and that of a computer system have several similarities ^{[118, 228, 229]}:

Both utilize a basic binary signal (0-1). In computers, current either flows or it does not. In the nervous system, this corresponds to the all-or-none law of action potentials.

Both consist of a hardware component (physical equipment for the computer and organs for the nervous system) and a software component ^[229] (programs for computer systems and higher cognitive functions for the nervous system) ^[228].

However, the nervous system is infinitely more powerful on several levels:

If we compare a transistor (the basic element of a microprocessor, which is the center of calculation and data processing in a computer) to a synapse, as both perform similar functions, we find that the highest-performance microprocessors designed today contain only about 3 billion transistors [Wikipedia, Transistor count]. In contrast, the nervous system possesses approximately 100 trillion synapses (100,000,000,000,000) ^{[4, 57]}, a number of neural connections within the human body that is as fascinating as it is extraordinary.

Our brain is a supercomputer that not only manages this vast number of neurons and synapses but does so at a minimal energy cost: approximately what is required to power an ordinary light bulb. If we were to build a supercomputer with the same number of transistors, it would require at least 100 megawatts of energy to operate - enough to power an entire city ^[139].

Not only is the brain superior in its number of connections and low energy consumption, but it is also far more efficient in managing those connections. In the brain, these operate in parallel: billions of pieces of information circulate simultaneously at any given moment. In contrast, microprocessors typically operate in serial mode, processing one piece of information after another.

Yet, the most extraordinary faculty of the nervous system is certainly not its raw power. The true and undeniable force of the nervous system is, and will always be, its flexibility and plasticity. Every day, we lose approximately 100,000 neurons ^[111], yet we continue to live as if nothing has changed. This is due to the formation of new connections that compensate for the deficit. Conversely, a microprocessor would possibly fail following the loss of a single transistor.

Discovery :

In 1890, the famous Russian physiologist Ivan Pavlov ^[76] noticed that dogs tended to salivate before actually coming into contact with food. He decided to investigate this "psychic secretion" thoroughly although he was a physiologist. He conducted a series of experiments in which he signaled every meal to a dog with a sound. After a few days, the dog began to salivate whenever it heard the signal. Pavlov concluded that physiological reflexes could be triggered by specific conditioning of the brain, and he introduced the concept of the conditioned reflex ^[54].

This experiment had a major impact on modern neurology and psychology. Later, the term neuroplasticity was coined by his student, Jerzy Konorski, who further developed Pavlov's research. We know today that conditioned reflexes are merely variations of a fundamental and essential property of the nervous system: neuroplasticity ^[97].

Neuroplasticity is the most remarkable and striking cerebral faculty; it is the power to change and adapt to environmental conditions and experience. It is thanks to neuroplasticity that we can memorize, forget, learn, develop, and recover from brain injuries that can sometimes be devastating.

Pavlov's discovery is just one example of what the nervous system is capable of. Indeed, the nervous system is in a state of perpetual change and development, and research in this field continues to amaze us every day with the incredible potential of neuroplasticity.

Thanks to modern techniques such as PET (positron emission tomography ^[67]) and functional MRI ^[76], which allow for the localization of brain regions responsible for specific functions, it has been shown that every person possesses a unique distribution of functional areas within their brain. While there are broad correspondences, there are nonetheless differences based on each individual's past and experience.

For example, a violinist has a brain region that is more developed for the muscles controlling the pinky finger compared to the other fingers. Similarly, an individual blind from birth or childhood who uses Braille to read develops significant activity in the visual cortex, even though they are unable to see.

Whenever a brain region becomes non-functional due to damage to a sensory or effector apparatus, it reallocates its neural resources to other functions. In the brain, nothing remains idle.

This explains how the blind can possess highly refined hearing and touch, how those who are deaf and mute develop impressive sign language communication skills, and how other individuals with disabilities manage to compensate by developing other aptitudes. Neuroplasticity also explains how we can become more intelligent over time, even though we lose tens of thousands of neurons every day without them being replaced.

Mechanisms :

Where does this flexibility and plasticity of the nervous system come from? In fact, there are several underlying mechanisms, occurring at both local and global scales.

At the synaptic level (synaptic plasticity):

If an action potential is triggered in a presynaptic neuron and the same stimulation is repeated several times ^[52], it is noted that the response of the postsynaptic neuron increases in intensity over time; thus, there is an improvement in synaptic efficiency. If, after a few days, the same presynaptic neuron is restimulated, the same intense response will be recorded postsynaptically.

This phenomenon is called Long-Term Potentiation (LTP) ^{[3, 38, 39]}. Whenever a synapse is repeatedly used, it becomes more reactive and efficient over a long duration. This may be due to:

• Increased secretion of neurotransmitters.
• An increase in the number of postsynaptic receptors or changes in their properties (phosphorylation), causing them to stay open longer.
• A decrease in neurotransmitter reuptake.

At the cellular level (neuronal plasticity):

A neuron can create new synapses (synaptogenesis) ^[74] or modify the structure of dendritic spines, which impacts the amplitude of synaptic excitation. The excitability threshold at the axon hillock can also vary according to several factors, particularly hormonal ones; a higher threshold makes it more difficult to generate an action potential.

Furthermore, though exceptional, low levels of neurogenesis can occur, most often in the hippocampus, where new neurons are born with new functions.

On a global scale (cerebral plasticity):

There can be a reorganization of neural networks and a redefinition of their connections ^[141]. We recall the famous phrase by Donald Hebb ^[140] (considered the father of neuroplasticity in the 1950s): “Neurons that fire together wire together” ^[140]. Whenever a neural circuit is repeatedly activated, it forms a robust network dedicated to a specific function.

Applications :

An increasing number of studies focus on neuroplasticity to elucidate its mechanisms, limits, and - most importantly - its potential. Today, neurosurgeons possess the knowledge to predict whether a function will be recovered following a surgical procedure on the brain.

This cerebral flexibility has begun to be exploited in recent years, particularly in the field of sensory loss.

For instance, devices now exist that allow people with certain forms of blindness to "see" using their tongue ^{[97, 141]}: cameras mounted on the forehead transmit visual data to a device placed on the tongue, where light signals are converted into mechanical signals. Because the tongue is highly sensitive, it allows the patient to discriminate between these "mechanical pixels." Over time, the brain adapts to this new function, and the visual cortex takes over the perception of this new sensory modality.

Another example involves a patient who lost her sense of balance due to labyrinthine toxicity caused by antibiotics. She was fitted with a device that sends regular vibratory signals to the floor of the mouth based on her spatial orientation. Gradually, her brain adapted to this new form of signal and integrated its function in the same way it integrates neurological data from the inner ear.

These practical applications of neuroplasticity in sensory loss prove once again that sensory organs are for the most part means of extracting data from the world. While they are very powerful and elaborate on their own, their peripheral failure can be bypassed through artificial means (sensory substitution ^[97] ), and the brain will successfully adapt to these new modalities.