Neuroplasticity: How the Brain Changes, and What It Cannot Do

Neuroplasticity definition: what it means

The clinical reference definition is “the ability of the nervous system to change its activity in response to intrinsic or extrinsic stimuli by reorganizing its structure, functions, or connections”. The idea is older than the word. William James wrote about plasticity in the nervous system in 1890; the Polish neurophysiologist Jerzy Konorski introduced the term neural plasticity in 1948, and Donald Hebb popularised it the following year.

Three points in that definition are easy to miss. Plasticity is not a special mode the brain enters — it is how all learning and memory physically happen. It works in both directions, weakening connections as well as building them. And its results are not automatically good.

How neuroplasticity works: the four mechanisms

“The brain rewires itself” covers several different processes that operate on very different scales.

Synaptic plasticity is the foundation. In 1973 Terje Lømo, a Norwegian physiologist, and Tim Bliss stimulated a pathway into the hippocampus of anaesthetised rabbits and found that the response of the receiving cells stayed amplified afterwards — in fifteen of eighteen animals, for periods ranging from 30 minutes to 10 hours. They had recorded long-term potentiation: a connection that becomes stronger because it has been used. It remains the clearest model of how experience is written into the brain.

Functional reorganisation is what happens when an area is damaged and other areas take over part of its work. After hemispherectomy — the removal of one cerebral hemisphere, usually for severe epilepsy in early life — imaging shows the remaining hemisphere reorganising to restore lost function. That the brain can do this at all is remarkable; that it does it best in children is the key limit, discussed below.

The evidence that adult brains change: London's taxi drivers

The best-known demonstration comes from drivers of London's black cabs, who must memorise the city's street layout to pass an exam known as “the Knowledge”. In 2000 Eleanor Maguire and colleagues scanned licensed drivers and found that their posterior hippocampi — a region involved in spatial memory — were larger than those of people who did not drive taxis, and that the size tracked how long each person had driven.

A comparison at one moment cannot show which came first. So Katherine Woollett and Maguire followed trainees over the four years of preparation, scanning them before and after. Those who qualified showed a selective increase in posterior hippocampal grey matter. Trainees who failed to qualify, and control participants, showed no structural change. The authors describe the trainees as average-IQ adults — which makes the study a direct answer to whether ordinary adult brains remodel themselves under sustained, demanding learning. They do, in the region the learning uses.

Does the adult brain grow new neurons?

This is the most quoted claim about neuroplasticity and the least settled. The evidence points in opposite directions.

• For: in 1998 Peter Eriksson's group in Gothenburg examined the brains of patients who had been given a marker that labels dividing cells and reported new neurons in the adult hippocampus. In 2013 a Karolinska Institute team led by Jonas Frisén measured carbon-14 from Cold War nuclear tests in the DNA of hippocampal neurons and estimated that about 700 new neurons are added to each hippocampus per day, with a modest decline with age.
• Against: in 2018 Shawn Sorrells and colleagues at UCSF found that young neurons in the human dentate gyrus decline sharply during the first year of life and were not detected at all in adults aged 18 to 77.
• For again: the same year Maura Boldrini's team reported thousands of immature neurons in people aged 14 to 79, and in 2019 a Madrid group found them in healthy people into their eighties.

The dispute is largely methodological: the markers used to identify a new neuron can also appear in immature cells that were not newly generated, and the Madrid group attributed its positive result to tightly controlled tissue processing. The honest summary is that adult human neurogenesis is possible but unproven, and its size, if it exists, is unknown. Any product or article that promises to “grow new brain cells” is claiming more than the research has shown.

At what age is neuroplasticity strongest?

Plasticity is greatest in early childhood and narrows with age. Pierre Paul Broca observed in the nineteenth century that speech returned more readily after brain damage in children than in adults, and that observation has held. The team that found no new neurons in adult hippocampi found young neurons in infants, but declining sharply during the first year of life.

That is not the same as saying the adult brain is fixed — the taxi driver studies show otherwise. Adult plasticity is real, but it needs more repetition, it is more local, and recovery after injury is less complete than it would be in a child.

Neuroplasticity is not always good

Plasticity follows use, not intention. Clinicians describe the outcome of reorganisation after injury as beneficial (function restored), neutral or negative. The negative form, maladaptive plasticity, is part of the explanation for chronic pain and phantom limb sensations: networks reorganise and strengthen, but around a symptom. The same mechanism that makes a practised skill automatic can make an unwanted pattern persistent.

How to increase neuroplasticity — what the evidence supports

The popular advice — learn a language, take a new route to work, brush your teeth with your other hand — is built on a true premise and an untested conclusion. The premise is that the brain changes with new demands. The conclusion, that these small novelties improve thinking in general, is not what the research on training shows.

The most thorough review of the question, by Daniel Simons and six co-authors in 2016, examined every peer-reviewed study cited by leading brain-training companies. It found extensive evidence that training improves performance on the trained tasks, less evidence for closely related tasks, and little evidence for distantly related tasks or everyday cognitive performance. That pattern is exactly what the taxi-driver studies predict: plasticity builds the circuitry the practice uses.

What this means in practice:

• Train the skill you actually want. Gains transfer poorly, so practise the target directly rather than a proxy for it.
• Make it sustained and demanding. The structural changes in trainee drivers built up over years of effortful learning, and only in those who reached the standard.
• Look after the conditions. Exercise, environmental factors, task repetition and motivation are listed as positive influences on synaptic plasticity. They support learning; they are not a substitute for it.

Neuroplasticity and IQ

Every lasting gain in ability depends on plasticity, so the useful question is not whether the brain can change but which experiences change it broadly enough to move a measure of general reasoning. The strongest answer so far is schooling. Stuart Ritchie and Elliot Tucker-Drob pooled 142 effect sizes from 42 data sets covering more than 600,000 people and estimated that each additional year of education raises IQ by about 1 to 5 points, an effect that persisted across the lifespan. They concluded that education appears to be the most consistent, robust and durable method yet identified for raising intelligence.

Education works where brain games do not because it is years of effortful practice in exactly the abilities an IQ test samples: reasoning, vocabulary, knowledge and sustained attention. The practical routes, and what each is worth, are set out in what the evidence says about increasing your IQ. The two kinds of ability that respond differently to learning are explained in fluid vs crystallized intelligence, and what a test actually measures is covered in how IQ tests work.

Where this sits among other ideas about the brain

Neuroplasticity is the rare brain idea that is true and still overstated. Its neighbours fare worse: the claim that people are left-brained or right-brained failed a direct imaging test, and learning styles failed direct teaching trials. Plasticity survives scrutiny because it is a well-measured physical process — what does not survive is the leap from “the brain can change” to “any mental exercise will change all of it”.

Trust and scope notes

This page is educational. It summarises published research on how the brain changes and is not medical advice, a diagnosis or a rehabilitation plan. Anyone recovering from a brain injury or stroke should follow the guidance of their own clinical team. Our IQ test does not assess brain health or plasticity.

IQ Revealed is not affiliated with, endorsed by or connected to the National Library of Medicine, the Karolinska Institute, Sahlgrenska University Hospital, the University of California, the Spanish National Research Council, or any of the universities, hospitals, journals or researchers named on this page. They are named so that each claim can be traced to the work that made it. Quoted phrases are taken from the published sources linked above.