Adult brains do change with practice, and the fine print matters

Somewhere on your phone, an app or an ad is probably promising to “rewire your brain.” The phrase borrows real science. The adult brain does physically change when you practice something hard. But the studies behind that fact tell a narrower, more interesting story than the marketing.

To earn a license to drive a London black cab, a trainee has to learn the city’s layout by heart: roughly 25,000 streets within six miles of Charing Cross station, plus thousands of landmarks. The training, known as “the Knowledge,” usually takes three to four years. About half of the people who start it never qualify.

In a study published in Current Biology in 2011, researchers at University College London scanned 79 male trainees as they began and again three to four years later, along with 31 men who were not training. At the start, the groups matched on IQ, memory and brain structure. At the end, only the 39 trainees who qualified showed growth in one part of the brain: the back of the hippocampus, a structure deep in the brain that handles memory and navigation. Trainees who dropped out or failed showed no change. Neither did the comparison group.

So the brain does change with use. The fine print covers where the change happens, what it costs, how long it lasts, and whether adults grow new brain cells at all.

Brain change is specific, and it has a price

Neuroplasticity means the brain’s capacity to alter its structure and function in response to experience. The National Academies of Sciences, Engineering, and Medicine, in its 2018 consensus report How People Learn II, concluded that the brain keeps developing throughout life, stays adaptable, and is shaped by learning as much as it shapes learning.

The taxi research shows three features that marketing rarely mentions.

The change is local. In the 2011 study, gray matter increased in the rear hippocampus of the qualifiers and nowhere else in the brain. An earlier study from the same group, published in PNAS in 2000, found that licensed cab drivers had larger rear hippocampi than men who did not drive taxis, but smaller front sections. Both differences grew with years spent driving a cab.

There may be a cost. In 2006 the team compared taxi drivers with London bus drivers, who have similar driving experience and report similar stress but follow fixed routes. The taxi drivers again had more gray matter toward the back of the hippocampus and less toward the front, and they were worse than the bus drivers at taking in new visual and spatial information. In the 2011 study, the newly qualified drivers judged the positions of London landmarks better than the comparison group did, but they were worse at recalling a complex drawing after a 30-minute delay.

It may not apply to everyone equally. The qualifiers had trained about twice as many hours a week as those who failed. The 2011 authors also raised the possibility that some people carry a genetic predisposition toward this kind of change. And the trainees followed over time did not show the shrinkage at the front of the hippocampus that the earlier comparisons had found. The researchers speculated it might come later.

Practice changes the brain fast, and the change can fade

Juggling studies show that brain changes from practice can appear quickly and then recede.

In 2004, a German team that included Christian Gaser and Arne May reported in Nature that 12 volunteers who spent about three months learning to juggle showed a temporary increase in gray matter in an area that processes visual motion. In 2008 several of the same researchers recapped that result in PLOS One, then repeated the experiment with 20 adults, average age 26.5, none of whom could juggle.

Each volunteer was scanned six times: before learning, three times during several weeks of practice, and two and four months after practice stopped. The motion area showed a trend toward growth within seven days. Then, as the authors put it, the increase “recedes when exercise is stopped, although the participants were still able to juggle.”

The skill outlasted the scan finding. Neither practice hours nor juggling performance predicted how much the brain changed; the authors suggested that learning something new matters more to brain structure than repeating it.

A measurement caveat applies to nearly every “this activity grows your brain” headline. A bigger gray-matter reading on an MRI scan does not mean new neurons. The 2008 authors listed other possibilities: larger cells, new glial cells (the brain’s support cells), more connection points between neurons, or shifts in blood flow and fluid. The taxi researchers said the same of their results. A scan can show that something changed. It usually cannot show what.

The decade-long fight over new neurons in adults

Adult neurogenesis, the birth of new neurons after development is complete, was first established in rodents. The question is whether the adult human hippocampus keeps making them, in a small region called the dentate gyrus. It is a lesson in how laboratory methods shape answers.

How the argument unfolded

2013. A team at Karolinska Institutet in Sweden used an unusual natural experiment. Nuclear bomb tests in the 1950s and 1960s raised atmospheric carbon-14, which ended up in the DNA of cells born at the time. Measuring it in brain tissue, the team estimated in Cell that about 700 new neurons are added to each hippocampus per day in adults, declining modestly with age. That is where the “700 new neurons a day” line comes from, and it is one method’s estimate, not a settled count.

2018. Two labs published opposite answers within weeks. A University of California, San Francisco team led by Arturo Alvarez-Buylla examined hippocampi from 17 adults who had died and 12 people who had epilepsy surgery, aged 18 to 77, and found no young neurons. Writing in Nature, they concluded that “neurogenesis in the dentate gyrus does not continue, or is extremely rare, in adult humans.” A Columbia University team led by Maura Boldrini examined whole hippocampi from healthy people aged 14 to 79 and reported thousands of immature neurons at every age.

2019. A Madrid team led by María Llorens-Martín used brains collected and processed under tightly controlled conditions. In Nature Medicine it reported thousands of immature neurons in healthy people into their 80s, with numbers falling as Alzheimer’s disease advanced.

2021 and 2022. The San Francisco group answered that doublecortin, the protein most studies use to spot young neurons, is not unique to newborn cells: it turns up in neurons of other adult brain regions, and most cells it labels in the adult hippocampus look mature. A Yale team then read the genetic activity of individual cells and found clear signatures of neuron production in adult mice, pigs and macaques, but not in humans. A University of Pennsylvania team using a similar method reported immature neurons across the lifespan and rare dividing precursor cells in adults.

2025. In July the Karolinska group reported in Science that it had identified dividing neural progenitors, the cells that give rise to new neurons, in adult hippocampi. That targeted the skeptics’ central objection. Its own summary closes modestly, saying only that the results contribute to understanding neurogenesis in adult humans.

2026. In February a University of Illinois Chicago team reported in Nature that a large single-cell study of post-mortem tissue had identified neural stem cells and immature neurons, with a distinct profile in “SuperAgers,” older adults with unusually strong memory.

Why careful labs disagree

In 2018, 18 neurogenesis researchers explained in Cell Stem Cell how the same question could yield opposite answers. Most of their reasons concern what happens to a brain after death:

• Delay before preservation. In rat studies, staining for doublecortin fades within a few hours after death. Many brains in the study that found no new neurons waited up to two full days before being chemically fixed, against a maximum of 26 hours in a comparison study.
• Time in preservative. Long storage in formalin can mask a second marker.
• The dying process. Death floods the body with stress hormones, which may suppress the marker.
• Starting point versus end product. Marker studies look for cells early in the process. Carbon dating looks for finished neurons.

Those commentators favored adult neurogenesis, so their list is an argument, not a neutral audit. A 2023 review in Neuron, led from the Netherlands Institute for Neuroscience, then reanalyzed the single-cell data sets and found that this newer method had also produced conflicting results and carried its own technical and biological pitfalls.

What the dispute means for you

In this library’s evidence terms, “adults make new hippocampal neurons” sits at positive evidence with a contested interpretation. The most defensible reading today is that some new neurons are probably made, in numbers that are uncertain, possibly small, and different from person to person.

Notice what nobody in this fight claims: that the adult brain cannot change. The taxi trainees, jugglers and stroke patients in this article changed without anyone counting new neurons. The argument is about how much one small region renews itself. A lower count in people with Alzheimer’s disease is also an association found after death; it does not show that raising neurogenesis would prevent or treat the disease. Exercise and the growth-factor research tied to it are taken up in our piece on exercise and cognition.

When the brain’s flexibility works against you

Plasticity is not automatically good. The capacity that lets a brain adapt can also help keep an unwanted sensation going.

Phantom limb pain, pain felt in a limb that has been amputated, is the classic example. In a 1995 Nature study, a Berlin-based team found a very strong correlation, 0.93 on a scale where 1.0 is a perfect match, between how far the brain’s touch map had shifted in people with arm amputations and how much phantom pain they reported.

A 2013 Oxford study led by Tamar Makin challenged the usual reading of that result. It found that persistent phantom pain went with a better-preserved representation of the missing hand, not a reorganized one, and proposed that the pain itself may keep the old map alive. Both studies are correlations, so neither proves cause. Central sensitization, chronic pain’s own version of this question, gets its fuller hearing elsewhere in this collection.

Tinnitus, a ringing or other sound with no outside source, is a second example. The National Institute on Deafness and Other Communication Disorders says one leading theory is that inner-ear damage changes the signals reaching the brain, and the sound is generated in the auditory cortex. Surveys estimate that 10 to 25 percent of adults have it. How hearing loss bears on memory and thinking is a separate subject, covered in our hearing-health report.

Rehabilitation is where this research meets patients

According to a 2020 fact sheet from the National Institute of Neurological Disorders and Stroke, about 800,000 people have a stroke each year, and roughly two-thirds survive and need rehabilitation. The same fact sheet says the brain can rewire its circuits after a stroke, supporting some recovery of function over months to years.

Constraint-induced movement therapy

One approach the NINDS describes is constraint-induced therapy. The less-affected arm is restrained, often with a mitt, so the person has to use the weaker arm through intensive, repetitive practice.

Its largest early test was the EXCITE trial, led by Steven Wolf and published in JAMA in 2006. It enrolled 222 people at seven US academic centers whose first stroke had occurred three to nine months earlier: 106 received two weeks of the therapy and 116 continued usual care. After a year, the time needed to complete a set of arm tasks had fallen 52 percent with the therapy, against 26 percent with usual care. On a 0-to-5 scale of how much people used the arm in daily life, scores rose from about 1.2 to 2.1 with the therapy, compared with 1.2 to 1.7 with usual care.

EXCITE also had limits. It studied a selected group at one stage of recovery, and the earlier studies it built on had involved people who still had some wrist and hand movement. Whether the approach suits a particular survivor depends on what that person’s arm and hand can do now, which is a judgment for the rehabilitation team.

On August 27, 2026, the American Heart Association and American Stroke Association published a new adult stroke rehabilitation guideline that replaces their 2016 version, and a correction followed in October. Its key updates include new and revised recommendations on fall prevention, caregiver support, and safe return to work and driving. Any summary that still treats the 2016 document as current is out of date.

Lazy eye after early childhood

Amblyopia, often called lazy eye, is poor vision in an otherwise healthy eye, caused by unequal or abnormal input to the brain during infancy and childhood. The National Eye Institute calls it childhood’s most common eye disorder, and notes that many eye care professionals once believed treating older children would bring little benefit.

An NEI-funded randomized trial by the Pediatric Eye Disease Investigator Group, published in 2005, tested that belief in 507 children aged 7 to 17 at 49 sites. All received glasses; some also received patching, close-up visual activities and, for younger children, eye drops. Among 7- to 12-year-olds, 53 percent of the treated group improved by at least two lines on the eye chart, against 25 percent with glasses alone. Among 13- to 17-year-olds never treated before, it was 47 percent against 20 percent.

Across all the 13- to 17-year-olds, including those treated before, the rates were 25 percent and 23 percent, a gap too small to rule out chance. The authors concluded that extra treatment appeared to offer teenagers little if their amblyopia had already been treated with patching. Most children who improved kept some vision deficit, and the trial could not yet say whether gains would last. The study’s co-chair, optometrist Mitchell M. Scheiman, said at the time that age by itself should not decide whether a child is treated.

Five questions for any “rewire your brain” claim

1. What changed? A region’s size on a scan, a test score, or something you would notice in daily life? Only the last is an outcome. A scan change is a mechanism.
2. In whom? Cab trainees studying for years, and stroke survivors months into recovery, are not a healthy adult using an app at lunch.
3. Compared with what? Was there a group that did something else, like the bus drivers? Were people randomly assigned, as in EXCITE and the amblyopia trial?
4. Did it last? The juggling gray matter receded within months. Ask whether anyone measured the effect after training stopped.
5. What did it cost? The taxi drivers’ gain came with weaker learning of new visual material. A pitch that lists only upside has probably left something out.

A product that cannot answer the first question with something you would notice sits closer to insufficient evidence than to proven. Commercial brain-training games, and the federal enforcement over their ads, are examined in our brain-training coverage.

When to get help instead of reading about plasticity

• Possible stroke. Any of five warning signs, per the Centers for Disease Control and Prevention, can mark a stroke when it comes on suddenly: numbness or weakness of the face, arm or leg, especially on one side; confusion or trouble speaking or understanding; trouble seeing; trouble walking, dizziness or loss of balance; and a severe headache without a known cause. Its advice: call 911 immediately, write down when symptoms started, and do not drive to the hospital.
• Arm weakness after a stroke. Whether intensive arm therapy such as constraint-induced training fits is a question for the rehabilitation physician or therapist, who can judge what movement there is to build on.
• A child’s eyes. If one eye seems weaker or drifts, whether and how to treat, at any age, is a decision for the child’s eye care professional.
• Tinnitus. The NIDCD says evaluation starts with a primary care visit to look for causes such as earwax or fluid from an ear infection; imaging may follow, especially when the sound pulses.
• Phantom limb pain. Which treatments to try, and in what order, is a decision for the amputation care team or a pain specialist.

The one thing worth remembering

Your brain responds to what you do with it, at any age. That part of the marketing is true. The evidence adds three conditions: the change is specific to what you practiced, it tends to fade when practice stops, and it can cost something elsewhere. The skill is what lasts and what matters. So judge any brain program by whether it improves something you need to do, compared with people who did something else, and whether the gain is still there months later.

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