The older picture
For much of the twentieth century, a common view held that the adult brain was essentially fixed. According to this picture, development in childhood set up the wiring, after which the brain mostly declined. Neurons that died were not replaced, and connections were thought to be largely stable. The great Spanish anatomist Santiago Ramon y Cajal, writing in the early 1900s, is often quoted as saying that in adult centers the nerve paths are fixed and immutable, though he also recognized that learning must involve some change. The prevailing emphasis was on stability.
Alongside this view was a strong emphasis on localization, the idea that particular functions live in particular places. Evidence from patients with brain injuries, starting in the nineteenth century, supported a role for specific areas, with language being the best-known example. The localization view was useful, but it sometimes left little room for flexibility.
Evidence for change
Over many decades, evidence accumulated that the brain is more adaptable than the older picture suggested. Several lines of work contributed.
- Learning changes connections. Experiments on synapses showed that their strength can increase or decrease with activity, a mechanism covered in the article on synaptic strengthening. Donald Hebb proposed in 1949 that cells that are active together tend to strengthen their connections, an idea often summarized as cells that fire together wire together.
- Maps can shift. Studies in animals found that the brain’s representation of the body or of sensory input can reorganize after injury or after repeated experience.
- Recovery after injury. Many people regain some abilities after stroke or brain injury, and rehabilitation is based on the idea that the nervous system can adapt, though recovery is often incomplete and varies widely.
- Skill learning in humans. Imaging studies have found differences in brain structure or activity linked to extensive practice, such as in musicians. A widely cited study of London taxi drivers reported differences in a memory-related region, the hippocampus, associated with navigation experience. Findings like these are suggestive, though many are correlational and it can be difficult to separate training effects from pre-existing differences.
- New neurons in some regions. Research suggested that new neurons can form in certain brain areas in adults, at least in some species. Whether and how much this happens in the adult human hippocampus has been debated, with different studies reaching different conclusions.
Together, these results led to a broader view: the brain changes throughout life, although the degree and speed of change differ by age, region and circumstance.
What neuroplasticity means
Neuroplasticity, or brain plasticity, is a general term for the ability of the nervous system to change in structure or function in response to experience, injury or other influences. It covers many different processes at different scales, from molecular changes at single synapses to large reorganizations of brain maps. A more detailed account of rewiring after learning or injury can be found in the article on rewiring the brain. Because the term covers so much, it is best to ask which kind of change is being claimed in any specific case.
Plasticity is not always beneficial. The same processes that support learning can also strengthen unhelpful patterns, as in chronic pain or in some aspects of addiction. Change is a feature of the system, not necessarily an improvement.
Common misconceptions
Plasticity means anything is possible. It does not. Change is constrained by age, biology and the amount of effort and time involved, and some losses cannot be fully reversed.
You can reshape your brain quickly. Some changes at synapses happen fast, but meaningful changes in skill and behavior usually require sustained practice. Products promising rapid rewiring should be viewed skeptically.
Children are plastic and adults are not. Young brains do go through periods of heightened sensitivity for some abilities, but adults continue to learn and adapt. The contrast is a matter of degree rather than an on-off switch.
Every brain scan change is meaningful. Small differences in imaging results between groups or over time do not automatically imply a particular behavioral benefit.
Other shifts in perspective
Plasticity is not the only change in how scientists think about the brain. There is growing attention to networks and connections rather than isolated regions, as described in the discussion of neural circuits. Glial cells, once thought of as mere support, are now studied for their roles in signaling and maintenance. And the brain is increasingly seen as part of a larger system, interacting with the body, the immune system and the environment.
Why the shift matters
A view of the brain as adaptable supports efforts in education, rehabilitation and mental health, and it encourages continued learning across the lifespan. It should be held with moderate optimism and humility. Much about the limits and mechanisms of change is still unknown, and researchers are working to determine which interventions meaningfully help, for whom, and by how much. The picture has moved from fixed to flexible, but a more precise description is that the brain is stable in some respects and changeable in others.