For centuries, the consensus of mainstream science and medicine was that the adult brain anatomy was fixed or ‘hardwired’, such that the brain only developed during childhood and declined in old age, remaining unaltered throughout midlife.  Today, it is well established that the brain changes in response to our environment throughout the lifespan, which is what we call brain plasticity.
Some scientists, including William James and Donald O. Hebb, already began to challenge this view in the late nineteenth century. [2,3] However, it took a long time before researchers were generally convinced by the idea that the adult brain is modifiable in response to the environment. So how did we get here?
In the 1960s, David Hubel and Torsten Wiesel discovered that if new-born kittens were deprived of visual stimulation in one eye during an early life period, they failed to develop vision in that eye. Thus, the kittens’ brain development clearly depended on experience of actually using their eyes to see things. Moreover, the brain area associated with the blind eye had now begun to process visual input from the functioning eye, suggesting that the brain had ‘rewired’ itself in response to the change in input. [4,5] Although these discoveries were groundbreaking, most scientists, including the authors, still opposed the idea that plasticity could occur in the adult brain.
However, with the emergence of MRI studies in the late 1900s, new discoveries of brain plasticity triggered the beginning of a decade of extensive investigation into the plastic potential of the brain.
In 2000, Eleanor Maguire and colleagues  showed that the hippocampi of London taxi drivers were larger relative to non-cabbies. This suggested that memorising the streets of London had led to structural changes in this brain region, which is well known to be particularly relevant for memory and navigation. [7,8] In 2004, juggling exercise was shown to increase brain grey matter in young adults . Interestingly, the observed grey matter changes receded in the absence of juggling exercise. Recently, my colleagues and I showed that memory training can improve brain connections in older adults, [10,11] but continuous training is a premise for maintaining such positive neural changes.
Today, adult brain plasticity is well established. Researchers are now asking questions about why some individuals benefit more than others from cognitive training programs,  and whether the brain's ability to adapt to the environment also make us vulnerable.
6. Maguire, E. A., Gadian, D. G., Johnsrude, I. S., Good, C. D., Ashburner, J., Frackowiak, R. S., & Frith, C. D. (2000). Navigation-related structural change in the hippocampi of taxi drivers. Proc Natl Acad Sci U S A, 97(8), 4398- 4403. doi:10.1073/pnas.070039597.
10. de Lange, A.-M. G., Bråthen, A. C. S., Rohani, D. A., Grydeland, H., Fjell, A. M. and Walhovd, K. B. (2017), The effects of memory training on behavioral and microstructural plasticity in young and older adults. Hum. Brain Mapp., 38: 5666–5680.
11. Engvig, A., Fjell, A. M., Westlye, L. T., Moberget, T., Sundseth, O., Larsen, V. A., & Walhovd, K. B. (2010). Effects of memory training on cortical thickness in the elderly. Neuroimage, 52(4), 1667-1676.
12. Lövdén, M., Brehmer, Y., Li, S.-C., & Lindenberger, U. (2012). Training-induced compensation versus magnification of individual differences in memory performance. Frontiers in Human Neuroscience, 6, 141.