Andrew Marr discusses how far the brain can change and adapt with the neuroscientist Heidi Johansen-Berg. Decades ago it was thought that the adult brain was immutable but later research has shown that even brains damaged by stroke have the capacity to adapt. The writer Ben Shephard looks back to the turn of the 20th century and the birth of modern neuroscience, while the novelist Charles Fernyhough asks whether knowing more about the way the brain works will have as big an impact as the findings of Darwin and Freud. The clinical psychologist, Mark Williams, is interested in how we can relieve the despair of feeling trapped in our thoughts, and is one of the pioneers of mindfulness-based cognitive therapy.