Dr Michael Mosley concludes his series exploring the brutal history of experimental psychology by looking at how experiments on abnormal brains have revealed the workings of the normal brain.

He meets remarkable individuals like Karen, who suffered from a rare condition - alien hand syndrome - which meant that one of her hands constantly attacked her. And Julia, who seems to have recovered from her stroke - until experiments reveal she is unable to recall the name of any object.

Michael explores the case of an amnesiac known for years only by his initials, HM, who became the most studied individual in the history of psychology and whose extraordinary case opened a window on how our memory works. He visits the centre which has been set up to map HM's brain down to the level of a neuron. But are the functions of our brain really as fixed as we think? Michael tries out a device which aims to make us see using our tongue. Show less