Why is the posterior hippocampus larger than normal in London taxi drivers?

Why is the posterior hippocampus larger than normal in London taxi drivers? According to Dr. Eleanor Maquire, Ph.D., professor of cognitive neuroscience at the University College London (UCL), the rigorous and grueling preparation that potential taxi drivers have to complete to pass what is called "the Knowledge", is the reason for these results. These first findings were discovered 20 years ago, and reinforced by a second study that followed drivers for four years as they went through the licensing process. The test is a series of oral examinations to test applicants' capacity to memorize the entirety of London's streets, thoroughfares, and other pathways, without the aid of maps or electronic devices. 

A recent issue of Brain and Life highlights this research and the implications for Alzheimer's research with regards to neuroplasticity, the ability of brains to grow new neural networks. Essentially, the memorization and development of a mental map of their immediate worlds as taxi drivers led to increased brain activity and new neural connections. Because spatial disorientation is common in Alzheimer's patients and has been linked to a shrinking hippocampus, new research employing the latest MRI technologies is trying to pinpoint the areas of specific growth in the brain.

The Taxi Brains Project, headed by Hugo Spiers, PhD, is the current iteration of this research trajectory to "help scientists fight dementia with the help of London's licensed taxi drivers." As Tom Hutley, a participant in the project who passed "the Knowledge" test in 2017 explains, since passing, "Once I've been to a place and come at it from a different direction, a switch goes off in my brain – the map gets filled in."