Navigators from the Planet Hippocampus

Hackney cab sign, London

If you’re a regular user of London taxis, you may have come across something called The Knowledge. A significant part of the world’s most exacting taxi qualification examinations, it tests drivers’ navigational knowledge of London’s streets and points of interest.

Not easy. London’s a labyrinth, 6 square miles of navigational hell. The Blue Book, or Guide to the Knowledge of London, contains all of London’s 25,000 streets, plus ‘points of interest’. It takes three years – an average of 12 attempts – to master the Blue Book. The dropout rate is enormous. It’s like learning the bible. Properly knowing it is a serious feat of memory, so serious, in fact, that it involves physically growing parts of the brain. No joke.

Our understanding of the function of memory is today in a good place. Old one-stop-memory shop models are out. Studies of damaged brains, aided by the rise and rise of magnetic resonance imaging technology, show how different parts of the human brain have evolved systems specific to types of memory. Many of these studies took place on one brain. It belonged to Henry Gustav Molaison. Molaison was an American epileptic whose correctional surgery – he was operated on in 1957 – included the removal of nearly all the hippocampus, an area located above both temples. The operation resulted in anterograde amnesia: Molaison was unable to commit new events and meaning to long term memory. He died in 2008.

Molaison’s case is useful to our understanding of the brains of London cabbies. The Blue Book works the hippocampus hard. This area of the brain – as Molaison’s surgery indicated – is largely responsible for what in the business is known as the declarative memory system. An example of a declarative memory is remembering that you went for a drive today (where you went, how long it took etc) – as opposed to knowing how to drive, which is a procedural memory function. Given, therefore, that a significant part of the training for The Knowledge consists of (motor)biking around London, committing tens of thousands of (brain manufactured)experiences to memory, it will come as no surprise (perhaps) to learn that a study in 1999 found that Blue Book brains have larger posterior hippocampi than the rest of us.

The posterior? Yes. It seems that this part of the hippocampus is responsible for storing a specific subset of the declarative memory system, one that is ‘a spatial representation of the environment.’ It maps space.  It’s our GPS. Logical, then, that people who rely heavily on navigational skills have bigger ones.

MRI showing growth (left to right) of the hippocampus in taxi drivers - image courtesy of Elenour Maguire, Wellcome Department of Cognitive Neurology, University College London

And that’s less than half the story. The study also showed that Blue Book brains have correspondingly smaller anterior hippocampi than the rest of us. Subsequent experiments – conducted by Hugo Spiers – suggest that this area keeps an eye on the straight line distance between you and your destination. In the same way that your Sat Nav goes bonkers once you go wrong – and especially if you go wrong several times – so ‘the activity in the front end of the hippocampus ramps up and up’ the further you get from your destination. That’s the rest of us. We get lost. Travel twice the distance. Blue Book brains – unless there’s traffic, or someone’s pulling a fast one – take the most direct route. Ergo, the Blue Book anterior hippocampus is smaller.

More. In the same way that neuroscientists discovered that aspects of Henry Molaison’s spatial memory remained intact post-surgery (he could draw a detailed floor plan of his house), and fairly surmised therefore that spatial representation is not solely the remit of the hippocampus, so the London taxi driver’s navigational brain operates from different localities. For example, a study using a bastardised version of Sony’s The Getaway – a simulated experience of Blue Book brain driving through London – shows that, once planned, the journey hardly uses the hippocampus. It goes into cruise control. Unexpected events are handled by the right prefrontal cortex; the expected – landmarks etc. – by the retrospenial cortex. The hippocampus only kicks into gear once there is a change in destination.

All of which means that next time you get into a London cab, know – please – that you are dealing with a superior life force. A brain grower. You are.

MRI scan of brain