So, memorization will change brain structure. Interesting.
I'd also bet that people who for various reasons had to memorize a lot (for example, if you weren't ever taught to read either now or years ago when it wasn't a common skill) also have different brain structure.
"The Knowledge" is not purely about memorization, there is also route planning to consider. Potential cabbies are not merely asked to quote street names and tell you where they are, they are also required to plan routes from point to point given various limitations such as road closures, etc.
It's not clear that the memorization alone would cause such changes in the brain's structure.
In addition to just memorizing facts, they are building a mental structure for all the connections within the city. I wonder if learning the structure of any large system would have the same effect? For example, a large software system, the economic system, or the political system -- where you understand the relationships of all the political players.
Perhaps the political system, or a complex ecosystem with a lot of relationships, but I think software and economic systems are generally much less complex than London. The thing about a city like London is it's pretty much 'incompressible' data. If you want to learn New York, large chunks of it compress by saying things like "the roads are mostly a grid", "these streets are numbered sequentially", etc. London doesn't show that kind of patterned, designed structure that allows other complex systems like software systems to be learned. Imagine trying to learn Unix when anything that would be referenced by a number is referenced by its own name.
Also cabbies need to know traffic patterns for the city to get around it quickly, which means in principle you're learning what the traffic is likely to be like on every road on every day of the week. That's a lot more compressible though.
I would assume that it is simply the amount of thought and focus that is required to memorize thousands of streets that would cause these increases. I wish there was a control or a group of people that were studying something besides memorization to see if there was truly a difference. The time frame here is 3 to 4 years.
Here you go: "Musical training-induced functional reorganization of the adult brain: functional magnetic resonance imaging and transcranial magnetic stimulation study on amateur string players."[1]
I'd also bet that people who for various reasons had to memorize a lot (for example, if you weren't ever taught to read either now or years ago when it wasn't a common skill) also have different brain structure.