Are we there yet?

'Are we there yet?' You may say excitedly when you are reaching your favourite playground or hangout. How do we know how far we are from where we want to be? Well, the hippocampus (Greek for seahorse) is a part of the brain that has been known for processing spaces and also in helping us find our way to places.

In this post, we will be learning about how training has an effect on the brain, specifically how training on processing spaces affects the hippocampus, at three different levels of difficulties.

Level 1 (4-9 years old)- Do parts of the brain grow bigger the more we use them?

Do you remember where your pencil case is in the house? How about your favourite stuffed toy? If you are able to find these things in your house, it is because of this small part of the brain known as the hippocampus (and it does more than just helping you find your things too)!

Here is a drawing of the hippocampus (in black) with the rest of the brain!

Taxi drivers in your town have a hard time finding the fastest way for you to get to where you need to go. For example, in the picture below, which is the fastest path from the home to where you want to go?

A taxi driver would be able to find the fastest way to get from your home (in purple) to where you need to be (in green) that you can see in red. You can see that there are other ways for the taxi driver to move from your home to where you want to go in blue but it may take you much longer. This means that the driver needs to know all the roads to find the fastest way.

Bus drivers find it easier because there is this one way when they drive the bus. The fixed path would have bus stops along the way for people, like you and I, to get on and off.

In a few studies, the researchers wanted to see if the size of the hippocampus will change in taxi drivers the more they drive around the city. The researchers used a machine to measure the size of the hippocampus in taxi drivers and bus drivers. The results showed that taxi drivers had a bigger hippocampus compared to bus drivers. The size of the hippocampus in taxi drivers was also bigger the more time they spent driving on the roads!

This means that the more experience a taxi driver has on the road, the part of the brain that processes space also increases.

Level 2 (10-14 years old)- Is there a correlation between driving experience as a taxi driver and size of the hippocampus?

Are you able to picture the layout of your house? Can you gauge how many steps it takes to get from the whiteboard to your desk in school? This is all thanks to a part of the brain known as the hippocampus (which does a lot of things other than processing space)!

Taxi drivers would require more processing of spaces compared to bus drivers as they would need to find the fastest way for you to get to your destination. As there are many different combinations in our starting point and ending point, the taxi driver needs to be able to flexibly use his/her knowledge of the roads to get us to our destination quickly. On the other hand, bus drivers have a set route that they have to take with the starting point at a bus interchange and the end point either at another bus interchange or the same interchange (with of course a few bus stops between them). Thus, the bus driver does not require as much effort to remember all the roads in the city, just the ones that he/she would need to drive on. It was hypothesised (theorised) that taxi drivers would have a bigger hippocampus compared to bus drivers (with the same years of experience controlled for).

In a few studies, the researchers wanted to investigate whether the hippocampus in taxi drivers differed in size compared to bus drivers. The amount of experience driving on the roads in London was also matched between taxi drivers and bus drivers (this means that the researchers are reducing the possibility that the differences in the amount of time spent on the road would affect the size of the hippocampus).

Independent variable (factor that the researcher is changing): Taxi drivers and Bus drivers

Dependent variable (factor that the researcher is measuring): Size of the hippocampus

The hippocampus of the taxi drivers and bus drivers was measured using a machine called the MRI (Magnetic Resonance Imaging).

The results of the study show that taxi drivers had a bigger hippocampus compared to bus drivers. Moreover, there was a positive correlation between the number of years spent as a taxi driver and the size of the hippocampus (the more years spent as a taxi driver, the bigger the size of the hippocampus).

Level 3 (15 years old and above)- Does the structure of the hippocampus change based on experience as a taxi driver?

The hippocampus is a part of the brain involved in spatial memory (where objects are in space; for example, knowing exactly where your school shoes are in the house). Spatial memory includes, and is not limited to, giving directions, locating objects in a different room (eg. I know that my pencil case is in my bag) and navigating roads.

There are two types of matter in the hippocampus, the white matter and grey matter. The grey matter in the hippocampus contains a lot of the neuronal cell bodies (which are basic units in the brain that sends information to other cells) while the white matter allows information to be sent more quickly in the brain.

The hippocampus can also be divided into two parts: the anterior (nearer the front) and posterior (nearer the back).

In London, taxi drivers would have to pass a test, known as the Knowledge, which measures the drivers ability to navigate the roads of London to find the fastest route from the starting point to a destination. Thus, taxi drivers would have to memorise all the roads in central London to be able to identify this ideal route. Bus drivers, on the other hand, have a fixed route that they drive on and so although they would need to remember the route, it is less taxing on the hippocampus to have a representation of the roads in London that they will be travelling on. It was hypothesised that there would be gray matter differences, that is due to expertise in navigating roads, in taxi drivers and bus drivers.

The papers investigated whether the structure of the hippocampus changes based on experience as a taxi driver. An MRI (Magnetic Resonance Imaging) machine was used to measure the structural differences of the hippocampus in taxi drivers and bus drivers. The MRI machine works by passing radiowaves through the skull with a computer capturing the detailed images of the structures in the brain. The amount of experience between drivers was matched to ensure that it was not a confounding variable (extra variable that may have an effect on the dependent variable).

Two-Tailed Hypothesis (the change in hippocampus may be that the gray matter may increase and the white matter may decrease or vice-versa): The structure of the hippocampus would change based on experience navigating the roads of London

Independent variable: Taxi drivers and bus drivers

Dependent variable: Structural changes in gray and white matter of the hippocampus

The results of the study showed that only taxi drivers had a change in the volume of gray and white matter in the hippocampus [more specifically, there was greater gray matter volume in the mid-posterior hippocampi (in the middle of the posterior hippocampi; ‘hippocampi’ is the plural form of hippocampus) and less volume of the anterior hippocampi].

There was also a positive correlation between navigational experience and the volume of gray matter in the posterior hippocampus on the right side of the brain. The researchers thus suggested that the posterior hippocampus may be involved in storing spatial representations of the environment and that this area of the brain can expand to accommodate for more detailed representation for people who depend highly on such navigational skills, like taxi drivers.

In conclusion, this raises the question about the extent of plasticity in the brain (the brain’s ability to undergo biological/structural changes based on experience) and whether such neuroplasticity (structural changes in the brain) differs in childhood and adulthood.

References:

MacKenzie, R. J. (2019, August 30). Gray Matter vs White Matter. Retrieved July 23, 2020, from https://www.technologynetworks.com/neuroscience/articles/gray-matter-vs-white-matter-322973

Maguire, E. A., Gadian, D. G., Johnsrude, I. S., Good, C. D., Ashburner, J., Frackowiak, R. S., & Frith, C. D. (2000). Navigation-related structural change in the hippocampi of taxi drivers. Proceedings of the National Academy of Sciences, 97(8), 4398-4403.

Maguire, E. A., Woollett, K., & Spiers, H. J. (2006). London taxi drivers and bus drivers: a structural MRI and neuropsychological analysis. Hippocampus, 16(12), 1091-1101.

MRI Scans: Definition, uses, and procedure. (n.d.). Retrieved July 23, 2020, from https://www.medicalnewstoday.com/articles/146309