Module 2

Lesson 2

by Dr. Irena O’Brien, PhD

Neuroplasticity

Let’s now go back to the London taxi drivers and look at it in more detail.

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Learning “The Knowledge”

takes 3 - 4 years

As I mentioned in lesson 1, this research is foundational in the area of neuroplasticity.

In order to qualify as a licensed London taxi driver, a trainee must learn the complex and irregular layout of London’s ~ 25,000 streets within a 6 mile radius of Charing Cross train station, and the locations of thousands of places of interest. This learning is known at “The Knowledge” and typically takes between 3 and 4 years, followed by stringent examinations which must be passed to obtain a license. These comprehensive training and qualification procedures are unique among taxi drivers anywhere in the world.

The following is one of the studies on the London taxi drivers carried out over many years by Eleanor Maguire from University College, London. The article is open access and I’ve included a link to it in the additional resources.

This particular study comprised 39 trainees who went on to qualify as licensed London taxi drivers, 20 trainees did not qualify, either because they ceased training or they failed their exams, and 31 control participants who had never driven a taxi. All participants were given a battery of memory, reasoning, and IQ tests and they were scanned at the beginning of the study - we call that Time 1) and three years later at Time 2. At the beginning of the study, none of the trainees had ever driven as mini-cab drivers. And there was no difference between the groups on any of the memory, reasoning, or IQ tests at Time 1. In other words, all three groups were equated.

Woollett, K. & Maguire, E. A. (2011). Acquiring "the Knowledge" of London's layout drives structural brain changes. Current Biology, (21) 2109-2114.

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Qualified trainees

have larger hippocampi

In the qualified trainees, the researchers found that gray matter volume in the posterior hippocampi bilaterally had increased between T1 and T2. For the trainees who did not qualify and for the control group, there was no difference in their hippocampi between T1 and T2.

This image shows different views of the brain and changes in hippocampi volume for the qualified trainees. In the mri scanner, images are taken in slices and the researcher can specify the direction of the slices, and the distance between the slices. The top panel, here , is an axial view, or horizontal view. The blue lines show where the slices were taken from and to the left are the slices themselves. The other two panels show sagittal, or vertical slices, taken from the right and left hemispheres. This image is for the qualified trainees only. You can see here that they showed an increase in gray matter volume in the posterior, meaning further back, hippocampi, bilaterally. The reason why the activations look so small, in addition to these being small images, is that they show only the change in hippocampi volume between T1 and T2. What the researchers did was subtract the T1 volume from the T2 volume, and what you see here is only the difference in volume between the two, not the entire volume. The hippocampi volume for the non-qualified trainees and the control group did not differ between T1 and T2.

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Qualified trainees

have larger hippocampi

This slide shows one of the voxels in the right hippocampus of the three groups. You can see here that only the qualified trainees showed a significant increase in posterior hippocampal gray matter between T1 and T2. A voxel is, in effect, a 3 dimensional pixel and again, when doing an mri scan, the researchers can choose the size of the voxel.

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Here’s an illustration of the hippocampus that clearly shows where the hippocampus is located, which you were not able to do from the earlier slide. The part within the yellow circle is the posterior hippocampus, the area that increased in volume for the qualified trainees. And recall that the increase was bilateral, in both the left and right hemispheres.

And the increased posterior hippocampal gray matter volume most likely occurred as a result of acquiring the detailed spatial representation of London’s layout. It cannot be attributed to the training procedures because all trainees, even those who failed to qualify, were exposed to the same training conditions.

These results indicate the close relationship between the hippocampus and spatial navigation and suggests that the hippocampus acts as a storage site for the spatial information acquired during “the Knowledge,” or as a processing hub for detailed navigation information.

In a separate study, they also found that hippocampal gray matter volume correlated with the increasing experience of the drivers, meaning that the drivers with a greater number of years of experience showed greater hippocampal gray matter volume.

The next question is what mechanism underpins this process?

Image by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body. Bartleby.com: Gray's Anatomy, Plate 739

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•Neurogenesis•Synaptogenesis•Dendritic arborization•Glial cells

When using structural MRI scanning in humans, it’s not possible to address the mechanism which underpins the growth in hippocampal gray matter. To address this question, we need to look at animal studies.

Research with rodents has shown that when learning requires cognitive effort and where learning actually takes place (meaning where material is remembered after a delay), there is an effect on the rate of hippocampal neurogenesis. Furthermore, the animals that learn best have more new neurons than those who do not learn, or do not learn efficiently. Neurogenesis means the growth of new neurons and the hippocampus is one of the only areas in the brain where neurogenesis occurs in adults.

The development of greater communication between neurons in the form of increased synaptogenesis might also be involved. As you may guess, synaptogenesis means the growth of new synapses.

Proliferation in dendritic arborization meaning, more dendritic branching, enhances communication between neurons, which could in turn increase memory capacity and also lead to changes in hippocampal volume.

Glial cells, which continue to be produced throughout adulthood, could also be implicated and have been shown to increase in volume with the addition of synapses following learning. Glial cells provide support and insulation to neurons.

So, all of the above may lead to increased hippocampal volume.

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• Structural changes in the human brain into adulthood

• Provides encouragement for lifelong learning

• Neurorehabilitation

• Memory improvement in one domain may be at the expense of memory performance elsewhere

• May be genetic limitations to memory performance and hippocampal volume

The results from the London taxi drivers research shows that there is a capacity for memory improvement and related structural changes to occur in the human brain well into adulthood.

These results offer encouragement for lifelong learning, and possibly a role in neurorehabilitation

But, memory improvement may come at the expense of memory performance elsewhere. One thing that the researchers also found was that there was poorer learning and memory for complex figures at T2 by the qualified trainees but not for the non-qualified trainees or the controls.

This study did not rule out that there may be a genetic limitation to memory performance and hippocampal volume. It’s possible that hippocampal plasticity may be expressed only in certain individuals. The trainees that qualified may have had a genetic predisposition toward plasticity that the non qualified individuals lacked. It may be that the non qualified trainees stopped training or failed their exams because they lacked a genetic predisposition to plasticity in the hippocampus.

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London Taxi Drivers Studies

• Foundational

• Programme of research

• Excellent study design

• Equated on nuisance variables

I’ve spent a lot of time on this study for a number of reasons:

1. The research by Eleanor Maguire is foundational in the area of human neuroplasticity. 2. This study is part of program of research. A single finding is not enough. It needs to be part of a program of research that produces consistent results.

And/or it needs to be replicated. 3. This was an excellent study design for her question. Her question was whether learning caused structural changes in the hippocampus. If she had just

scanned the participants one time and showed that the qualified trainees had larger hippocampal volume, that would not have shown that it was due to learning. They may have had larger hippocampi before they even started learning “the Knowledge.” By scanning them at the start of their training and again at the end of the study, she showed that the growth in hippocampal volume was due to learning. She also included a group of trainees that did not qualify, and a matched control group. This is important for comparison purposes. If she had not included these groups, it would have been impossible to conclude that the hippocampal growth was due to what was different between the groups: that the qualified trainees had studied “the Knowledge” for 3+ years and passed their exam.

This is one of the few studies that has used a longitudinal design like this, where they follow participants over a long course of study and practice from the start. This is the only way that we can conclude that the growth in hippocampal volume was due to learning.

4. She also equated them on demographics, intelligence, memory, and reasoning variables. By equating them at T1, she showed that the growth in hippocampal volume was not due to any of these other factors.

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Neuroplastic change

can be reversed

Plasticity may go both ways. If a skill is no longer practiced, there may be a decrease in gray matter volume. This is exactly what Woollett and Maguire found: London taxi drivers who had been retired for about 3 years showed worsened London navigation skills and less posterior hippocampal gray matter than currently employed taxi drivers. This study was preliminary in that the number of participants was very small (8 in each group) and they were scanned at one time only. So an unanswered question is whether their hippocampal gray matter had decreased since retirement, or did this group have smaller hippocampi to begin with. Chickadees, for example, experience seasonal volume changes in hippocampal gray matter where increases in volume occur during the autumn/winter months when demands on spatial memory are also increased due to migration. So plasticity may be maintained only when necessary for adaptational purposes arising from demands placed by interactions between the animal and the environment. It may be the same for the retired taxi drivers: their hippocampal volume decreased when they no longer needed to remember London streets and landmarks.

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Longitudinal design

to determine change

I’ve described the London taxi drivers study in detail because it forms part of a foundational program of research. But many other domains have been studied and found associations between expertise and neuroanatomy. Some of the domains studied have been musicians, jugglers, dancers, professional simultaneous interpreters, painters, chess players, professional car divers, karate experts, perfumers, and athletes. However, few of the studies were longitudinal in design and we’ve seen that the only way to determine whether the training caused the brain change is to have a longitudinal design.

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© 2016 The Neuroscience School

Increased gray matter

after 3 months of juggling

The next two studies did have longitudinal designs. Draganski and colleagues asked participants to practice juggling. After 3 months of practice, the researchers observed an increase in gray matter in the temporal cortex and parietal cortex compared to a group of non jugglers, and the amount of gray matter was related to juggling expertise. They also scanned participants 3 months after the jugglers stopped practiced juggling and the grey matter in these areas had decreased. We saw this with the London taxi drivers, where their hippocampal gray matter was correlated with their experience. And how the hippocampal volume of retired taxi drivers was smaller than that of current drivers.

Hyde and colleagues compared a group of children with no musical training to a group that was just starting musical training. After 15 months of musical training, the researchers found an increase in frontal and corpus callosum volume, compared to children with no musical training. At T1, there were no group differences in brain volume.

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•Goal•Effortful•Repetition•Reversal

Conditions for Neuroplasticity

What can we learn from this research?

• There needs to be a goal: In the studies I’ve presented, all of the participants had a goal, to pass “the Knowledge,” learn how to juggle, learn how to play music, etc.

• The learning needs to be effortful: The learning did not happen by accident. The participants put in great effort to learn “the Knowledge,” learn to play music, juggle, learn to play chess, become dancers, etc.

• It needs to be repeated over a period of time: Again, in all the research I presented or mentioned, structural brain changes occurred after the participants trained over a period of time. For the London taxi drivers, it was over 3 years or more. For the jugglers, it was only 3 months. The amount of time depends on the difficulty of the domain. Learning “the Knowledge” is infinitely more difficult than learning how to juggle.

• The brain changes may be reversed if training is stopped. We saw that with the retired taxi drivers, the chickadees, and the jugglers.

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