Core Studies
Navigation-related structural change in the hippocampi of taxi drivers
Maguire et al (2000)
Citation:
Maguire, E. A., Gadian, D. G., Johnsrude, I. S., Good, C. D., Ashburner, J., Frackowiak, R.S. and Frith, C. D. (2000). 'Navigation-related structural changes in the hippocampi of taxi drivers.' Proceedings of the National Academy of Science, USA. 97. p4398–403.
Exam Board / Level:
OCR AS
Short Summary
This study investigated the differences between the hippocampi of taxi drivers and a control group of non-taxi drivers. This was to see whether the hippocampi, which are linked to navigation and spatial awareness, were different in London taxi drivers. The researchers used two different analysis techniques to measure the volume of the left / right anterior and posterior hippocampi in 16 London taxi drivers, all male, aged between 32 and 62. The results showed that the taxi drivers had a larger posterior hippocampus, whereas the control group had a larger anterior hippocampus. There was also a correlation between the size of the right posterior hippocampi and length of time spent as a taxi driver. The results suggest that the volume of the hippocampus changes relating to environmental stimuli, indicating that this region of the brain has plasticity and differences are acquired.
Long Summary
Abstract
Maguire et al used Structural Magnetic Resonance Imaging (MRI) to compare the brains of licensed London taxi drivers to a control group who did not drive taxis. The results showed that there was a significant difference in the size various parts of the hippocampi of taxi drivers: the posterior hippocampi were larger in taxi drivers, whereas the anterior hippocampi was larger in control subjects. The volume of the hippocampi also correlated with how long the subject had been a taxi driver.
This evidence supports the theory that the posterior hippocampi store a spatial representation of the environment and is ‘plastic’, responding to the individuals’s needs in response to their environment.
Background
Previous research has demonstrated that in small mamals and birds who require navigation skills (for eg. To find to stored food) the hippocampi volume (in relation to body size) is larger and in some species even changes in volume during different seasons.
In healthy humans studies have shown individual differences in hippocampal volume related to gender, musicianship etc. but have not demonstrated whether these differences are innate or determined by environmental stimulation.
Functional MRI scans indicate that the hippocampi are heavily involved in navigational tasks and spatial memory, although its precise role is unknown.
Therefore Magure et al’s study aimed to investigate whether the environment related changes to brain structure seen in smaller mammals and birds are also found in humans and they hypothesised that the hippocampi would be the most likely area of the brain to show these changes.
In order to investigate this, the researchers recruited London Taxi drivers who must go through extensive training to learn “The Knowledge” (about 2 years) and must also pass a set of police tests. Drivers of varying levels of experience allowed them to investigate the effect of this experinece on brain structure.
The researchers used VBM – voxel based morphometry, which produces grey-scale images of the whole brain, showing which areas of the brain are densest – this allows for an overall view that is not biased to one particular area. They also counted the pixels of the image of the hippocampi, then compared the brain scans of taxi drivers to a comparison group who did not have extensive navigation skills.
Methodology
Subjects
- 16 right handed, male, licensed London taxi drivers, mean age 44 yrs, range 32-62 yrs who had all been taxi drivers for at least 1.5 yrs (mean 14.3 yrs, range 1.5-42 yrs). All had good general health.
- Control group consisted of 50 scans taken from the MRI database where the taxi drivers were scanned, after excluding females, left-handed males, those not aged 32-62 and those with any health problems. The mean age and age range were the same as for the taxi drivers and the ages were also spread across the whole range.
Measurement and Analysis
Data was collected using images from Structural MRI scans which create a three-dimensional sequence of 108 images (slices of 1.5 mm).
VBM analysis was based on voxels (3D pixels) of 1.5X1.5X1.5 mm. Each image was standardised to a template image to adjust for differences in overall brain size. The template was based on 50 healthy males scanned on the same scanner (13 of which were in the control group). The images were then separated into gray matter, white matter and cerebrospinal fluid, then the gray matter images were smoothed to reduce the confounding effect of differences in brain anatomy. The gray matter of the taxi drivers was then compared to the control group based on a standard definition of the different hippocampal regions. Differences were analysed statistically using a t test and linear model (p<0.05).
Pixel Counting also used the standard definition of the different hippocampal regions and the images of the taxi drivers were compared to 16 of the control group of exactly the same ages. 26 slices for each hippocampus (left and right) of each subject were analysed by a person experienced in using the MRI technique who did not know whether the images were from the experimental or control group. The 2 most posterior slices were excluded due to variance, leaving 24 slices: 6 posterior, 12 body and 6 anterior. The total volume for each area of the hippocampi was calculated by counting the pixels in each slice and then multiplying this by the depth of the slice (1.5mm). The data was corrected for differences in ICV (intracranial volume – overall brain size).
Changes with Navigational Experience was measured by correlating the length of time in months spent as a taxi driver against the amount of gray matter measured in both techniques.
Results
VBM
Significantly greater volume of gray matter was found in only one brain area in the taxi drivers: the left and right hippocampi. There were no other observed differences anywhere else in the brain.
The posterior hippocampus had more gray matter bilaterally (both sides) in taxi drivers than in controls.
The anterior hippocampus had more gray matter bilaterally in the controls than in the taxi drivers.
Pixel Counting
Overall hippocampus volume did not differ significantly between the two groups (this was true for both the corrected and uncorrected data).
However, there were significant differences in the volume of specific areas of the hippocampi. Analysis of Variance (ANOVA) demonstrated that there were differences related to the groups (taxi drivers or controls) and also to the side of the brain:
Changes with Navigational Experience
A positive correlation was found between the VBM measured volume of the right posterior hippocampus and length of time spent as a taxi driver (r=0.6, p<0.05).
This correlation is supported by a positive correlation between pixel counting measurement of the same area and time spent as a taxi driver (r=0.5, p<0.05).
A negative correlation was found between the volume of the anterior hippocampus and time spent as a taxi driver (r=-0.6, p<0.05). This result was also the same when corrected for age (by using the percentage of their age that they had spent as a taxi driver).
The data from the oldest subject was not included in the correlations as he had been a taxi driver for 42 years, and the next longest serving taxi driver had been one for 28 years. However, the data for this subject was in line with the correlations found.
Discussion
The results of the two techniques show a significant difference in the gray matter of the hippocampi of taxi drivers compared to controls who do not have extensive need for navigational skills. Specifically, taxi drivers have more gray matter in the posterior hippocampi, whereas the control group had more gray matter in the anterior hippocampus. These results show that the gray matter is distributed differently across the hippocampi for both groups, as overall the same amount of gray matter was found across the whole of both left and right hippocampi.
To assess whether these differences are innate and indicate that some individuals may be neurologically predisposed towards becoming a taxi driver, the researchers also used correlations, which demonstrated that the longer the individual had been a taxi driver, the greater the volume of their right posterior hippocampus and the smaller the volume of their right anterior hippocampus. This indicates that these changes in the brain are acquired, indicating that there is ‘local plasticity’ in this region of the brain that is related to environmental stimuli.
Previous studies have shown that the posterior hippocampus of rodents and monkeys is involved in spatial navigation and also in human studies of retrieving navigational information. In patients with hippocampal lesions results indicate that routes learned before the lesion are left in tact, although further evidence would be required in order to assess whether this means that the hippocampus stores cognitive maps. However, the findings of Maguire et al’s study suggest that the posterior hippocampus is involved in navigation based on previously learned spatial information, whereas the anterior hippocampus may be more involved in the learning of new spatial information.
The findings of both measurement techniques show that both the left and right hippocampi are involved in navigation, although the left hippocampus may be involved in storing memories of people and events that are related to the spatial information but do not depend on a full spatial map. This suggestion by Maguire et al is based on the increase in volume of the right posterior hippocampus over time, indicating that it increases in volume to accommodate new information. The human hippocampus has also evolved to store episodic memories, but this area is part of the old brain and is found in all species of mammals and birds.
The authors highlight that their study can not show exactly how these changes occur within the brain, but point out that the findings of this study have positive implications for those who have suffered brain injury or disease. Further research is required to find out if the plasticity found in the hippocampus is also found in other areas of the brain.
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