Maguire et al. (brain scans)
Maguire, E. A., Gadian, N. G., Johnsrude, I. S., Good, C. D., Ashburner, J., Frackowiak, R. S., & Fith, C. D. (2000) Navigation-related structural changes in the hippocampi of taxi drivers.
Maguire et al. were attempting to demonstrate the plasticity of the brain. Plasticity (or neuro-plasticity) refers to changes that occur in the organization of the brain as a result of experience.
The researchers studied the hippocampus of London taxi drivers because they were interested to see if the hippocampus would change because of the taxi drivers high dependence on navigational skills
The hippocampus is a brain structure and humans and other mammals have two hippocampi, a left and right one. The hippocampus is located in the medial temporal lobe and belongs to the limbic system.
The limbic system is the set of brain structures that forms the inner border of the cortex.
There is evidence to suggest that the hippocampus plays major roles in short term memory and spatial navigation.
Maguire et al. note that there is much evidence to suggest that the role of the hippocampi is to facilitate spatial memory in the form of navigation.
For example Maguire et al. refer to evidence that increased hippocampal volume relative to brain and body size has been reported in small mammals and birds who engage in behaviour requiring spatial memory, such as food storing.
Furthermore brain differences between distinct groups of participants such as males and females and musicians and non-musicians have been found.
The authors also note that lesion work and functional neuro-imaging work confirm the involvement of the human hippocampus in spatial memory and navigation.
Interestingly Maguire et al. do refer to evidence that might suggest the plasticity of the hippocampus when they note that in some species of small mammals and birds, hippocampal volumes actually enlarge during seasons when demand for spatial ability is greatest.
Maguire et al. note that from previous studies it is impossible to know whether differences in brain anatomy are predetermined or whether the brain is susceptible to plastic change in response to environmental stimulation.
Maguire et al. set out to discover whether morphological (changes in form and shape) changes could be detected in the healthy human brain associated with extensive experience of spatial navigation. Their prediction was that the hippocampus would be the most likely brain region to show changes.
To test this prediction the researchers decided to study London taxi drivers because they rely heavily on spatial navigation skills in their working lives. London taxi drivers have to undertake extensive training known as ?The Knowledge? and during this time they have to acquire a vast spatial memory of the roads of central London.
The aim of the study was to investigate whether changes could be detected in the brains of London taxi drivers and to further investigate the functions of the hippocampus in spatial memory.
The participants for this study were 16 healthy, right-handed male licensed London taxi drivers. The taxi drivers were compared with the scans of 50 healthy right-handed males who did not drive taxis
The mean age of the taxi drivers was 44 and the range of ages was 32?62 years.
All of the participants had been licensed London taxi drivers for more than one and a half years (mean time as taxi driver was 14.3 years, and the range was 1.5?42 years).
The average time spent training to be a taxi driver before passing the licensing tests fully (i.e., time on ?The Knowledge?) was 2 years (the range was 10 months to 3.5 years). Some of the taxi drivers trained continuously and some part time.
All of the taxi drivers were described as having healthy general medical, neurological, and psychiatric profiles.
The scans of the 50 control participants were selected from the structural MRI scan database at the same unit where the taxi drivers were scanned. Participants below 32 and above 62 years of age were excluded as were females, left-handed males, and those with any health problems.
Both the mean age and the age range did not differ between the taxi driver and control groups. The researchers were also careful to ensure an even spread of participants in each decade (for example, 41?50 years or 51?60 years) up to the upper limit of the oldest taxi driver. Therefore participants were not clustered at one end of the age scale.
The study is an example of a quasi or natural experiment because the researchers are comparing the data of taxi drivers and non-taxi drivers. The researchers have no control of this variable as it is naturally occurring.
The researchers also make use of correlation analysis as they are investigating a relationship between the brain scans and length of time the taxi drivers have been licensed.
The data were collected using structural magnetic resonance imaging (MRI) which collects data about the structure or anatomy of the brain. The MRI scanner works by exposing a participant?s brain to a strong magnetic field and radio waves to produce detailed pictures of the brain.
Computer software converts the information into a 3D image of the brain and in this study the analysis can calculate the amount of grey (or gray) matter in the hippocampus.
Unlike white matter, Grey matter contains neural cell bodies. To use a computer analogy, the gray matter can be thought of as the actual computers themselves, whereas the white matter represents the network cables connecting the computers together
The data were measured using two different techniques ? voxel-based morphemetry (VBM) and pixel counting. .
Voxel-based morphology (VBM) was used in this study to measure the density of grey matter in the brain. VBM provides a three dimensional measurement of volume of an area.
Pixel counting was carried out on the scans of the 16 taxi drivers and 16 age-matched controls taken from the 50 control participants.
Pixel counting consists of counting the pixels in the images provided by the MRI scans. A pixel is simply a two dimensional measurement of an area. Areas were calculated by taking images of slices of the whole length of the hippocampus.
The scan was separated into 26 slices ? 24 of these focused on the hippocampus. Six slices were of the posterior (rear) hippocampus, twelve slices were of the hippocampus body and 6 slices were of the anterior (front) hippocampus.
The pixel counting was carried out by a researcher who was experienced in this technique and importantly was not aware of the previous VBM results and whether the slices being counted were scans of taxi drivers or controls.
The first main findings of the research were that the posterior hippocampi of taxi drivers were significantly larger relative to those of control subjects and that the anterior hippocampal region was larger in control subjects than in taxi drivers.
The second main finding was that hippocampal volume correlated with the amount of time spent as a taxi driver (positively in the right posterior and negatively in the right anterior hippocampus).
The VBM analysis found that significantly increased grey matter volume was found in the brains of taxi drivers compared with those of controls in only two brain regions, namely the right and the left hippocampi. No differences were observed elsewhere in the brain.
In the taxi drivers the increase in hippocampal gray matter volume was focal and limited to the posterior hippocampus bilaterally. In other words the increase was limited to both the left and right posterior hippocampi.
In the controls there was a relatively greater gray matter volume bilaterally in anterior hippocampi relative to those of taxi drivers. That is, the taxi drivers had a decrease of grey matter in both their left and right anterior hippocampi.
The pixel counting revealed that there was no significant differences between taxi drivers and the controls in the overall volume of the hippocampus but did confirm that there were regionally specific differences.
The control group had higher hippocampul volume in the right anterior hippocampus than their left
The control group had higher hippocampul volume in the right anterior hippocampus than taxi drivers.
There was no significant difference in overall hippocampus body between taxi drivers and controls although taxi drivers had higher volume in the left side.
The taxi drivers had a higher volume in their posterior hippocampus than the non-taxi driver control group.
The correlational analysis found that length of time spent as a taxi driver correlated positively with volume in the right posterior hippocampus.
A negative correlation was found between length of time spent as a taxi driver and volume in the right anterior hippocampus.
Length of time as a taxi driver included both training on The Knowledge and time as a qualified taxi driver. The results of the correlations were also supported by the pixel counting.
One taxi driver?s data was not included in the correlational analysis as he had 42 years experience of cab driving and this was an outliner score as the driver with the next highest length of experience was 28 years. However the researchers note that his scores were consistent with the general findings from the taxi drivers.
Maguire et al. argue that this study demonstrates the plasticity of the hippocampus in response to environmental demands.
They argue that the posterior hippocampus stores a spatial representation of the environment and that in the London taxi drivers the volume of the posterior hippocampus expanded because of their dependence on navigation skills.
The authors believe that this study suggest that the changes in hippocampal gray matter?at least on the right?are acquired. This finding indicates the possibility of local plasticity in the structure of the healthy adult human brain as a function of increasing exposure to an environmental stimulus.
Maguire et al. argue that the anterior and posterior hippocampus have different roles. The posterior hippocampus is involved when previously learned spatial information is used, whereas the anterior hippocampal region may be more involved (in combination with the posterior hippocampus) during the encoding of new environmental layouts.
They also suggest that the left and right sides of hippocampus have different functions. They speculate that the right holds mental maps whereas the left hippocampus complements its partner by storing memories of people and events.
In terms of evolution Maguire et al. note the hippocampus is a phylogenetically old part of the brain, with a circuitry that may have evolved to deal with navigation. In humans the functions of the hippocampus have adapted to accommodate other types of memory, such as episodic memory, but the hippocampus still retains an ability to store large-scale spatial information.
The authors note that this study has implications for the rehabilitation of those who have suffered brain injury or disease. If similar environment related plasticity is possible in other regions of the human brain outside of the hippocampus then such people may be helped in the future.
Evaluation of Procedure
It is normally difficult to make conclusions form natural or quasi experiments as any differences found between the two groups (e.g. volume of grey matter) may not have occurred because of the independent variable (taxi driver or non-taxi driver). For example, it could be possible that people with certain brain differences are attracted to certain jobs. However, the researchers carried out a correlational analysis which clearly demonstrated that there was a positive correlation between length of time the participants had been driving taxi cabs and their posterior hippocampul volume. This suggests that length of time taxi driving is associated with grey matter within the hippocampus.
The MRI scans provided the researchers with vast amounts of quantitative data relating to the volume and size of the hippocampus enabling them to carry out statistical analysis such as correlational analysis.
Although the task of being scanned in an MRI scanner is hardly ecologically valid it would not be possible for the participants to respond to demand characteristics.
MRI scanning is of course a costly technique in terms of expensive equipment and researchers? time, although the use of computers does make very sophisticated analysis possible.
MRI scanning technology does not pose any health risks to the participants and all of the participants gave informed consent. None of the participants should have been negatively affected by their experience.
An obvious weakness of the sample is that the participants were all male. There may be difference between males and females in terms of spatial memory. We could apply the same argument to left handed people
The study employed many controls. For example the samples were matched for age, sex and handedness. The pixel counting was carried out by a researcher who was experienced in this technique but ?blind? to the participants? condition.