Maguire et al. (2000)

Background

Previous case studies have shown that specific parts of the brain are associated with memory. The most significant case was the case of HM. HM was a 27-year-old man who under when experimental brain surgery in the 1950s to treat his severe epilepsy. The surgery involved removing parts of his brain, including the hippocampus. HM survived the operation and his epilepsy became more manageable, however he had profound amnesia and ended up with a memory span of just a few minutes. This case highlighted a clear link between the hippocampus and memory. 

The hippocampus (the latin name for seahorse) is a brain structure which exists in each hemisphere of the brain (the left and right). Maguire et al. wanted to investigate whether  the hippocampus was responsible for spatial memory (navigation skills).

parts-of-the-brain-hippocampus

Maguire et al. also wanted to demonstrate neuroplasticity: the ability for specific structures within the brain to change and adapt to meet the needs of specific environments. Previous research has shown that, in small mammals, hippocampal volume increases during seasons when demand for spatial memory is at its greatest.

To test their hypothesis, Maguire et al. investigated the hippocampal volume (i.e. size) of London taxi drivers to determine whether environmental demands placed on them by their job, enhanced the volume of their hippocampus and thus their navigation skills.

In this study, brain scanning techniques were used to analyse the hippocampal volume of participants. This was done through the use of MRI scans. MRI scans produce an extremely detailed 3D still image of the brain by rotating magnetic fields around the head. Other types of brain scans include the CAT scan and the PET scan.

London Taxi driver must take a test called “The Knowledge” before they can get the licence to become a taxi driver. This test requires them to have a detailed knowledge of  25,000 streets within a six-mile radius of Charing Cross as well as a more general knowledge of the major routes throughout the rest of London.

Aim

The aim was to find out whether changes in the brain, as a response to learning (or neuroplasticity), could be detected in those with extensive navigation experience.

Hypothesis

  1. The hippocampus in London Taxi drivers will be structurally different to the hippocampus in non-taxi drivers
  2. Hippocampal changes will increase with time spent as a taxi driver and thus are likely to be caused by the environment

Method & Design

Independent variable: Whether the participant was a taxi driver or not

Dependent variable: Structure and volume of different regions of the hippocampus

A quasi-experiment was used, as the independent variable was naturally occurring and thus the researchers had no control over it. Independent Measures Design with participants naturally falling into one of two groups.

A correlational analysis was also undertaken between time spent as a taxi driver (variable 1) and hippocampal volume of specific areas (variable 2)

Sample

The sample consisted of two groups of participants: taxi drivers and a control group.

The taxi drivers included16 right-handed London taxi drivers who were all male, aged between 32-62 years (mean age = 44) with healthy medical profiles. Participants had been licensed for at least 1.5 years. Time spent as taxi driver ranged from 1.5 – 42 years (mean time as a taxi driver = 14.3 years). All taxi drivers had passed “the knowledge” test to achieve their license.

The control group included 50 non-taxi drivers matched for handedness, health, gender and age. This group had no extreme navigational experience and were chosen from the MRI database at the Welcome Department of Cognitive Neurology.

Procedure

London taxi drivers brains and the control group had their brains scanned using an MRI scanner to look at the anatomy of the brain. These scans were analysed and compared.

The measurement of the brain differences used two techniques: VBM (Voxel-based Morphometry) and Pixel counting

VBM was used on the 16 taxi drivers and all 50 controls. VBM is a 3D measurement of volume. It allows a computer to calculate the grey matter volume/density and general brain differences between the taxi drivers and non-taxi drivers in structural MRI scans. This technique produces pictorial representation and quantitative data.

Pixel counting was used on the 16 taxi drivers and 15 identically matched age controls from the sample of 50 controls. A pixel is a 2D slice. The pixels were counted in photographic slices from the scans of the hippocampus. The technician performing the pixel analysis did not know who were the controls and who were the taxi drivers, therefore a ‘blind’ design was used in order to reduce any experimenter bias. The technician was also blind to the VBM results.

Results

The VBM found… significantly increased grey matter volume  in the brains of taxi drivers compared to those of non-taxi drivers in 2 brain regions, namely the right and left hippocampi. No differences were observed elsewhere in the brain. In the taxi drivers the increased grey matter volume was found in the right and left posterior hippocampi whereas in the non-taxi drivers, there was relatively more grey matter volume in the right and left anterior hippocampi.

The Pixel Count found… that non-taxi drivers had greater anterior right hippocampal volume than taxi drivers. The body of the hippocampus was larger on the right than the left in the brains of non-taxi drivers compared to those of taxi drivers. Taxi drivers had a greater posterior hippocampal volume than non-taxi drivers.

The correlational analysis found… a positive correlation between time spent as a taxi driver and the volume of the right posterior hippocampus and a negative correlation between length of time as a taxi driver and the volume of the anterior right hippocampus.

Conclusion

The results  indicate a relationship between navigational skills and the distribution of grey matter in the hippocampus.

This supports the idea that the brain has changed in response to the demands of being a taxi driver, not a pre-existing structure that predisposes a person to become taxi driver.

The fact that the hippocampal volume correlates positively with the length of time as a taxi driver suggests that the hippocampal changes occur in line with the demands on spatial memory made by their job. This suggests that the human brain has plasticity, i.e. the ability to change to meet the demands of the environment.

Controls in the study: The expert conducting the analysis did not know whether the MRI scan was of a taxi driver brain or not, all the participants were male, right handed, aged between 32-62, from London and were healthy, a control group was used, all participants were scanned using the same MRI scanner.

Evaluation

Strengths

+ VBM and pixel counting provide quantitative data which allows easy comparison between the taxi drivers and the control group through correlation and statistical analysis.

+ The use of highly technical equipment (MRI/VBM) allows for precise measurement of data, which cannot be falsified by the participant, and thus the procedure is more reliable as easy to repeat.

+ High validity as high control was used thus extraneous variables, such as researcher bias were controlled for. Also two different measures were used to establish differences in hippocampal volume (VBM and pixel counting) plus correlation analysis to support findings.

+ The research was ethically sound and conducted within BPS guidelines, therefore no harm was caused to participants.

+ Quasi-experiments have high ecological validity, since a natural change (one not induced by the experiment) occurs in a natural environment, thus findings contain very little bias or demand characteristics

Weaknesses

– The structural MRI scans do not reveal exactly what is happening in the hippocampus that leads to changes, but that structural changes in hippocampi of taxi drivers has occurred, thus validity may be an issue.

– Brain scanning techniques are subject to human error, and as only one technician analysed the scans, the ability to replicate the study and produce consistent findings is decreased

– There may have been individual differences in the plasticity of the hippocampus within individuals, this was not taken into account.

– An unrepresentative sample was used which consisted of only males and right-handed individuals, therefore we cannot generalise the findings to the general population as there may be structural differences within the hippocampus of females and left-handed individuals.

– Ethnocentric sample used as the experiment was carried out on London taxi drivers by Western psychologists.

– Correlations do not allow for cause and effect therefore we cannot know for sure whether the taxi drivers had increased volume due to the environment or whether they already had an enlarged hippocampus thus leading them to a job where they can put their skills into place.

– Quasi-experiments tend to have little control over extraneous variables therefore they are difficult to replicate exactly

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