Navigation-related structural change in the
hippocampi of taxi drivers
Eleanor A. Maguire*
, David G. Gadian
, Ingrid S. Johnsrude
, Catriona D. Good
, John Ashburner
,
Richard S. J. Frackowiak
, and Christopher D. Frith
Wellcome Department of Cognitive Neurology, Institute of Neurology, University College London, Queen Square, London WC1N 3BG, United Kingdom;
and Radiology and Physics Unit, Institute of Child Health, University College London, London WC1N 1EH, United Kingdom
Communicated by Brenda Milner, McGill University, Montreal, Canada, January 28, 2000 (received for review November 10, 1999)
Structural MRIs of the brains of humans with extensive navigation
experience, licensed London taxi drivers, were analyzed and compared
with those of control subjects who did not drive taxis. The
posterior hippocampi of taxi drivers were significantly larger
relative to those of control subjects. A more anterior hippocampal
region was larger in control subjects than in taxi drivers. Hippocampal
volume correlated with the amount of time spent as a taxi
driver (positively in the posterior and negatively in the anterior
hippocampus). These data are in accordance with the idea that the
posterior hippocampus stores a spatial representation of the environment
and can expand regionally to accommodate elaboration
of this representation in people with a high dependence on
navigational skills. It seems that there is a capacity for local plastic
change in the structure of the healthy adult human brain in
response to environmental demands.
One important role of the hippocampus is to facilitate spatial
memory in the form of navigation (1). Increased hippocampal
volume relative to brain and body size has been reported in
small mammals and birds who engage in behavior requiring
spatial memory, such as food storing (2). In some species,
hippocampal volumes enlarge specifically during seasons when
demand for spatial ability is greatest (2, 3). In the healthy human,
structural brain differences between distinct groups of subjects
(for example, males and females, ref. 4, or musicians and
nonmusicians, ref. 5) have been documented. From existing
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.
Furthermore, although lesion work (6, 7) and functional neuroimaging
work (8) confirm the involvement of the human hippocampus
in spatial memory and navigation, there is still debate
about its precise role. Given the propensity of lower mammalian
avian hippocampi to undergo structural change in response
to behavior requiring spatial memory (2, 3), the present study
addressed whether morphological changes could be detected in
the healthy human brain associated with extensive experience of
spatial navigation. Our prediction was that the hippocampus
would be the most likely brain region to show changes.
Taxi drivers in London must undergo extensive training,
learning how to navigate between thousands of places in the city.
This training is colloquially known as ``being on The Knowledge''
and takes about 2 years to acquire on average. To be licensed to
operate, it is necessary to pass a very stringent set of police
examinations. London taxi drivers are therefore ideally suited
for the study of spatial navigation. The use of a group of taxi
drivers with a wide range of navigating experience permitted an
examination of the direct effect of spatial experience on brain
structure. In the first instance, we used voxel-based morphometry
(VBM) to examine whether morphological changes associated
with navigation experience were detectable anywhere in the
healthy human brain. VBM is an objective and automatic
procedure that identifies regional differences in relative gray
matter density in structural MRI brain scans. It allows every
point in the brain to be considered in an unbiased way, with no
a priori regions of interest. The data were also analyzed by using
a second and completely independent pixel-counting technique
within the hippocampus proper. Comparisons were made between
the brain scans of taxi drivers, who had all acquired a
significant amount of large-scale spatial information (as evidenced
by passing the licensing examinations), and those of a
comparable group of control subjects who lacked such extensive
navigation exposure.
Methods
Subjects. Right-handed male licensed London taxi drivers (n
16; mean age 44 years; range 32­62 years) participated. All had
been licensed London taxi drivers for more than 1.5 years (mean
time as taxi driver 14.3 years; range 1.5­42 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
(range 10 months to 3.5 years; some trained continuously, some
part time). All of the taxi drivers had healthy general medical,
neurological, and psychiatric profiles. The scans of control
subjects were selected from the structural MRI scan database at
the same unit where the taxi drivers were scanned. Those
subjects below 32 and above 62 years of age were excluded as
were females, left-handed males, and those with any health
problems. After the application of these exclusion criteria, the
scans of 50 healthy right-handed males who did not drive taxis
were included in the analyses for comparison with the taxi
drivers. Both the mean age and the age range did not differ
between the taxi driver and control groups. We were also careful
to ensure an even spread of subjects in each decade (for example,
41­50 years or 51­60 years) up to the upper limit of the oldest
taxi driver, such that subjects were not clustered at one end of
the age scale.
Image Acquisition. Structural MRI scans were obtained with a 2.0
Tesla Vision system (Siemens GmbH, Erlangen, Germany) by using
a T1-weighted three-dimensional gradient echo sequence (TR 9.7
ms; TE 4 ms; flip angle 12°; field of view 256 mm; 108 partitions;
partition thickness 1.5 mm; voxel size 1 1 1.5 mm).
Image Analysis Method 1: VBM. Data were analyzed by using VBM
implemented with Statistical Parametric Mapping (SPM99, Wellcome
Department of Cognitive Neurology) executed in MATLAB
(Mathworks, Sherborn, MA). Detailed descriptions of the technique
are given elsewhere (9, 10). Briefly, the subjects' data were
spatially normalized into stereotactic space (11) by registering each
of the images to the same template image by minimizing the
residual sums of squared differences between them. The template
Abbreviations: VBM, voxel-based morphometry; ICV, intracranial volume.
*To whom reprint requests should be addressed. E-mail: e.maguire@fil.ion.ucl.ac.uk.
The publication costs of this article were defrayed in part by page charge payment. This
article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C.
§1734 solely to indicate this fact.
Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073 pnas.070039597.
Article and publication date are at www.pnas.org cgi doi 10.1073 pnas.070039597
4398­4403 PNAS April 11, 2000 vol. 97 no. 8
was generated from the structural scans of 50 healthy males
acquired in the same scanner used to collect the data for the current
analysis (the scans of 13 of the control subjects used in the VBM
analysis were included in the creation of this template). The
spatially normalized images were written in voxels of 1.5 1.5
1.5 mm and segmented into gray matter, white matter, and cerebrospinal
fluid by using a modified mixture cluster analysis technique.
To reduce confounds caused by individual differences in
gyral anatomy, the gray matter images were smoothed by using an
isotropic Gaussian kernel of 4-mm full width at half maximum. The
statistical model included a measure of total amount of gray matter
in each brain as a confound (essentially the original values before
normalization). Statistical tests involved locating regionally specific
differences in gray matter between subject groups and were based
on t tests and the general linear model. Significance levels were set
at P 0.05 (small volume correction for multiple comparisons, with
62 resolution elements comprising the volume of interest). To be
consistent across the two analysis techniques, we defined the
hippocampal regions the same way in both cases as described below.
Image Analysis Method 2: Pixel Counting. The three-dimensional
images from the 16 taxi drivers and a precisely age-matched
sample of 16 normal controls taken from the 50 used in the VBM
analysis were submitted for region-of-interest-based volumetric
measurement of both hippocampi by using a well established
pixel-counting technique (12, 13). The images were analyzed by
one person experienced in the technique and blinded to subjects'
identity as taxi drivers or controls and the VBM findings. Briefly,
the images were reformatted into 1.5-mm-thick contiguous
sections in a tilted coronal plane that was perpendicular to the
long axis of the hippocampus. Hippocampal cross-sectional areas
were measured for each slice along the whole length of the
hippocampus. There were at least 26 contiguous slices (each
1.5-mm-thick) for each hippocampus of each subject, giving a
total length of approximately 4 cm. Total hippocampal volume
was calculated by summing the cross-sectional areas and multiplying
by the distance between adjacent slices (i.e., 1.5 mm).
These volumes were then corrected for intracranial volume
(ICV), which was measured from the sagittal slices of the original
three-dimensional data sets. Measurements for the most posterior
slices violated assumptions of homogeneity of variance and
were therefore not considered in the analyses, leaving the
measurements for 24 slices. These were grouped into three
regions (14): posterior hippocampus (6 slices), body (12 slices),
and anterior (6 slices). Values for the component slices for a
particular region were averaged, and the data were analyzed by
ANOVA and correlations (significance level set at P 0.05). We
report the analyses on data uncorrected for ICV (when data were
corrected for ICV, the results did not differ).
Results
VBM. Significantly increased gray 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 (Fig. 1
a and b). No differences were observed elsewhere in the brain.
The increase in hippocampal gray matter volume was focal and
limited to the posterior hippocampus bilaterally. The voxel of
peak difference in gray matter density in the right hippocampus
was at (x, y, z) 31, 22, 13 (Z 4.34) and, in the left
hippocampus, was at 31, 28, 10 (Z 4.19). The area of
difference extended in the y axis in the right hippocampus from
17 mm to 33 mm and in the left hippocampus from 18 mm
to 34 mm. Controls showed a relatively greater gray matter
volume bilaterally in anterior hippocampi relative to those of taxi
drivers (Fig. 1c). The voxel of peak difference in gray matter
density in the right hippocampus was at 32, 7, 28 (Z 2.65)
and in the left hippocampus was at 34, 7, 26 (Z 2.65). The
anterior changes did not survive correction for multiple comparisons
but were confirmed in the second analysis technique as
described below.
Pixel Counting. Hippocampal volumes (both corrected and uncorrected
for ICV) and ICV did not differ significantly between
Fig. 1. VBM findings. (a Left) Sagittal section of an MRI scan with the
hippocampus indicated by the red box. (a Right) Coronal section through the
MRI scan, again with the hippocampi indicated. (b) The group results are
shown superimposed onto the scan of an individual subject selected at random.
The bar to the right indicates the Z score level. Increased gray matter
volume in the posterior of the left and right hippocampi (LH and RH, respectively)
of taxi drivers relative to those of controls, shown in the top of the
figure in sagittal section. Underneath, the areas of gray matter difference are
shown in coronal sections at three different coordinates in the y axis to
illustrate the extent of the difference down the long axis of the hippocampus.
(c) Increased gray matter volume in the anterior of the left and right hippocampi
of controls relative to those of taxi drivers, shown in sagittal section.
Note that, although the Talairach and Tournoux (11) coordinate system was
used to describe the locations of VBM differences in stereotactic space, the
images were normalized with respect to a template based on a large number
of brains scanned in the same scanner used to collect the current data (see
Methods). Thus, the coordinates given refer to our brain template and only
approximately to the Talairach and Tournoux template.
Maguire et al. PNAS April 11, 2000 vol. 97 no. 8 4399
PSYCHOLOGY
the two subject groups [right hippocampus: taxi drivers, uncorrected
4,300 mm3
( 432), corrected 4,159 mm3
( 420);
controls, uncorrected 4,255 mm3
( 424), corrected 4,080
mm3
( 325); left hippocampus: taxi drivers, uncorrected
4,155 mm3
( 410), corrected 3,977 mm3
( 350); controls,
uncorrected 4,092 mm3
( 324), corrected 3,918 mm3
(
224); ICV of taxi drivers 1,521 mm3
( 145); controls 1,540
mm3
( 107)].
Although the analysis revealed no difference in the overall
volume of the hippocampi between taxi drivers and controls, it
did reveal regionally specific differences in volume (Fig. 2).
ANOVA (group by side) on anterior hippocampal volumes
revealed a main effect of group, with control volumes significantly
greater than those in taxi drivers [df (1, 30); F 5; P
0.05], a main effect of side, with the anterior right hippocampus
being larger than the left [df (1, 30); F 4.5; P 0.05], and no
interaction. Analysis of the volumes for the body of the hippocampus
showed no interaction and no effect of group but a
main effect of side, again right greater than left [df (1, 30); F
5.4; P 0.05]. Finally, analysis of volumes of the posterior
hippocampus revealed a main effect of group, with taxi drivers
having a greater posterior hippocampal volume than controls [df
(1, 30); F 4.1; P 0.05]. Neither the interaction nor the main
effect of side was significant.
Changes with Navigation Experience. We examined the correlation
between volume and amount of time spent as a taxi driver
(including both training on The Knowledge and practicing as a
qualified taxi driver). We found that length of time spent as a taxi
driver correlated positively with volume in only one brain region,
the right posterior hippocampus (Fig. 3a). Correction for age was
made by including it as a confounding covariate. The voxel of
peak correlation in the right hippocampus was at 22, 33, 3 (Z
5.65). The extent of the area in the right hippocampus ranged in
the y axis from 29 mm to 36 mm. The spatial extent of this
area overlapped with the area showing greater volume in the
categorical comparison of taxi drivers with controls. The VBM
changes as a function of time (in months) spent as a taxi driver
(corrected for age effects) and global gray matter are plotted in
Fig. 3b, where the data were considered within a linear model
Fig. 2. Volumetric analysis findings. The orientation of the slices measured in the volumetric analysis with respect to the hippocampus is shown (Top Right Inset).
A, anterior; B, body; P, posterior. (Upper) The mean of the cross-sectional area measurements (uncorrected for ICV) for the three regions of the left hippocampus
(LH). (Lower) The means for the right hippocampus (RH). Taxi drivers had a significantly greater volume relative to controls in the posterior hippocampus, and
controls showed greater hippocampal volume in the anterior. There was no difference between the two groups in the hippocampus body. *, P 0.05.
4400 www.pnas.org Maguire et al.
(r 0.6; P 0.05). The correlation between time spent as a taxi
driver (corrected for age) and the pixel-counting data indicated
a similar relationship for the posterior hippocampus (r 0.5; P
0.06). The pixel-counting data also showed a negative correlation
between the time spent as a taxi driver and the volume of
anterior hippocampal tissue (r 0.6; P 0.05) as plotted in
Fig. 3c. When time as a taxi driver was corrected for age by
expressing it as a percentage of age, the result was identical (r
0.6; P 0.05). The data of one taxi driver were not included
in the correlation analyses. He had been a taxi driver for 42 years,
and the next nearest length of time was 28 years; thus, he was
treated as an outlier and removed. The data of this subject were
completely in line with the relationships as plotted; for example,
his VBM response measure was 13.7.
Discussion
The data presented in this report provide evidence of regionally
specific structural differences between the hippocampi of licensed
London taxi drivers compared with those of control
subjects. Taxi drivers had a significantly greater volume in the
posterior hippocampus, whereas control subjects showed greater
volume in the anterior hippocampus. The converging results
Fig. 3. Correlation of volume change with time as a taxi driver. (a Left) Sagittal section. (a Right) Coronal section. The VBM group results are shown
superimposed onto the scan of an individual subject selected at random. The bar to the right indicates the Z score level. The volume of gray matter in the right
hippocampus was found to correlate significantly with the amount of time spent learning to be and practicing as a licensed London taxi driver, positively in the
right posterior hippocampus (b) and negatively in the anterior hippocampus (c).
Maguire et al. PNAS April 11, 2000 vol. 97 no. 8 4401
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from these two independent analysis techniques indicate that the
professional dependence on navigational skills in licensed London
taxi drivers is associated with a relative redistribution of gray
matter in the hippocampus. We further considered whether the
volume differences between the groups could be incidental and
unassociated with the navigational requirements of the taxi
drivers. For example, could this particular arrangement of
hippocampal gray matter predispose individuals to professional
dependence on navigational skills? Although this explanation
would be fascinating in itself, we tested this notion directly by
examining the correlation between volume and amount of time
spent as a taxi driver. Right hippocampal volume correlated with
the amount of time spent as a taxi driver (positively in the
posterior and negatively in the anterior hippocampus). We
believe that these data suggest that the changes in hippocampal
gray matter--at least on the right--are acquired. As such, 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.
Previous rodent and monkey studies have found the dorsal
(posterior) hippocampus to be preferentially involved in spatial
navigation (15­18). Such an involvement may also be true for the
rostral part of the hippocampus in birds (19). There are also
many more cells with spatial correlates--place cells (20)--in the
dorsal than in the ventral rat hippocampus (21). Functional
neuroimaging studies of navigation in humans show that the
retrieval or use of previously learned navigational information is
associated with activation of the posterior hippocampus (22­24).
Patients with lesions of the hippocampus that spare the posterior
aspect have been reported to be unimpaired at recalling routes
learned before lesion onset (6). Exactly how much posterior
hippocampus needs to be spared to support the recall of ``olď'
cognitive maps is not clear. For instance, it has been reported
recently that a patient with extensive damage to the medial
temporal region bilaterally, including the hippocampus, could
recall the town where he grew up many years before (25). The
authors of that report inferred from this fact that the medial
temporal region is not the repository of spatial maps. However,
they also allude to some sparing of the hippocampal tissue near
the lateral ventricles bilaterally. It is therefore not clear whether
this tissue was contributing to the recall of the patienťs spatial
map. Unfortunately, the patient was not tested by using functional
neuroimaging during the recall of these old memories,
which could have confirmed whether the remaining hippocampal
tissue was still functional. We would predict that it was.
Our finding that the posterior hippocampus increases in
volume when there is occupational dependence on spatial navigation
is evidence for functional differentiation within the
hippocampus. In humans, as in other animals, the posterior
hippocampus seems to be preferentially 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.
A basic spatial representation of London is established in the
taxi drivers by the time The Knowledge is complete. This
representation of the city is much more extensive in taxi drivers
than in the control subjects. Among the taxi drivers, there is, over
time and with experience, a further fine-tuning of the spatial
representation of London, permitting increasing understanding
of how routes and places relate to each other. Our results suggest
that the ``mental map'' of the city is stored in the posterior
hippocampus and is accommodated by an increase in tissue
volume. These results challenge the traditional view that the
hippocampus has a transient role in memory (26) at least in
relation to spatial navigation and the posterior hippocampus.
The need to navigate is a basic cross-species behavior. The
hippocampus is a phylogenetically old part of the brain, with an
intrinsic circuitry that may have evolved to deal with navigation.
Undoubtedly, in humans, the functions of the hippocampus have
adapted to accommodate other types of memory, such as
episodic memory (27­29), but the hippocampus retains an ability
to store large-scale spatial information (1).
Our findings from two independent measurement techniques
are consistent with patient (6) and functional neuroimaging (22)
reports of bilateral hippocampal involvement in successful navigation.
Unlike right hippocampal volume, however, left hippocampal
volume did not correlate with years of taxi-driving
experience, suggesting that the left hippocampus participates in
spatial navigation and memory in a different way from the right
hippocampus. We may speculate that the left hippocampus
complements its partner by storing memories of people and
events that occur in the rich context of taxi driving in the real
world, where an over-arching framework--such as integrating
information into an existing map--is not required (30).
Although the hippocampus does not support navigation in
isolation from other brain regions, it seems to be crucial to the
storage and use of mental maps of our environments. The
prolonged accumulation of other types of nonnavigational information
may also produce similar hippocampal changes. Our
present findings, however, corroborated as they are by the results
of patient and neuroimaging studies, suggest that space and the
posterior right human hippocampus are intimately linked.
Given the macroscopic level of our analyses, the data do not
speak directly to the microscopic mechanisms, such as neurogenesis
(31­33), that might underlie the structural change we
report herein. The differential changes in posterior and anterior
hippocampus may represent two separate processes. The most
parsimonious explanation, however, is that our findings reflect
an overall internal reorganization of hippocampal circuitry (34)
in response to a need to store an increasingly detailed spatial
representation, where changes in one hippocampal region are
very likely to affect others. On a broader level, the demonstration
that normal activities can induce changes in the relative
volume of gray matter in the brain has obvious implications for
rehabilitation of those who have suffered brain injury or disease.
It remains to be seen whether similar environment-related
plasticity is possible in other regions of the human brain outside
of the hippocampus.
E.A.M., I.S.J., C.D.G., J.A., R.S.J.F., and C.D.F. are supported by the
Wellcome Trust. D.G.G. thanks the Wellcome Trust for its support at
the Institute of Child Health. We are grateful for the assistance of Helen
Gallagher, Amanda Brennan, and Jon Galliers. We also thank Andrew
Holmes, Cheryl Johnson, and David McGonigle.
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