Individual Variation of Cortical Surface
Area Asymmetries
Asymmetries in the size of cortical regions are regularly associated
with functional lateralization. Assessment of cortical asymmetry is
often confounded by measurement artifact and a lack of information
about the normal variance of asymmetry in regions that are functionally symmetric. In order to measure hemispheric asymmetries in
the surface area of cortical gyri, magnetic resonance (MR) images
were acquired from 10 normal, right-handed males. Computer
representations of the cortical surface in all 20 hemispheres were
reconstructed from the images by first creating a white matter
model and then ‘inflating’ it to approximate the cortical surface. The
advantage of this approach is that it accurately models the deep
sulci as well as the cortical surface. Surface area measurements of
the whole hemisphere, the postcentral and the cingulate gyrus were
collected from each subject. For each region an asymmetry score
was computed based on the difference in the surface area of the left
and right regions. Many subjects showed asymmetries in these two
gyri; however, the mean asymmetry scores were never significantly
asymmetric. The large variability of individual asymmetry scores
indicates that cortical asymmetries may be present even in the
absence of clear functional asymmetry. An understanding of the
degree of asymmetry in structures that do not show clear functional
lateralization is critical for interpreting data gathered from cortical
regions that are functionally asymmetric.
Introduction
The diversity of cortical morphology has been well documented
from a variety of sources. Recently, efforts to co-register brain
images into stereotaxic coordinate space, or an idealized brain
template, have revealed tremendous interindividual variability in
the size, shape and configuration of cortical gyri and sulci
(Keyserlingk et al., 1988; Talairach and Tournoux, 1988;
Damasio and Damasio, 1989; Steinmetz et al., 1989a, 1990;
Mazziotta et al., 1991; Evans et al., 1992; Thompson et al., 1996).
More sophisticated algorithms for matching brain images to a
template have only been successful for subcortical structures
that are less variable in size and shape than the gyri of the
cerebral cortex (Bajcsy et al., 1983; Bohm et al., 1989;
Bookstein, 1989; Dann et al., 1989; Greitz et al., 1991; Gee et al.,
1993). Studies of cortical topography have described both supernumerary gyri, ‘missing’ gyri, varying distributions of extrasulcal
and intrasulcal tissue, as well as diverse patterns of sulcal
branching (Stensaas et al., 1974; Ono et al., 1991). Matching the
neocortex to a template can control for global size differences,
but not these local shape differences.
Studies of gyral anatomic asymmetry that are motivated by
the functional lateralization of language have focused on the
perisylvian region and frontal operculum (Geschwind and
Levitsky, 1968; Teszner et al., 1972; Witelson and Pallie, 1973;
Wada et al., 1975; Rubens et al., 1976; Chi et al., 1977a,b;
Galaburda et al., 1987; Albanese et al., 1989; Steinmetz et al.,
1989b; Falk et al., 1991; Steinmetz and Galaburda, 1991). There
have also been numerous assessments of cortical asymmetry
Jeffrey J. Hutsler, William C. Loftus1 and Michael S. Gazzaniga
Center for Cognitive Neuroscience, Dartmouth College,
Hanover, NH and 1Center for Neuroscience, University of
California, Davis, CA, USA
outside the putative language zones, but these have employed
relatively coarse anatomic delineations without explicit
reference to gyral topography (LeMay and Culebras, 1972;
Hochberg and May, 1975; Chui and Damasio, 1980; Habib et al.,
1984; Bear et al., 1986; Glicksohn and Myslobodsky, 1993).
Given the prevailing view that a close association exists between
functional and gross anatomical asymmetry, characterizing what
constitutes normal asymmetry in other regions is a prerequisite
for understanding the relationship between normal variation in
asymmetry and individual differences in cognitive ability.
This study examines normal variation in interhemispheric
asymmetr y at the scale of gyral anatomy. Cortical surface
models were constructed for 10 right-handed, male subjects.
Subjects were homogeneous with respect to age, gender and
handedness in an effort to eliminate the potentially ‘canceling’
effects of these variables (Steinmetz et al., 1991; Witelson and
Kigar, 1992). Geometric artifacts resulting from slice level
and image orientation (Zipursky et al., 1990; Glicksohn and
Myslobodsky, 1993) were circumvented by using a contiguous
three-dimensional computer model of the hemispheres (Loftus et
al., 1993, 1995) . The resulting three-dimensional surface is a
more accurate gauge of tissue area than conventional twodimensional, or ‘quasi’ three-dimensional measures, of surface
area since the delineation of gyri subsumes the sulcal walls as
well as the superficially exposed gyral crowns (Zilles et al., 1989;
Armstrong et al., 1991; Loftus et al., 1993). The questions
addressed are: (i) do individual brains have consistent
asymmetries at the level of individual gyri; (ii) can the direction
and/or degree of asymmetry be related to theories of asymmetric
development; and (iii) what is the range of asymmetry in areas
that are not thought to be functionally asymmetric?
Materials and Methods
Anatomical magnetic resonance images were collected from 10
right-handed, male subjects. Subjects were between the age of 21 and 29
and ever y subject reported being strongly right-handed. Edinburgh
laterality indexes confirmed this result for 9 of the 10 subjects ( χ = 78.51,
SD = 13.35, range 61.9–100.0), while an Edinburgh score was not
available for the tenth subject. Three millimeter, gapless, T1-weighted
images were acquired in the coronal plane for each subject using a
General Electric 1.5 T magnet that produced images 256 × 256 pixels in
size. Pixel resolution within these matrices was 0.937 mm. Images were
obtained with 3-D Flash and a TE/TR = 20 ms. The head was aligned in the
magnet so that a horizontal laser marked the intercanthal line and a
vertical laser intersected the midpoint of the nasion and philtrum.
Alignment was checked by verifying the presence of midline structures
on the same midsagittal locator image. These images were stored on
magnetic tape and then transferred to a Silicon Graphics workstation.
Model Construction
Since each image is averaged over 3 mm of thickness, the gapless image
set forms a volume that is composed of ‘bricks’. Each brick is 0.94 × 0.94
× 3 mm , or the size of an individual voxel within an image multiplied by
Cerebral Cortex Jan/Feb 1998;8:11–17; 1047–3211/98/$4.00
Figure 2. Three-dimensional model of one subject’s brain showing the cingulate gyrus.
Since the posterior boundary is often ambiguous, ‘artificial’ posterior boundaries were
defined as the coronal section that corresponded to the rostral border of the corpus
callosum (gray vertical line). Arrow shows the location of the subparietal sulcus.
Figure 1. Three-dimensional model of one subject’s right hemisphere showing the
postcentral gyrus. These models can be fully rotated and labeling of the region of
interest can be viewed on-line by the operator. The operator also has simultaneous
access to the two-dimensional image set in the original scan plane and in two
computer-generated orthogonal planes. Small segments of the model can be viewed in
conjunction with the underlying MR scan for ease of labeling the deep sulcal walls.
the distance between images. A white matter model was created by
defining a threshold near the gray–white matter border for all of the
images simultaneously. These thresholds were then edited by the
operator to include only the white matter under the cortical mantle. A
triangular mesh was then applied to the resulting volumetric data set
using a ‘marching cubes’ algorithm (Lorensen and Cline, 1987). To
eliminate errors produced in the automatic creation of the hemispheric
cortical surface, preliminar y surface models were first calculated
between adjacent coronal sections. These preliminar y models were
edited to remove surfaces that corresponded to non-cortical structures.
When editing was complete, a white-matter surface model was created
simultaneously for the entire hemisphere. The advantages of creating a
white-matter model, rather than a direct surface model, lie mainly in its
ability to map accurately the cortical surface that is present within sulci.
A white-matter model is not prone to the loss of sulci running parallel to
the slice plane, since white-matter sulci are much larger than the slice
plane thickness. This allows the acquisition of relatively thick slices (3
mm) that increase the definition of the gray–white matter boundary, and
significantly reduce scan time, thus reducing artifacts that are due to head
movement.
The resulting model was then ‘inf lated’ (Dale and Sereno, 1993) to
approximate the true cortical surface. The model can be viewed as a
triangular mesh that is composed of individual vertices (points) that form
the apices of the triangles. The algorithm used for inf lation moves an
individual vertex within the model toward a gray value in the original
volumetric MR data set that lies on the cortical surface (surface vector). In
order to create a smooth surface a ‘smoothness’ vector is also applied to
the model which constrains vertex movement by taking into account the
position of neighboring vertices (Dale and Sereno, 1993). A weight was
specified for each vector to control their relative inf luence during
inf lation. These same weights and thresholds were used for both
hemispheres of any individual subject.
Regional Identification
To facilitate gyral identification individual cortical locations could be
labeled on the whole-hemisphere model with simultaneous reference to
12 Variability of Regional Asymmetry • Hutsler et al.
the original MR images and small sections of the model that revealed the
depths of the cortical sulci. Two cortical locations that could be easily
defined were chosen to assess the degree and direction of asymmetry.
These were the postcentral gyrus and the cingulate gyrus. The
postcentral gyrus was bordered dorsally by the longitudinal fissure,
ventrally by the lateral (Sylvian) fissure, anteriorly by the central sulcus
and posteriorly by the postcentral sulcus (Fig. 1). In cases where the
postcentral gyrus was not continuous, minor surface variation was used
to connect the two sulci. Double postcentral sulci (Ono et al., 1991) were
not encountered in our sample. The cingulate gyrus (Fig. 2) was bounded
dorsally by the cingulate sulcus, ventrally by the corpus callosum,
anteriorly by the anterior end of the cingulate sulcus and posteriorly by a
combination of the subparietal sulcus and the anterior portion of the
calcarine fissure. Due the variation in these posterior sulci, a second set of
measures was taken in which the posterior boundary of the cingulate
sulcus was redefined in both hemispheres by the coronal slice that
corresponded to the rostral edge of the splenium of the corpus callosum
just anterior to the marginal branch of the cingulate sulcus (see Fig. 2).
Surface Area/Asymmetry Computation
The surface area of each triangle in the model, and each triangle within a
labeled cortical region, was automatically computed by dividing the cross
product of its sides by two. These triangles were then summed to obtain
the surface area of the region of interest (ROI). An asymmetry coefficient
was then computed for each region of each subject based on the surface
area of left (L) and right (R) homologues corrected for region size
(L – R)/[(L + R)/2] (Galaburda et al., 1987; Leonard et al., 1993). These
asymmetr y coefficients can range between 2 and –2, with the sign
indicating the direction of asymmetry and the value indicating the degree
of asymmetry. Perfectly symmetric regions have a value of zero.
Results
Individual subjects often showed marked size asymmetries of
either the cingulate or postcentral gyrus. Compared to these
regions the whole brain was remarkably symmetric. Figure 3
shows each subject’s asymmetry coefficient for the whole brain
(A), the postcentral gyrus (B), and the cingulate gyrus (C).
Overall measures of hemispheric surface area were not asymmetric (df = 9, t = 0.533, P = 0.6068), and the mean asymmetry
coefficients for the postcentral and cingulate gyri did not differ
from symmetry (postcentral: df = 9, t = 1.492, P = 0.1700;
cingulate: df = 9, t = -1.986, P = 0.0783). Although the group did
not show consistent asymmetries many individual cases were
asymmetric. If an asymmetry value of ±0.1 is used to evaluate
individual asymmetries, 7 out of 10 subjects had an asymmetric
Figure 4. Scattergrams indicating the degree of agreement between the two raters for
the postcentral (A) and cingulate (B) of eight subjects.
Figure 3. Asymmetry plots showing the degree and direction of asymmetry for each
of the ten subjects (x axis). (A) Whole brain; (B) postcentral gyrus; (C) cingulate gyrus.
The mean asymmetry score is indicated by the thick solid line while the lower and upper
95% confidence intervals are indicated by the dashed lines.
cingulate gyrus, while 4 out of 10 had an asymmetric postcentral
gyrus. Even if the threshold values are corrected for sample size
and variance (see below), 4 out of 10 subjects show asymmetries
in the cingulate, and the same proportion show asymmetries in
the postcentral gyrus.
Inter-rater Reliabilities
To assess the possibility that error variation in the delineation of
cortical regions resulted in significant individual asymmetries,
inter-rater reliabilities were conducted for 8 of the 10 subjects. A
second set of reliability measurements was not obtained for the
two remaining subjects due to the unavailability of the second
rater. Each rater was unaware of the other’s results. Figure 4
shows the scattergrams of asymmetry coefficients for the two
raters. The postcentral gyrus was labeled with a high degree of
reliability (r = 0.855, P = 0.0044). The cingulate gyrus was also
labeled reliably (r = 0.708, P = 0.0485), but only when the
posterior boundar y was defined by the coronal slice that
coincided with the rostral edge of the splenium of the corpus
callosum. Surface area measurements that utilized ambiguous
cortical landmarks to define the posterior boundary of the
cingulate, such as the subparietal and cingulate sulci, were not
reliable (r = 0.543, P = 0.1740). Due to this low reliability all
analyses of the cingulate gyrus reported here are based on
measurements that truncate the cingulate at the section plane
that corresponds to the anterior border of the splenium.
Raters also showed a marked degree of agreement in the
direction of asymmetry. In the case of the postcentral gyrus
there were six agreements and two disagreements, and for the
cingulate the two raters were always in agreement as to the
direction of hemispheric asymmetr y. Given the degree of
agreement between raters, only the first rater’s values were used
for the remaining statistical analyses.
Individual Asymmetries
It is often desirable to point to individual brains as being either
symmetric or asymmetric. Typically a threshold of either ±0.1 or
±0.2 is used to determine if an individual’s asymmetry coefficient
differs significantly from symmetry (Galaburda et al., 1987). The
number of individuals that had marked asymmetries in either the
cingulate or postcentral gyrus varied depending on whether a
threshold of 0.1 or 0.2 was chosen. As stated above, with an
asymmetry threshold of ±0.1 four individuals had an asymmetric
postcentral gyrus and seven had an asymmetric cingulate gyrus.
If a threshold of ±0.2 is utilized, the number of asymmetric
Cerebral Cortex Jan/Feb 1998, V 8 N 1 13
subjects drops to three and three, respectively. A statistically
significant threshold for individuals should take into account the
variability in the size of the region of interest and the number of
cases this variance is based upon. Since size variance is not stable
across cortical regions, an arbitrary threshold such as ±0.1 can be
either too conservative or too liberal. Threshold values for
significant asymmetry coefficients can be derived empirically
given the sample size and variance of the structure being
measured. Using a P value of 0.05 the critical values for the
present sample are ±0.122 for the postcentral gyrus and ±0.185
for the cingulate gyrus.
Ontogeny of Asymmetry
Asymmetric brain regions can develop due to either a unilateral
enlargement or reduction of a cortical region while its
contralateral homologue is unchanged (Galaburda et al., 1990).
If asymmetry is the result of a size reduction in one hemisphere,
then as the absolute asymmetry values increase, the combined
size of the homotopic regions should decrease. Conversely, if
asymmetry results from a size increase in one hemisphere, then
as absolute asymmetry values increase, the combined size of the
homotopic areas should also increase. If, on the other hand, both
regions contribute to the final asymmetry there will be no clear
relationship between their combined areas and the degree of
asymmetry. Neither the postcentral (r = –0.238, P = 0.5205) nor
the cingulate gyrus (r = 0.043, P = 0.9087) showed significant
correlations between the absolute value of the asymmetry
coefficients and the combined areas. The overall size of the two
hemispheres was also not correlated with the absolute value of
the asymmetry coefficients (r = –0.022, P = 0.954).
Handedness was related to the size of certain regions in the
analysis. Most notably the higher an individual’s Edinburgh score
(the stronger their right-handedness), the smaller their left
postcentral gyrus (r = –0.832, P < 0.01 two-tailed, n = 9;
corrected for multiple correlations). Although intriguing, the
limited range of handedness scores (+61 to +100 out of a range of
–100 to +100) precludes interpretation of this result.
Discussion
The present results demonstrate that there is substantial
individual diversity of left–right gyral asymmetries in the
cerebral cortex. Many individuals showed marked asymmetries
in the two regions examined; however, neither region was
consistently asymmetric across subjects. Information regarding
the amount of asymmetry present in structures other than those
that are considered to contain a large degree of functional
lateralization is critical for a balanced interpretation of data
gathered from cortical regions associated with language function
— a strongly lateralized ability. The large individual variability
indicates that cortical asymmetries may be present even in the
absence of clear functional asymmetry.
Mean Asymmetry Scores
Neither area was significantly asymmetric when all of the
subjects were combined; however, the present findings agree
with previous reports that the right cingulate gyrus may be
slightly larger in the general population (Noga et al., 1995). As
with this previous study, the trend was non-significant. It is
difficlut to compare these two studies, since the previous study
utilized vastly different methods to our own.
The failure to find any statistically significant mean
asymmetries in our study could be accounted for by insufficient
statistical power engendered by the relatively small sample size
14 Variability of Regional Asymmetry • Hutsler et al.
(10). It has also been suggested that non-significant effects can
be due to combining subgroups that show different patterns of
asymmetr y (Witelson and Kigar, 1992). The possibility that
previously reported subgroup effects, such as age, sex and
handedness, may have obscured asymmetries must be rejected
since the present sample is homogeneous with respect to these
potentially confounding factors. This does not, however,
eliminate the presence of other undocumented subgroup
effects. Finally, the parcellation of gyri employed in the present
study may have grouped asymmetric substructures inside symmetric superstructures and thereby obscured the underlying
asymmetries. Recent investigations on morphological asymmetries in the region of the planum temporale (Rubens et al.,
1976; Steinmetz et al., 1990; Witelson and Kigar, 1992; Leonard
et al., 1993; Loftus et al., 1993) have shown that as the vertically
oriented ‘parietal’ bank of the planum gets larger, the
horizontally oriented ‘temporal’ bank gets smaller. Thus, the
combined intrahemispheric surface area of the temporal and
parietal banks surface is conserved despite its configuration,
and overall left–right symmetry of the total planar surface is
maintained even though the temporal and parietal banks are
respectively leftward and rightward asymmetric. It is conceivable that other reciprocal asymmetries are hidden in the present
data.
Where significant individual asymmetries exist reciprocal
asymmetries (asymmetries of opposite valence) may be present
in one or more other cortical locations since there is no overall
asymmetry in the surface area of each hemisphere. Evidence that
such a mechanism could exist comes from experiments in the
modification of gyral patterns in the developing monkey cortex
(Goldman-Rakic and Rakic, 1984). Prenatal resection of the
occipital lobe on one side produces a compensatory enlargement and concomitant asymmetry of the inferior parietal lobule.
One proposed mechanism for this effect is that afferent fibers
compete for synaptic space within cortical zones of the
hemisphere. Successful competition for connections results in
an increase in cortical area for one zone at the expense of a
decrease in cortical area in a rival zone. Regional anatomic
asymmetries could emerge if the outcome of competition within
one hemisphere differed from that of the other hemisphere.
Anatomical Diversity
The use of surface landmarks to circumscribe the boundaries
around a cortical region can often be fraught with numerous
difficulties. Perhaps the most important is that the location of
sulci may not consistently correspond to the border of the
cytoarchitectonic region of interest (Ono et al., 1991; Hutsler
and Gazzaniga, 1995). In addition, where surface landmarks are
variable or ambiguous boundaries are subject to the interpretations of the operator. The interpretation of ambiguous landmarks
is probably inf luenced by an idealized conception of what a
region should look like. These idealized concepts could reduce
the variance both between hemispheres and between subjects.
Both of the regions examined here are composed of multiple
areas as defined by gross histology (cytoarchitectonics) and comparative physiology. Cytoarchitectonically, Brodmann divided
the postcentral gyrus into four separate regions: area postcentralis intermedia (field 1), area postcentralis caudalis (field
2), area postcentralis oralis (field 3) and area subcentralis
(field 43). Areas 3, 1 and 2 form cortical strips that run parallel
to the orientation of the postcentral gyrus. Area 1 lies directly on
the visible surface of the postcentral gyrus while area 3 abuts it
anteriorly and area 2 abuts it posteriorly. Area 43 covers the
ventral end of the postcentral gyrus and spills into the dorsal
bank of the Sylvian fissure (Brodmann, 1909). Von Economo’s
description of this same region is remarkably similar; however,
he explicitly subdivides Brodmann’s area 3 into an area PB and an
area PA (area postcentralis giganto-pyramidalis) that covers up to
one-third of the surface of the anterior wall of the postcentral
gyrus near the bottom of the central sulcus and retains
characteristics of the adjoining precentral gyrus (some large
Betz cells in layer V). Von Economo’s area PC (area postcentralis
intermedius) and area PD (area postcentralis caudalis) are similar
to Brodmann’s areas 1 and 2 respectively (von Economo, 1929).
Physiologically, each of these cytoarchitectonically defined
regions appears to contain a complete representation of the body
surface and receives inputs primarily from a single type of
sensory receptor. The dorsally located area 42 is a secondary
auditory area (SII).
The cingulate gyrus is also composed of several cytoarchitectonic regions. Brodmann defined the anterior cingulate
as being composed of two regions: area 33, which borders the
corpus callosum at its anterior boundary, and area 24, which
covers a larger area on the exposed surface of the cingulate gyrus
and continues further posteriorly than area 33. In our study a
uniform posterior boundary was utilized to minimize the impact
of the highly variable and difficult to define posterior boundary
of this region. This boundary through the coronal slice that
corresponds to the anterior (rostral) boundary of the splenium of
the corpus callosum crosses two other Brodmann’s areas: 31 and
23 (Brodmann, 1909). Von Economo’s description includes
several additional subdivisions, but his scheme can be
summarized as an agranular cortical type in the anterior portions
of the cingulate, a granular type posterior to this (just behind the
imaginary extension of the central sulcus) and a region of
allocortex lying in a narrow band at the transition between the
isocortex of the cingulate gyrus and the corpus callosum (von
Economo, 1929).
Individual Asymmetry Coefficients
As suggested above, the number of individuals showing
asymmetrical cortical regions can be greatly exaggerated (or
underestimated) in the absence of data concerning cortical
variability. Typically, an arbitrary asymmetry coefficient ranging
from 0.1 to 0.2 has been used to assess individual cases. If the
variance in the size of cortical regions is high, then this value is
an overestimation of the number of individuals in a group
demonstrating significantly large asymmetry scores. If, however,
the variability is low, these same threshold values may be overly
conser vative. We instead used the variance in the sample
population to determine the minimal threshold necessary to
achieve significance. A lternatively, if a particular asymmetry
threshold is desired (i.e. ±0.1), then a power analysis can be set
to determine the number of subjects required to achieve this
threshold.
previous contour-based approaches. Surface areas range from
1399 to 1871 cm2 using the old method, while the present
results yield a range from 1695 to 2174 cm2. In addition, surface
area measurements taken from post-mortem brains (Hofman,
1985: 2430 cm2) more closely match the results that we report
with the present technique. This correspondence notwithstanding, we would suggest that there are several computational
improvements that would make the present method far superior
to the contour approach. These include the incorporation of a
volumetric thresholding technique that would more accurately
model the gray–white matter boundar y, and the addition of
further mathematical constraints to the inf lation algorithm. The
present technique (Dale and Sereno, 1993) represents the
beginning of a new approach to modeling methods that by
incorporating a volumetric white-matter model will more
accurately conform to the convoluted walls of the cerebral sulci.
Function and Implications
We can only speculate on the functional significance, if any, of
regional asymmetry scores. There is evidence that cortical
morphology ref lects underlying connectivity and architectonic
field boundaries (Welker, 1990; Watson et al., 1993; Rademacher
et al., 1993) and that these are specified by genetic factors
(Weinberger et al., 1992). Surface area techniques, similar to
those used in the present study, have also revealed genetic
inf luences on cortical morphology (Tramo et al., 1995).
Classically, cortical size asymmetries have been associated with
specialized function; however, many of these studies are
confounded by measurement artifact (Loftus et al., 1993).
Although one might suspect that sensorimotor regions could
show a leftward asymmetry due to the prevalence of
right-handedness in the general population, our findings, in
agreement with those reported previously (White et al., 1997),
indicate that this is not the case. The multiple functions that have
been proposed for the cingulate gyrus, including its involvement
in emotional behavior, memor y processing and attention to
visual stimuli, do not appear to be highly lateralized in the
normal brain.
These findings have important implications for the
development of standardized brain templates used for the
purposes of intersubject averaging of data gathered with PET or
MRI. While it is feasible to develop a so-called ‘average’ template
separately for each hemisphere based on the mean size and
shape of each homologue, the resulting whole-brain model
would be atypically symmetric. The use of such averaged models
raises serious concerns for interpreting the results of cortical
locations that show marked variance in the distribution of their
surface area. Additionally, the large degree of variability in the
size of asymmetr y coefficients suggests caution in the
interpretation of size asymmetries in areas that are presumed to
be functionally asymmetric.
Notes
Previous Methodology
Most previously utilized methods for modeling the cortical
surface have relied upon the tracing of two-dimensional
contours directly from the MR image set. The length of these
contours can either be multiplied by the distance between
sections, or, using a true three-dimensional approach, can be
‘stitched’ together by triangulation (Loftus et al., 1993).
Measurements of the same subjects utilized in the present study
reveal that surface areas are consistently smaller with these
This research was supported by ONR N00014–89-J-3035, NIH/NINDS P01
NS17778–10 and the McDonnell-Pew Foundation. Some of these results
were presented at the Annual Meeting of the Society for Neuroscience
held in Washington, DC in November 1996. The authors gratefully
acknowledge Dr Mark J. Tramo and Dr Ronald L. Green for subject
recruitment; Dr John Weaver for defining and implementing the MR
sequences; Robert Ferranti for carrying out the MR acquisitions.
Address correspondence to Jeffrey J. Hutsler, Center for Cognitive
Neuroscience, Psychology Department, 6162 Silsby Hall, Dartmouth
College, Hanover, NH 03755, USA. Email [email protected].
Cerebral Cortex Jan/Feb 1998, V 8 N 1 15
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