BRAIN
AND
COGNITION
6, 15-23 (1987)
Volumetric Asymmetries of the Human Brain:
Intellectual Correlates
RONALD
A. YEO
University of New Mexico
ERIC TURKHEIMER
The University of Texas at Austin
NAFTALI RAZ
University of Health Sciences, The Chicago Medical School
AND
ERIN
D. BIGLER
Austin Neurological Clinic
Volumetric measures of the brain and ventricles were derived from CT films
and related to intellectual variables from the Wechsler Adult Intelligence Scale
(WARS). Subjects were patients referred for neurological examination for headache
or somatic complaints, sometimes accompanied by anxiety or dysphoric affect
(N = 41), for whom a comprehensive neurological work-up revealed no evidence
of abnormality. The asymmetry of hemispheric volume (left minus right over
total, times 100) was correlated (r = .57, p < ,001) with Verbal IQ minus
Performance IQ within subjects. No relationship was observed between total
brain or hemispheric volumes and IQ scores. Brain and ventricular volumes were
larger for the left hemisphere than the right. o 1987 Academic press, IK.
The search for intellectual correlates of human brain size and shape
has a long history (Van Valen, 1974; Gould, 1981), but has been severely
hindered by difficulties in obtaining direct measures of both the brain
and behavior of intact subjects. Typically, either autopsy data were
available and cognitive functions inferred, or psychometric data were
Reprint requests should be addressed to Dr. Ronald A. Yeo, Department of Psychology,
University of New Mexico, Albuquerque, NM 87131.
15
0278-2626187$3.00
Copyright
0 1987 by Academic Press, Inc.
All rights of reproduction
in any form reserved.
16
YE0
ET AL.
collected and brain parameters assessedinferentially. Correlations between
various measures of head size and various measures of intellectual ability
have ranged from r = .08 to r = .22 (Van Valen, 1974). Furthermore,
the range of brain and behavioral variables examined has been quite
restricted. The overall level of ability has received much more attention
than patterns of abilities, and total brain size has received much more
attention than measures of hemispheric size or hemispheric asymmetry.
With the advent of computerized axial tomography (CT), detailed measurements of brain parameters can be obtained from intact subjects who
have been administered standardized, reliable, aptitude tests. Two recent
studies suggest that hemispheric asymmetry, as determined from CT
scans, may be related to patterns of intellectual ability, as assessed by
the relative superiority of Verbal IQ (VIQ) or Performance IQ (PIQ) on
the Wechsler Adult Intelligence Scale (WAIS). Both studies measured
the widths of the occipital lobes, either 5 mm (Rosenberger & Hier, 1980)
or 8 mm (Luchins, Weinberger, & Wyatt, 1982) anterior to the occipital
poles. Among individuals with learning disabilities, preselected for large
VIQ-PIQ difference scores, the correlation between the difference score
and hemispheric asymmetry (left occipital width minus right occipital
width over total) was r = .38 (Rosenberger & Hier, 1980). Among schizophrenics, the mean VIQ-PIQ differences were greater among those
with normal (i.e., left larger) asymmetries than among those with reversed
asymmetries (11.8 vs. - 8.0, respectively) (Luchins, Weinberger, & Wyatt,
1982).
These findings must be interpreted with caution for several reasons.
First, both studies used only cognitively deviant subjects. Second, sampling
procedures in each study were such that generalizability is uncertain.
Rosenberger and Hier (1980) selected subjects with large VIQ-PIQ discrepancies, and in the Luchins et al. (1982) study IQ scores were available
for only a subset of the schizophrenics (26 of 79), raising the possibility
of a sampling bias. Third, hemispheric width may not be the optimal
linear index of functional asymmetry. Pieniadz and Naeser (1984)discovered
that occipital length, but not width, correlated with planum temporale
asymmetry at post mortem. Finally, the CT asymmetry measures in each
study were limited to a single point on a single CT slice. While hemispheric
width is related to hemispheric volume, the relationship may well be
nonlinear. If there is any relationship between anatomical asymmetry
and patterns of abilities, then volumetric measures, which more closely
approximate the true configuration of brain structures (Penn, Belanger,
& Yasnoff, 1978), may demonstrate the relationship better than the linear
measures previously employed.
The association of anatomical asymmetries with patterns of cognitive
ability represents an important step in our efforts to understand the
functional significance of cerebral lateralization (Geschwind & Galaburda,
VOLUMETRIC
ASYMMETRIES
AND ABILITY
17
1985). Asymmetries measured from CT scans, in cor@ast to those obtained
at post mortem, can be related to psychometric analyses of patterns of
cognitive ability. The present study sought to evaluate the hypothesis
that anatomical asymmetries of the cerebral hemispheres are associated
with verbal vs. nonverbal intellectual superiority, using volumetric CT
scan measures in cognitively normal subjects.
METHOD
Subjects. Subjects were selected from the outpatients of the Austin Neurological Clinic
according to the following criteria: (1) their CT scans were judged to be normal, (2) no
abnormal findings were evident on neurological examination, (3) no schizophrenic or major
affective disorder had ever been diagnosed, and (4) the WAIS had been administered.
Forty-one subjects (27 females) met these criteria. Presenting complaints of the subjects
were headaches (N = 19, 46% of the sample), somatic problems (N = 8, 20%), perceived
changes in consciousness (e.g., concentration or memory complaints, dizziness; N = 7,
17%), somatic and psychological (e.g., anxiety, dysphoric affect) complaints (N = 5, 12%),
and dysphoric affect (N = 2, 5%). The mean age of the sample was 38.4 years (SD =
14.1) and the mean number of years of education was 14.2 (SD = 3.8). WAIS means (and
standard deviations) were Full Scale IQ (FSIQ)= 107.6 (ll.l), VIQ = 109.4 (13.3), PIQ =
104.9 (10.2), and VIQ minus PIQ = 4.5 (11.4). Some subjects (N = 13) received an
incomplete WAIS (averaging 3.6 Verbal and 2.5 Performance scale subtests), and for these
subjects IQ scores were prorated. Minnesota Multiphasic Personality Inventory (MMPI)
scores were available for a subset of the sample (N = 36) and indicated that the subjects
were above average in terms of psychological distress and neuroticism, especially hypochondriasis. (Mean T scores for those subjects given the MMPI were L = 48, F = 58,
K = 58, Hs = 74, D = 70, Hy = 70, Pd = 64, Pa = 63, SC = 71, Ma = 61, and Si =
57.) An elevation on the hypochodriasis (Hy) scale is expected for the general medical
population and is not a specific characteristic of the current sample (Welch & Dahlstrom,
1956).
CT scan analysis. All CT scans were obtained with an EM1 1010 scanner, producing
eight (in seven subjects) or nine transaxial slices along the orbito-meatal line, each g-mm
thick. The quantification procedure involves several steps (Yeo, Turkheimer, & Bigler,
1983). First, prior to the collection of the psychometric data, the brain, ventricles, and
interhemispheric fissure were traced from the CT films. The lowest slice used was at the
level of the supracellar cistern, on which the temporal poles were isolated, and on that
slice the cerebellum was not traced. Second, the tracings were digitized on a digitizing
tablet (Summagraphics “Bit Pad”). A stylus was manually moved around the perimeter
of a structure (i.e., hemisphere or ventricle), writing x and y coordinates at the rate of 5
per second, up to 200 per inch. Areas of structures were then computed using an interactive
program written in APLSF (Turkheimer, Yeo, & Bigler, 1983). Volumes were computed
using the trapezoidal rule. In a previous investigation (Yeo et al., 1983) the interrater
reliability of the technique across both digitizing and tracing steps, but using the same CT
film, was r = .99.
Volume measures were obtained for the brain and ventricles in each of four quadrants
(left and right anterior, left and right posterior). Volumes of each hemisphere were computed
separately, the interhemispheric fissure dividing the brain into left and right halves. On
lower slices the interhemispheric fissure does not extend through the middle portion of
the brain. For these slices, the fissures were traced to the fullest extent possible, and then
a line was drawn connecting the most posterior portion of the anterior fissure line to the
most anterior portion of the posterior fissure line. Next, each hemisphere was divided into
anterior and posterior halves. Essentially, a perpendicular bisector of the interhemispheric
18
YE0 ET AL.
fissure defined anterior and posterior halves. The derivation of this bisector is somewhat
complicated, however, by the fact that the interhemispheric fissure is not a straight line.
A straight line segment, approximating the interhemispheric fissure, was defined for the
purpose of obtaining an accurate perpendicular bisector. Two lines were drawn (see Fig.
l), one connecting the most anterior extensions of the frontal lobes (LFP and RFP, in
Fig. l), and one connecting the most posterior extensions of the occipital lobes (LOP and
ROP). The interhemispheric fissure was extended anteriorly and posteriorly so that it would
intersect with these lines (at points A and B, respectively). Line segment AB was bisected,
providing the horizontal line dividing each hemisphere into anterior and posterior halves.
Note that the mesial border of the hemispheres was defined by the interhemispheric fissure,
not segment AB, which was only used to divide the hemispheres already defined by the
interhemispheric fissure. Asymmetry measures for all variables were expressed as left
minus right volume over total volume, times 100 (Asymmetry Quotient, or AQ).
Staristical analyses. Partial correlations were used to express the relationship between
brain and behavioral measures. In all analyses reported below the number of slices obtained
on the CT film was treated as a control variable, in order to ensure that volumetric measures
were based on an equivalent number of slices for all subjects. Multiple regression procedures
relating the quadrant volumes to the WAIS measures were not used because of the problem
of multicolinearity arising when each predictor in a regression equation is a nearly linear
combination of the other predictors (Darlington, 1968). As a result, unstable estimates of
regression parameters are obtained, and the regression equation becomes uninterpretable.
RESULTS
The asymmetry of the volumes of the hemispheres was correlated with
VIQ-PIQ differences at r = 37 0, < .OOl; see Fig. 2). The asymmetry
effect was a product of both anterior and posterior asymmetries; the
anterior AQ correlated with VIQ-PIQ differences at Y = .27, vs. r =
.29 for the posterior AQ. The magnitude of the correlation of AQ with
FIG. 1. Determination of anterior and posterior hemispheric quadrants. See text for
explanation.
VOLUMETRIC
-4
ASYMMETRIES
19
AND ABILITY
-2
3
nEnISPnOER*C
AS:MrlETRr
tNoEX
&J
FIG. 2. Scatterplot of the relationship between VIQ-PIQ difference scores and volumetric
hemispheric asymmetry (r = S7, p < 301).
VIQ-PIQ split did not significantly differ in males (r = .68) and females
(r = JO). To evaluate the possibility that the above average degree of
psychopathology revealed on the MMPI may have influenced the correlation
between AQ and VIQ-PIQ split, the MMPI F scale (an overall index of
psychopathology) was partialled out. The magnitude of the correlation
was unchanged, suggesting that the degree of psychopathology did not
significantly influence our results. Correlations between AQs and demographic variables (age, sex, education) did not significantly differ from
zero.
None of the measures of brain size (total brain volume, left or right
hemisphere volume, or anterior or posterior volumes) was significantly
correlated with any of the intellectual variables (FSIQ, VIQ, or PIQ).
The correlations among these measures are shown in Table 1. When
ventricuhu- volumes were subtracted from hemispheric volumes, producing
a net hemispheric volume measure, correlations with IQ variables remained
nonsignilkant. Ventricular volumes were also unrelated to ability measures.
In addition to investigating relationships between brain morphology
and behavior, the current investigation provided data regarding the physical
asymmetries of the brain. The left hemisphere was found to be larger
than the right, in both anterior and posterior halves (see Table 2). The
total left hemisphere ventricular volume was also greater than the right.
Each of these asymmetry measures (anterior AQ, posterior AQ, and
ventricular AQ) differed significantly from zero (p < .Ol). Asymmetry
indices for non-right-handers were somewhat attenuated, though not significantly different from right-handers. These data should be interpreted
with caution due to small sample size (N = 6 for non-right-handers,
N = 32 for right-handers).
YE0 ET AL.
TABLE 1
CORRELATIONS
OF BRAIN
VOLUME
MEASURES WITH
IQ
SCORES
Volume
VIQ
PIQ
Volume Total
Left
Right
Anterior
Posterior
FSIQ
VIQ
PIQ
Total
Left
Right
Anterior
.92*
.a2*
.07
.08
.05
- .08
.23
.56*
.12
.16
.06
- .03
.28
.06
- .03
.05
-.ll
.13
.98*
.98*
.91*
.86*
.95*
.90*
.85*
.89*
.85*
.58*
Note: N = 41.
*p<.OO1.
DISCUSSION
Current results suggest that physical asymmetry of the cerebral hemispheres is associated with verbal vs. nonverbal intellectual superiority.
The bivariate distribution of the VIQ-PIQ difference scores and AQ was
roughly triangular in shape. Thus, if AQ exceeds a value of four, VIQ
is very likely to exceed PIQ. Smaller AQs were less reliably associated
with patterns of cognitive abilities. The observed correlation was larger
than that observed by Rosenberger and Hier (1980) in learning disabled
subjects, even though their subjects were preselected for large VIQ-PIQ
difference scores. The difference in the magnitude of the correlation may
be attributed to differences in CT scan measurement techniques, the
subjects, or both.
What is the mechanism behind the asymmetry effect? Two speculations
might be considered. Perhaps VIQ might exceed PIQ simply because
there is a relatively greater volume of that part of the brain that specializes
in verbal reasoning, i.e., the left hemispere. This notion presupposes a
volume-ability relationship, which would be revealed by significant correlations between left hemisphere volume and VIQ, and between right
hemisphere volume and PIQ. These correlations, however, were not
TABLE 2
ASYMMETRY
QUOTIENT
MEANS (AND
SD)
BY HANDEDNESS’
Asymmetry
Total sample
(N = 41)
Right-handers
(N = 32)
Non-right-handers
(N = 6)
Hemispheric
Anterior
Posterior
2.70 (1.8)
2.91 (4.1)
2.53 (4.4)
2.80 (1.9)
2.95 (4.3)
2.67 (4.6)
2.15 (1.5)
2.66 (2.1)
1.75 (3.1)
a The handedness of three subjects was not known.
VOLUMETRIC
ASYMMETRIES
AND ABILITY
21
significant. Perhaps, then, the observed asymmetry serves as an indicator
of the relatively greater efficiency or competence of one hemisphere. A
relatively larger right hemisphere might confer greater nonverbal ability,
because the hemisphere that is more specialized for nonverbal functions
is relatively more competent.
Reduced or reversed asymmetry of hemispheres has recently been
found to be associated with better recovery from global aphasia (Pieniadz,
Naeser, Koff, & Levine, 1983), and, in the same subjects, with better
recovery from hemiplegia (Schenkman, Butler, Naeser, & Kleefield, 1983).
The magnitude of initial postlesion impairment was not, however, related
to the degree of asymmetry. Subjects with reduced or reversed asymmetry
were no less aphasic or hemiparetic upon first testing. These results
could be interpreted in at least two ways. Subjects with reduced or
reversed asymmetry may have greater right hemisphere representation
of language and motor control. Henderson, Naeser, Pieniadz, and Chui
(1983) have recently noted that reversed asymmetries could not predict
crossed aphasia, raising doubts about the ability of CT asymmetries to
serve as a marker of language dominance. Alternatively, the relatively
larger right hemisphere of those patients who recovered better may simply
denote a more efficient or competent hemisphere. That is, in those patients
with a better recovery, the right hemisphere may have been better at
learning a new task, rather than having a greater degree of preexisting
linguistic or motor representation. This hypothesis is consistent with the
current results as well as the observation of no initial postlesion language
and motor differences in subjects with reduced or reversed asymmetry;
a difference in patterns of hemispheric representation should have been
manifest in differences in symptom severity upon initial testing.
No significant relationship was observed between total brain volume
and ability. Van Valen’s review (1974) noted that the median correlation
between measures of brain size and ability was r = . 11. As most of the
ability measures in the studies he reviewed were verbal in nature, the
most analogous correlation from the present study would be between
total brain size and VIQ, which was r = .12 (NS). The more direct
measures of brain size in the current study did not serve to reveal a
relationship of greater magnitude.
The volume of the left hemisphere was found to be greater than that
of the right. The only other CT study examining volumetric asymmetries
reported similar results (Zatz, Jernigan, & Ahumada, 1982). Autopsy
studies have observed either a slight left hemisphere size advantage
(Aresu, as cited by von Bonin, 1962)or no hemispheric difference (Miller,
Alston, and Corsellis, 1980). Several CT studies have examined asymmetries using linear measurements. A larger left occipital area has been
consistently observed (LeMay, 1977; Chui & Damasio, 1980; Pieniadz,
et al., 1983). A larger right frontal area has also been noted by some
22
YE0 ET AL.
investigators(LeMay, 1977)but not others (Chui & Damasio, 1980;Pieniadz
et al., 1983). These studies measured frontal and occipital widths and
lengths at discrete points near the tips of the hemispheres. In the current
investigation the entire cerebral hemisphere (or anterior or posterior
halves) contributed to the asymmetry measures.Together with the results
of Zatz et al. (1982),the current data suggestthat the greaterleft hemisphere
size is not limited to the occipital lobes.
Ventricular volume also showed a rather consistent asymmetry: 35 of
our subjects (85%) had larger left than right lateral ventricles. Zatz et
al. (1982) reported that 73% of their subjects showed this asymmetry,
also supporting earlier pneumoencephalographic studies (e.g., McRae,
Branch, & Milner, 1968).It is interesting to note that the right hemisphere
is often found to weigh more than the left (Crichton-Browne, 1880; von
Bonin, 1962). These findings are not necessarily discrepant from the
observation of greater left hemisphere volume. The asymmetry in ventricular size would serve to decrease the overall density of the left hemisphere, explaining why it does not weigh more, despiteits greater volume.
The major limitation of the present study lies in the nature of the
subjects. While neurological examination revealed no evidence of abnormalities and WATSscoreswere representativeof the generalpopulation,
the subjects did have more psychological and medical difficulties than
average. Partial correlation analysis revealed that the degree of psychopathology did not influence our results, and it is difficult to imagine a
systematic effect resulting from the heterogenous medical complaints of
the subjects, Definitive analysis of these issues must await replication
with normal volunteers. It seemsmost likely, however, that these subject
characteristics would serve to obscure relationships rather than introduce
a systematic bias. The major conclusion of this study, that anatomical
asymmetry is related to patterns of intellectual ability, appears to be
quite robust. This is suggested by both the magnitude of the observed
correlation in the present study and the fact that this relationship has
now been observed in three rather disparate samples.
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