Associations of Brain Size and Verbal Performance Depend on Handedness
Christine
1
Chiarello ,
Suzanne
1
Welcome ,
University of California,
Stephen
2
Towler ,
1
Riverside ,University
Ronald
of Florida,
3
Otto ,
& Christiana M.
2
Gainesville , CDIC
2
Leonard
3
Riverside
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CONSISTENT HANDERS
Method
PARTICIPANTS:
• 100 male, 100 female native English speakers
• 18-34 years of age
• 103 consistent-handed (exclusive preference for one hand), 97 mixed-handed
PSYCHOMETRIC MEASURES:
• Verbal IQ estimated from Wechsler Abbreviated Scale of Intelligence (Wechsler, 1977)
• Reading subscales from Woodcock Reading Mastery Test - Revised (Woodcock, 1998)
• Word Identification (word reading)
• Word Attack (nonword reading)
• Passage Comprehension
DIVIDED VISUAL FIELD TASKS:
Composite Average score (averaged over LVF and RVF) and Composite Asymmetry
score across the following tasks calculated separately for accuracy and reaction time:
• Lexical Decision
• Masked Word Recognition (2 AFC procedure)
• Word Naming
• Nonword Naming
• Semantic (manmade vs natural) Decision
• Verb Generation
• Category Generation
BRAIN MEASUREMENTS FROM MRI:
• Gray, white and cerebrospinal fluid (csf) volumes of each cerebral hemisphere were
estimated by outlining every fifth sagittal image starting at the midline. The brainstem was
excluded by transection in the midcollicular plane. Inter-rater reliability of this measure is
> .98 (intra class correlation).
 As shown below, correlations of brain volume and verbal IQ for the
sex/handedness groups revealed positive correlations for all groups,
contrary to Witelson, et al. (2006). Indeed, the strongest correlation
was observed for mixed-handed males, the group for which Witelson,
et al. failed to find an association (significant correlations in bold).
ConsistHanders
Mixed Handers
Males
.271
.395
Females
.302
.246
Correlations of total brain volume with each of the performance
measures are shown below by handedness group (significant
correlations in bold).
ConsistHanders MixedHanders
NONLATERALIZED MEASURES
Word Identification
.181
.270
Word Attack
.118
.112
Passage Comprehension
.096
.261
Composite Lexical Accuracy
.077
.266
Composite Lexical RT
.078
-.135
LATERALIZED MEASURES
Composite Lex Accuracy Asym
.194
-.017
Composite Lex RT Asymmetry.280
-.081
• In consistent-handers only, as brain volume increases, so does
asymmetry of lexical processing - larger volume is associated with
greater left-hemisphere processing advantages (figure, left)
• In mixed-handers only, as brain volume increases, nonlateralized verbal
performance increases (figure, right)
• Most of the correlations remained significant when either gray or white
matter volume was examined
• Because the handedness groups were not perfectly balanced by sex
(consistent-handers: 44 male, 59 female; mixed-handers: 56 male, 41
female), we partialed out the effects of sex using multiple regression and
examined the percent of variance in brain volume accounted for by
performance in the two handedness groups. Most of the correlations
reported above remained significant when the effects of sex were
removed (italicized in the table above).
Acknowledgment
This research was supported by NIDCD grant 5R01DC6957.
1.5
R2 = 0.0784
RT ASYMMETRY
Brain volume varies substantially across individuals, and we recently
demonstrated that individual differences in brain volume, in both men and
women, account for sex differences in the relative sizes of various brain regions
(Leonard, et al., in press). Brain volume is known to correlate with some
measures of cognitive performance. Witelson, Beresh, & Kigar (2006) reported
that verbal IQ correlated with brain size in women and in consistent right-handed
men, but not in mixed-handed men. These authors suggested that functional
asymmetry, indexed by handedness, moderated the relation between verbal
intelligence and brain size in men, but not in women.
Here we re-examine the relationship between brain volume, handedness,
and verbal performance using a wider range of verbal measures (VIQ, reading
subtests, RT and accuracy on 7 lexical tasks), and behavioral measures of
language lateralization in a large sample of young adults. We included lateralized
tasks in order to have a measure of functional asymmetry that is more directly
related to language processing.
Results
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R = 0.0065
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R2 = 0.0707
R = 0.0051
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AVERAGE ACCURACY
Introduction
MIXED HANDERS
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BRAIN VOLUME
Conclusions
• We did not replicate the prior finding of Witelson, et al. (2006) that mixed-handed
males fail to show a VIQ-brain volume association. Our sample size was nearly
twice as large, but had a more restricted age range. We also used a more stringent
criteria for consistency of handedness.
• However, handedness does appear to moderate the relationship between brain
volume and verbal performance. For mixed-handers, greater brain volume is
associated with higher scores in word recognition and comprehension, but not with
asymmetry. In contrast, for consistent-handers greater brain volume is not
associated with better verbal performance, but rather with increased left hemisphere
specialization.
• Is verbal performance better in those with a larger brain? For those with a strong
phenotypic expression of handedness, the answer is no. However, larger brains
appear to function more asymmetrically, as suggested by some (Ringo, et al., 1994).
• In contrast, for those with more weakly expressed handedness, larger brains are
associated with better verbal performance, but not necessarily more asymmetric
performance.
• Consistency of handedness appears to be an important moderator of brainbehavior relationships.
References
Leonard, C.M., Towler, S., Welcome, S., Halderman, L.K., Otto, R., Eckert, M.A., & Chiarello, C. (In press).
Size matters: Cerebral volume influences sex differences in neuroanatomy. Cerebral Cortex.
Ringo, J.L., Doty, R.W., Demeter, S., & Simard, P.Y. (1994). Time is of the essence: A conjecture that
hemispheric specialization arises from interhemispheric conduction delay. Cerebral Cortex, 4, 331-343.
Wechsler, D. (1999). Wechsler Abbreviated Scale of Intelligence. San Antonio, TX: The Psychological
Corporation.
Witelson, S.F., Beresh, H., & Kigar, D.L. (2006). Intelligence and brain size in 100 postmortem brains: sex,
lateralization and age factors. Brain, 129, 386-398.
Woodcock, R.W. (1998). Woodcock Reading Mastery Test-Revised Normative update (WRMT-R). Circle
Pines, MN: American Guidance Service, Inc.