The left planum temporale is a marker of left hemisphere language specialization. We investigated the effect of individual handedness and familial sinistrality on left planum temporale surface area and found the size is reduced in proportion with the number of left-handed immediate family members and is lowest when one&#x27;s mother is left-handed. This reduction is independent of an individual&#x27;s handedness or sex and has no counterpart in the right hemisphere

Bernard Mazoyer

Emmanuel Mellet

Fabrice Crivello

Gregory Simon

Laure Zago

Laurent Petit

Nathalie Tzourio-Mazoyer

English

Nature Precedings

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Brain, language, and handedness: a family affair 
 
Bernard Mazoyer1,2,3, Gregory Simon1,2, Fabrice Crivello1, Laure Zago, Laurent Petit, 
Emmanuel Mellet, Nathalie Tzourio-Mazoyer1* 
 
1CI-NAPS, CNRS, CEA, Universités Caen & Paris Descartes,  
2CHU Caen, France 
3IUF Paris, France 
 
*corresponding author 
 
 
The left planum temporale is a marker of left hemisphere language specialization.  We 
investigated the effect of individual handedness and familial sinistrality on left planum 
temporale surface area and found the size is reduced in proportion with the number of 
left-handed immediate family members and is lowest when one’s mother is left-
handed.  This reduction is independent of an individual’s handedness or sex and has 
no counterpart in the right hemisphere. 
 
Left hemisphere dominance in language is unique to mankind.  Some have proposed it must 
be genetically encoded and its disturbance may be related to diseases such as 
schizophrenia1.  This salient feature of the human brain was discovered in 1861 by a French 
neurosurgeon, Paul Broca, after observing “loss of articulation” in a patient with a lesion of 
his left hemisphere.  One hundred years later, Geschwind and Levitsky discovered, on post-
mortem human brain pictures, a portion of the superior temporal cortex, named the planum 
temporale (PT), was larger in the left hemisphere compared to the right2.  As this piece of 
cortex was known to be involved in the processing of language sounds3, this asymmetry was 
thought to be a marker of left hemisphere dominance in language, which was later confirmed 
by modern neuroimaging techniques4,5.  At the population level, PT remains the only brain 
area which is consistently larger on the left side. 
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Since most human beings are right-handed, they also use the left part of their brain to 
move their right (preferred) hand.  This co-occurrence of left hemisphere dominance for hand 
and language led to hypothesize these two characteristics are inherited jointly.  However, 
studies of aphasic patients and measures of PT in left-handers have shown variable  
relationships between hand preference and language area asymmetry6.  As first postulated 
by Hécaen, factors other than one’s handedness may thus shape brain lateralization for 
language, including familial sinistrality (FS), the existence (FS+) or absence (FS-) of left-
handers among close relatives.  More than 30% of human subjects7 are FS+, which has 
been associated with less severe language deficits after left hemisphere brain lesions8 and, 
therefore, lower strength of hemispheric specialization for language.  
We investigated the effect of FS on PT surfaces to examine possible factors involved 
in left hemisphere dominance for speech (see Supplemental Methods and Results online).  
To test the hypothesis that a subject’s left PT surface area is dependent on the presence of 
left-handed relatives, we measured the surface of the left and right PT9 with brain magnetic 
resonance images obtained from 274 subjects.  In the left hemisphere, the PT surface area 
was reduced by 10% (P=0.03) in FS+ subjects (n=104) compared to FS- subjects (n=170).  
This effect was independent of subjects’ handedness, with both right and left-handed FS+ 
subjects having lower left PT areas (Table 1).  Moreover, the left PT size appears to 
decrease with the number of left-handed first degree relatives (P=0.04, adjusted for family 
size10) and to be lowest when one’s mother is left-handed (P=0.014, Fig. 1).  Notably, the left 
PT surface area was independent of the subject’s handedness and sex (Table 1).  In 
addition, FS had no effect on the right hemisphere PT size.  
To test the influence of FS on a quantitative measure of hand lateralization, subjects’ 
manual asymmetry was quantified with the finger tapping test (FTT).  Interestingly, FS had 
no significant effect (P=0.34), or interaction with sex (P=0.84) or handedness (P=0.23), on 
FTT asymmetry (see Supplementary Table 1 online).  As expected, the difference in manual 
skills between the preferred hand and non-preferred hand was larger in right-handers than in 
left-handers, due to poor left hand performances by right-handers (P< 0.0001, ANOVA). 
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The present results reveal familial sinistrality is a key factor in the anatomical 
development of the left temporal cortex phonological areas, affecting left hemisphere 
dominance for speech sound processing.  The presence of left-handed, immediate family 
members correlates with decreased leftward specialization for speech, independent of the 
individual’s handedness and without any effect on manual lateralization11.  This suggests 
different heritability mechanisms exist for handedness and hemisphere language dominance.  
This finding is in agreement with a report of familial aggregation, where the strength of the 
asymmetry of language functional networks was found to be independent of subject’s 
handedness12.  Recent genome-wide analyses, of human cortical patterning during 
development, have uncovered genes that shape the posterior temporal cortex and its 
asymmetries13.  Such genes are clear candidates for language hemispheric dominance 
heritability, but a link to the FS trait has not been demonstrated yet.  Other genes, including 
some linked to the X chromosome, are likely to be involved in hemisphere dominance as the 
offspring of left-handed mothers have the lowest PT surfaces areas.  
 
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(2002). 
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26-49 (1999). 
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(2000). 
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 13.      Abrahams,B.S. et al. Proc. Natl. Acad. Sci. U. S. A 104, 17849-17854 (2007). 
 
 
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Table 1. Planum temporale (PT) surface area according to sex (F: female, M: male), self-
reported handedness (RH: right-hander, LH: left-hander), and familial sinistrality (FS).  FS is 
defined as the presence (FS+) or absence (FS-) of left-handed close relative(s).  Values are 
mean ± s.d. 
 
Sex Handednes 
Familial 
sinsitrality
PT surface area (mm2) 
   Left  Right  
F 
RH 
FS- 631 ± 150 468 ± 153 
FS+ 552 ± 158 420 ± 132 
LH 
FS- 691 ± 134 411 ± 178 
FS+ 563 ± 128 478 ± 186 
M 
RH 
FS- 682 ± 208 478 ± 185 
FS+ 673 ± 164 503 ± 208 
LH 
FS- 718 ± 209 502 ± 201 
FS+ 619 ± 168 482 ± 196 
 
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Figure 1  Impact of familial sinistrality on the left planum temporale (PT) surface area.  (a) 
Outline of a left PT (in yellow) on the oblique slice tangential to the temporal lobe upper 
surface.  (b) Top: Left PT surface (mean ± s.d.) in subjects either without left-handed close 
relatives (FS-) or with a left-handed father, sibling, or mother (FS+).  Bottom: linear 
regression analysis of the left PT surface as a function of the number of left-handed relatives, 
adjusted for family size. 
 
 
 
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Supplementary Methods and Results 
 
Subjects  
 
All subjects gave their written informed consent and the study was approved by our local 
Ethics Committee.  The sample consisted of 274 subjects (199 males and 75 females), 
including 194 right-handers (134 men, 60 women) and 80 left-handers (65 men and 15 
women).  Handedness was self reported by the subjects.  The occurrence of left-handedness 
in our sample was higher in men (33%) than women (20%).  Familial sinistrality (FS), also 
self-reported by the subjects, was defined as the presence of at least one left-hander among 
the subject’s parents and siblings; 170 subjects had no left-handed relatives (FS-) whereas 
104 had at least one (FS-).  FS was independent of sex (P=0.41), handedness (P=0.38), and 
sex by handedness (P=0.19).  
The mean age was 24.3 years (s.d. = 6.1 years, range = 18-53).  Left-handers were 
significantly younger than right-handers (2.3 years difference, P = 0.024, ANOVA) with no 
other significant effect due to sex or FS.  The average education level of subjects was 14.7 
years (s.d. = 2.2 years, range = 9-20), with no significant effect of sex or FS, but a small 
significant difference in favor of right-handers (0.8 year, P=0.030, ANOVA).  
In the sub-sample of FS+ subjects, the average adjusted FS proportion (aFS)10 was 
0.22 (s.d. = 0.088) and independent of sex or handedness.  Occurrences of a single first 
degree relative were 28% for a brother, 23% for a mother, 13% for a father, and 12% for a 
sister.  No effect of sex or handedness was found on the distribution of the various possible 
left-handed relatives.  An effect of the parents’ gender was observed; 58 (55%) of the FS+ 
subjects had a left-handed parent, more often their mother (65%) than their father (35%), and 
the proportion of men having a left-handed mother was higher (73%) than for women (43%, 
P = 0.044).  
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 Evaluation of manual skill 
For each subject, the finger tapping test (FTT) was used to evaluate manual skill.  For each 
hand, tappings were recorded in five sessions of 10 seconds each.  The mean value is 
reported (Supplementary Table 1).  
 
 
Image acquisition and analysis  
 
Brain anatomical images were acquired on a 1.5 Tesla scanner (General Electric Signa) 
using a 3-dimensional T1-weighted sequence (0.9357 x0.9357x1.0 mm3 voxel size).  The PT 
surfaces were measured in the left and right-hemispheres after manual delineation, thanks to 
the knife cut method9.  All images were processed by the same highly-trained operator 
whose reproducibility was assessed in a subset of 174 subjects.  The average difference 
between the 2 measures was 10 ± 60 mm2 and -2 ± 66 mm2 and the correlation coefficient 
was 0.945 and 0.936, for the right and left PT, respectively. 
 
Statistical methods 
 
Analysis of covariance (ANCOVA) was used to assess the main effects of sex, handedness, 
and FS and their first order interactions on PT surface and FTT values.  A repeated measure 
design was used with “hemisphere” (left or right) as the within-subject factor.  Age, cultural 
level, and brain volume were included as covariates.  To study the relationship between 
intensity of familial sinistrality and PT surface, a second ANCOVA was performed in the 
subgroup of FS+ subjects using the adjusted familial sinistrality proportion (aFS) as a 
covariate.
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Supplementary Table 1.  Manual skill according to sex, self-reported handedness, and FS.  
Manual preference was assessed with the Edinburgh Inventory, manual skill with the Finger 
Tapping Test.  Values are mean ± s.d.  (F, female; M, male; RH, right-hander; LH, left-
hander; FS, familial sinistrality; FS+, presence of a close left-handed relative; FS-, absence 
of a close left-handed relative).  The number of subjects (n) is shown in parentheses. 
 
 Sex Handedness
Familial 
sinistrality
Manual skill 
    Right hand Left hand 
ALL 
(274) 
F 
(75) 
RH 
(60) 
FS- (38) 50 ± 4 43 ± 5 
FS+ (22) 50 ± 7 43 ± 6 
LH 
(15) 
FS- (10) 48± 6 50 ± 5 
FS+ (5) 48 ± 3 49 ± 2 
M 
(199) 
RH 
(134) 
FS- (90) 57 ± 6 50 ± 6 
FS+ (44) 56 ± 5 50 ± 5 
LH 
(65) 
FS- (32) 51 ± 6 54 ± 6 
FS+ (33) 54 ± 6 55 ± 7 
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Brain, language, and handedness: a family affair

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Brain, language, and handedness: a family affair

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