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Behavioural Brain Research 207 (2010) 447–451
Contents lists available at ScienceDirect
Behavioural Brain Research
journal homepage: www.elsevier.com/locate/bbr
Visual experience affects handedness
Sebastian Ocklenburga,∗, Corinna Bürgera, Christine Westermanna, Daniel Schneidera,
Heiner Biedermannb,1, Onur Güntürküna,1
aInstitute of Cognitive Neuroscience, Biopsychology, Department of Psychology, Ruhr-University of Bochum, 44780 Bochum, Germany
bPrivate Practice, Huhnsgasse 34, 50676 Cologne, Germany
a r t i c l ei n f o
Received 15 August 2009
Received in revised form 20 October 2009
Accepted 26 October 2009
Available online 4 November 2009
a b s t r a c t
In birds, a lateralised visual input during early development importantly modulates morphological and
a combination of inborn and experience-driven factors by analysing sidedness in children suffering from
congenital muscular torticollis. These children display a permanently tilted asymmetric head posture to
the left or to the right in combination with a contralateral rotation of face and chin, which could lead to
an increased visual experience of the hand contralateral to the head-tilt. Relative to controls, torticollis-
children had a higher probability of right- or left-handedness when having a head-tilt to the opposite
side. No statistical significant relation between head position and direction of functional asymmetries
was found for footedness and eye-preference, although the means show a non-significant trend in the
same direction as was observed for handedness. Thus, an increased visual control of the hand during
early childhood seems to modulate handedness and possibly other lateral preferences to a lesser extent.
These findings not only show that human handedness is affected by early lateralised visual experience
but also speak in favour of a combined gene-environment model for its development.
© 2009 Elsevier B.V. All rights reserved.
Like all amniotes, birds have a genetically determined embry-
onic preference for a right head turn . Inside the egg, this head
left eye is covered by the body and is therefore visually deprived
. This asymmetrical visual stimulation importantly affects the
just developing visual pathways and modulates the emergence
and organisation of visuocognitive asymmetries . Incubation
of eggs in total darkness prevents the development of anatomical
asymmetries within ascending visual pathways, while abolishing
or modulating behavioural left–right differences in object discrim-
ination [6,48,51]. Occlusion of the right eye and exposition of the
at birth, this reversal of lateralisation is only possible in embryos in
a period of time shortly before hatching [49,50]. In contrast, altri-
∗Corresponding author at: Abteilung Biopsychologie, Institut für Kognitive
Neurowissenschaft, Fakultät für Psychologie, Ruhr-Universität Bochum, Univer-
sitätsstraße 150, 44780 Bochum, Germany. Tel.: +49 234 32 26804;
fax: +49 234 32 14377.
E-mail address: email@example.com (S. Ocklenburg).
1These authors shared senior authorship.
at birth and still develops after hatching. Therefore, it is possible to
stimulation of the left eye and deprivation of the right eye . Just
a few days of early asymmetrical light input are enough to shape
behavioural asymmetries for the entire lifespan of the individual
behavioural asymmetry in humans is handedness. In contrast to
the development of behavioural asymmetries in birds, it is widely
assumed that environmental factors do not play a large role in
determining individual handedness and a number of different
single-gene models have been proposed to explain the distribu-
tion of left- and right-handedness [1,9,27,28,35,36]. Recently it has
been reported that the gene LRRTM1 on chromosome 2p12 is the
that is heavily discussed since it has been publicised [10,16,37].
Nevertheless, it is relatively undisputed that handedness is, to
a large extent, genetically determined. However, there are some
observations that cannot be readily explained by means of a
purely genetic model of handedness and therefore highlight the
importance of also integrating non-genetic factors into models of
handedness. For example, the frequent observation of discordant
environmental factors are taken into account , since exclu-
sively genetic models would predict genetically identical twins to
have identical handedness. Moreover, in societies where use of
0166-4328/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
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S. Ocklenburg et al. / Behavioural Brain Research 207 (2010) 447–451
the left hand is associated with bad luck or being unclean, such
as most Muslim African countries, there is a lower incidence of
left-handedness than in more liberal countries [12,53], showing
that cultural factors also influence handedness. Furthermore, in an
meta-analysis of a number of studies concerning handedness as
a function of period of birth Jones and Martin  found a sig-
nificantly higher incidence of left-handers among persons born in
spring and ensuing months (March to July in the northern hemi-
sphere) than among persons born during the rest of the year. A
ranging from differences in patterns of nutrition and disease to
seasonal variations in prenatal exposure to testosterone [26,42].
does not have a significant role in causing left- or right-handedness
[41,47]. However such purely non-genetic models of handedness
are unable to explain the dominance of right-handedness (despite
culture-dependent variations) in all human societies . There-
fore, taken together, the most promising approach seems to be an
integrative model that takes genetic as well as environmental or
primarily through parental influences have an impact on handed-
Interestingly, another possible process in which non-genetic
influences can shape handedness has been proposed. Humans are
amniotes just like birds are and, comparable to them, they also
display a preference for a right head turn before and after birth
. Indeed, it has been suggested that a right-sided bias in infants
may contribute to right-handedness by generating an asymmetry
in visual experience of the hands . This theory is supported
by findings that infants’ preferential head-turning direction corre-
lates with hand use . Although this strong infant right-turning
bias of the head has been reported to disappear at 15 weeks of
age , a similar turning bias is still visible in adults  and
correlates with adult handedness . Even though these studies
suggest a relationship between head-turning bias and functional
lateral preferences, they do not yield a causal proof. In principle,
the observed head–hand relation could also be explained by a cen-
tral asymmetry of motor organisation without further causal links
between its constituents. The best way to test the influence of a
skewed visual control on the development of handedness would
be to test children that are born with a permanent motor bias of
the head which subsequently increases the probability to visually
follow one hand. Children with congenital muscular torticollis pro-
vide this opportunity. In this congential orthopaedic condition, the
ing to an ipsilateral tilt and a contralateral rotation of face and chin
[13,20]. The head-tilt can be either to the left or the right side and
a head-tilt to the right is combined with a rotation of face and
chin to the left and vice versa. It has therefore to be noted that
this condition is not directly comparable to the right-sided head-
turning bias in healthy infants . Michel  assessed supine
head-orientation preferences in infants and in a supine position, a
head-orientation preference to the right side would clearly result
in an increased visual experience of the right hand. In torticollis
children, however, the head-tilt in combination with the rotation
of face and chin to the opposite side results in an increased visual
experience of the hand contralateral to the head-tilt (see Fig. 1).
In adults, no link between handedness and left or right torticol-
lis has been found . However, in this study, adult patients were
tested without asking if the condition was congenital or for how
of an underlying muscular, osseous, ocular, psychiatric, or neuro-
early sensitive period of plasticity for asymmetry , it is vital to
only incorporate participants for whom the condition exists since
delivery. Therefore, we assessed hand-, foot- and eye-preference
in children diagnosed with congenital muscular torticollis with a
right or left head-tilt and compared them to healthy controls. We
assume that children with a head-tilt to the right have a stronger
left-sided bias than the controls, whereas children with a tilt to the
left have a stronger right-sided bias than the controls.
A total of 117 children (54 girls and 63 boys) with a mean
age of 8.21 (SD=1.11, range: 7–11 years) were tested. The EG
(experimental group) consisted of 58 children which were newly
diagnosed with KISS (Kinematic imbalances due to suboccipital
strain) syndrome [2–4], a syndrome that includes congenital mus-
Participants were recruited in the private practice of the co-author
clinical assessment, shortly (less than half a year) before the test-
ing took place. Therefore the age of diagnosis was between the 7th
and 11thyear of life (depending on the participants age). To make
sure that the condition existed since birth, a diagnostic interview
with the parents including an inspection of baby pictures of the
participants (if available) for typical signs of congenital muscular
torticollis (see Fig. 1), was conducted before testing. The partici-
pants in the experimental group had no psychiatric, neurological,
muscular or orthopaedic disorders or any injuries that might have
caused an acquired torticollis  or might otherwise have influ-
enced functional lateralisation.
The experimental group was subdivided in EG Right (27 chil-
dren with a right head-tilt) and EG Left (31 children with a left
head-tilt). The CG (control group) consisted of 59 healthy children
recruited from local schools. Parents were present during test-
ing but were instructed not to interfere. All parents gave written
informed consent that their children were allowed to participate in
the study. The study was approved by the ethics committee of the
Fig. 1. A 4-month old boy with torticollis. The head-tilt in combination with the
of the hand contralateral to the head-tilt.
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S. Ocklenburg et al. / Behavioural Brain Research 207 (2010) 447–451
University of Bochum and all participants were treated according
to the declaration of Helsinki.
To test the children’s handedness, footedness and eye-
preference, they were asked to perform a number of different
behavioural tasks which were based on commonly used question-
naires to assess these lateral preferences in adults. The tasks used
to assess handedness were based on the Edinburgh Handedness
of paper; 2. Throw a ball; 3. Cut-out a nameplate using scissors; 4.
Use a toothbrush; 5. Cut a piece of play dough with a knife; 6. Use
a spoon to eat puffed rice; 7. Play a C-major scale on a xylophone.
To keep testing time and stress for the children to a minimum, the
three items ‘Drawing’, ‘Using a broom’ and ‘Opening a box’ of the
EHI were excluded from the test, since the remaining seven items
are sufficient to provide an internally consistent and valid measure
of handedness . Task seven was used to substitute the item
‘Striking a match’ of the EHI as an assessment of fine motor skills,
since the original item seemed unsuitable for children. The tasks
used to assess footedness were based on the Waterloo Footedness
Questionnaire . Children had to: 1. Kick a soccer ball towards a
goal; 2. Stand on one foot; 3. Smooth a carpet with one foot; 4. Step
up onto a chair; 5. Stomp on a fast-moving cloth ball; 6. Balance
on one foot on a wooden ledge; 7. Pick up a marble with the toes
of one foot; 8. Hop on one foot; 9. Stomp on the ground. The tasks
used to assess eye-preference were mainly obtained from Porac
and Cohen . Children had to sight through a hole in wooden
plate and down a kaleidoscope. Moreover, their dominant eye was
also assessed with the Miles test  and the Dolman test . The
counterbalanced across subjects.
For each behavioural test, a laterality quotient (LQ) was cal-
culated according to the method by Oldfield . The LQ’s range
was between −100 and +100, with positive values indicating a
right-sided preference and negative values a left-sided preference.
Participants were considered right-handed if the LQ was larger
than zero and left handed if it was smaller than zero. There was
no participant with an LQ of zero in the present sample.
No differencesin gender(?2(2)=1.08;
(?2(8)=8.06; p=.43) were observed between the groups. All
31 (100%) children in the EG Left were right-handed, whereas
in the in the EG Right 21 (77.78%) children were right handed
and the remaining 6 (22.22%) children were left handed. In the
CG 54 (91.53%) children were right-handed and 5 (8.47%) were
left handed (see Fig. 2). Children in EG Left were significantly
more often right-handed than those in the EG Right when those
two groups were compared directly (?2(1)=7.68; p<.01; effect
size ϕ=0.36). Also, when all three groups were compared, right-
handedness was significantly more prevalent in the EG Left
than in the CG and the EG Right (?2(2)=8.49; p<.05; effect size
ϕ=0.27). For footedness (?2(2)=1.69; p=.43) and eye-preference
(?2(2)=2.33; p=.31) no significant differences between the three
groups were observed.
To assess whether the three groups also differ in regard to con-
tinuously measured strength of their lateral preference, mean LQ’s
for handedness, footedness and eye-preference (see Fig. 3) were
For handedness, the means suggest that the children in the
EG Left (90.32) had a higher LQ than those in the CG (79.58),
which in turn had a higher LQ than the children in the EG
Fig. 2. Percentage of children that have a right-sided hand-, foot- or eye-preference
in the EG Left (white bars), the CG (grey bars) and the EG Right (black bars).
Fig. 3. Mean LQ’s for handedness, footedness and eye-preference in the EG Left
(white bars), the CG (grey bars) and the EG Right (black bars). Error bars show
Right (59.22). Children in EG Left had a significantly higher LQ
than those in the EG Right when those two groups were com-
pared directly (t(56)=−2.27; p<.05; effect size Cohen’s d=−0.61).
However, the overall effect when all three groups were tested
only approached significance (F(2,114)=2.74; p=.069). For footed-
ness (F(2,114)=0.72; p=.49) and eye-preference (F(2,114)=1.41;
p=.25) no significant differences between the three groups were
The primary finding of the present study was that increased
visual experience of one hand affects an individual’s handedness.
Relative to controls, torticollis-children had a higher probability of
right- or left-handedness when having a contralateral head-tilt in
trol group, 91.5% of the children were right-handed which is nearly
the same distribution as in the overall population . A higher
incidence of right-handedness was observed in children with a
head-tilt to the left and a rotation of face and chin to the right,
since a remarkable 100% of them were right-handed. In contrast,
the probability of being right-handed was reduced to 78% in chil-
dren with a head-tilt to the right and a rotation of face and chin to
the left. Therefore, more than twice as many as the about 10% one
would expect in the overall population  of the children in this
group were left-handed. No statistical significant relation between
head position and direction of functional asymmetries was found
for footedness and eye-preference. However, it should be noted
that the means (see Fig. 2) show a non-significant trend in the
same direction as was observed for handedness, at least when the
two experimental groups were compared, since there was a higher
Author's personal copy
S. Ocklenburg et al. / Behavioural Brain Research 207 (2010) 447–451
number of participants with a rightward preference for footedness
and eye-preference in the EG Left than in the EG Right.
A similar pattern emerged for the LQ’s. Torticollis-children with
a head-tilt to the left and a rotation of face and chin to the right had
the right and a rotation of face and chin to the left when those two
groups were compared directly. Although the overall difference
only approached significance when all three groups were com-
pared, there was a non-significant trend that torticollis-children
with a head-tilt to the left had higher LQ’s than controls and that
torticollis-children with a head-tilt to the right in turn had lower
LQ’s than controls. As for the absolute preferences, no statistical
significant relation between torticollis and LQ was found for foot-
edness and eye-preference was observed, but again, there seems
to be a non-significant trend in the same direction as was observed
The finding of an influence of torticollis on handedness in chil-
did not find such an influence in adult patients. However, these
authors did not account for whether the torticollis was congenital
are early sensitive periods of plasticity for asymmetry . It may
be possible that some of the participants tested in the Stejskal and
cates that there may be a sensitive period early in life after which
torticollis does not have an impact on handedness.
The present results clearly show an impact of a non-genetic,
experience based factor on handedness. In principle, this is in line
with research on the development of asymmetries and lateralised
preferences in other vertebrate species, such as birds in which
visual stimulation of one eye during early development is critical
for the development of functional and morphological asymmetries
. However, it is clear that the mechanism reported here is not
identical to that in pigeons or chicks. While the lateralised visual
the two hemispheres of the brain, lateralised visual control of the
left or the right hand due to torticollis is assumed to affect the hand
to this system.
There is also evidence for an impact of a non-genetic experience
based factor on behavioural asymmetries in non-human mam-
mals. For example, pawedness in mice is strongly influenced by
an environmental bias that favours a left- or right-sided prefer-
ence . Only about 10% of the mice in this experiment showed
a paw preference inconsistent with the environmental bias. How-
observed a larger genetic than non-genetic effect in the present
data, as 78% of the children with a head-tilt to the right and a rota-
tion of face and chin to the left were right-handed, although their
visual experience of their hands would favour left-handedness.
Therefore, a model incorporating both genetic and non-genetic
the present data. Such a model has been developed by Laland et al.
the probability of becoming left- or right-handed is influenced by
D alleles is than calculated by adding p, a factor that represents the
being right-handed. In DC individuals p is weighted by a parameter
h specifying the dominance of the two alleles and in CC individuals
p is excluded from the formula. In addition, they also assume that
parental handedness influences children’s handedness. In children
in right-handedness caused by having these parental influences is
also added to the formula. In children with two left-handed par-
mixed handedness a factor ? representing the changes in handed-
findings may add to the Laland model as they show that other
non-genetic factors than familial handedness have an impact on
handedness. Specifically, we would suggest that the values for the
handedness, but also take early asymmetries in visual experience
of the hands, as might be caused by head-turning preferences, into
account. This view is in accordance with the model of Michel 
who suggested that side biases in infants’ head-turning prefer-
ences generate an asymmetry in visual experience of the hands
that in turn influences handedness. Also, there is other experimen-
tal evidence for this assumption, such as the correlation of infants’
preferential head-turning direction with hand use  or the find-
ing that preferred head-turning direction during kissing in adults
is related to handedness .
edness is not limited to humans. A preference for a right head turn
and in chimpanzees it has been reported to be predictive of juve-
nile hand preferences . Based on these findings in primates,
Hopkins  developed a gene-environment model of handedness
that is very similar to the one we propose, namely that early posi-
tioning biases such as head-turning preferences lead to differential
stimulation of the two hands and that this, in turn, influences later
a very slight head-tilt with about 67% having the left eye higher
and 33% the right eye higher . However, it is not possible
to assess the exact impact of these minor degrees of asymme-
try on handedness in healthy populations based on the present
dataset. Torticollis-children exhibit a permanently fixed head-
posture, whereas in the overall population a clear head-turning
preferences are only evident in specific situations, such as if an
infant is brought into a supine position or during kissing. Also, it
is not known how often and how long normal adult exhibit the
asymmetries in eye height reported by McManus and Tomlinson
 in everyday life. This probably results in stronger asymmetries
in visual experience of the hand in torticollis-children than in the
Taken together, our findings showed an effect of visual expe-
rience of the hands on handedness and possibly on other lateral
preferences, which is not possible to explain with a purely genetic
approach. Therefore, the present data strongly suggest a combined
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