SCIeNTIFIC REPORTS | 7: 16804 | DOI:10.1038/s41598-017-16827-y
The origin of human handedness
and its role in pre-birth motor
, Romain Brasselet
, Stefania Zoia
, Maria Bulgheroni
The vast majority of humans are right-handed, but how and when this bias emerges during human
ontogenesis is still unclear. We propose an approach that explains postnatal handedness starting from
18 gestational weeks using a kinematic analysis of dierent fetal arm movements recorded during
ultrasonography. Based on the hand dominance reported postnatally at age 9, the fetuses were
classied as right-handed (86%) or left-handed, in line with population data. We revealed that both
right-handed and left-handed fetuses were faster to reach to targets requiring greater precision (i.e.,
eye and mouth), with their dominant (vs. non-dominant) hand. By using either movement times or
deceleration estimates, handedness can be inferred with a classication accuracy ranging from 89 to
100% from gestational week 18. The reliability of this inference hints to the yet unexplored potential of
standard ultrasonography to advance our understanding of prenatal life.
If human beings are congenitally prepared to handedness, if they are – at least to some extent - wired to be right-
or le-handed as some genetic theories have proposed
, one might hypothesize that their motor system, before
birth, is capable of dierently programming and executing movements according to the hand used. If such a pre-
diction is correct, then the natural questions are: when do the rst signs of human handedness emerge and which
factors modulate it? Besides this basic research interest, answering these questions holds promise for research
aiming at the early detection and treatment of disorders characterized by atypical brain asymmetries, such as
schizophrenia or autism spectrum disorder
, pathologies to date lacking reliable biomarkers.
e advent of ultrasonography started the investigation of fetal lateralized motor behaviors, which appear
and reach full repertoire by GW14
. Ultrasonographic evaluations of the frequency of lateralized
thumb-sucking has been used as a proxy for postnatal hand dominance
, revealing a right-side handedness
population bias (~85%), conrmed also among other types of arm movements
. e denition of such a bias is
based on cross-sectional evidence
, leaving unclear whether motor lateralization is stable within the gestational
period as well as postnatally. Furthermore, analyses of handedness through movement frequency or the qualita-
tive evaluation of general movement patterns
may not fully characterize the role of handedness on lateralized
fetal kinematics. Indeed, fetal reaching shows a surprisingly advanced prociency in motor planning and con-
. Reaching toward the eyes, a target requiring highly precise movements (i.e., small and delicate), takes
longer and necessitates a prolonged deceleration phase as compared to reaching towards the mouth or the uterine
wall. Based on the known eects of handedness in postnatal life
, we expect the endpoint of an action to aect
the kinematics of lateralized arm movements even in utero.
To uncover the prenatal development of signatures of handedness, we explore goal-directed motor programs
using kinematic analysis, a technique able to characterize the spatio-temporal features of movements in utero
We longitudinally measured the arm kinematics of 29 fetuses by using four-dimensional ultrasonography. ree
types of movements performed with either the right (RH) or the le hand (LH) were isolated: two self-directed
hand movements to the eyes (Fig.1A) and to the mouth (Fig.1B) and an outer-directed movement [i.e., ngers
touching the uterine wall (Fig.1C)]. With this multi-target approach, we aim to determine whether and how the
International School for Advanced Studies (SISSA), Trieste, Italy.
Struttura Complessa Tutela Salute Bambini
Adolescenti Donne Famiglia, Azienda Sanitaria Universitaria Integrata di Trieste, Trieste, Italy.
Company, Milano, Italy.
Department of General Psychology, University of Padova, Padova, Italy.
Neuroscienze, University of Padova, Padova, Italy.
Centro Linceo Beniamino Segre, Rome, Italy. Correspondence
and requests for materials should be addressed to V.P. (email: firstname.lastname@example.org) or U.C. (email: umberto.castiello@
Received: 2 August 2017
Accepted: 17 November 2017
Published: xx xx xxxx