Louise Rönnqvist
Erik Domellöf
Department of Psychology, Umeå
University, SE-901 87 Umeå, Sweden
E-mail: louise.ronnqvist@psy.umu.se
Quantitative Assessment of Right
and Left Reaching Movements in
Infants: A Longitudinal Study
from 6 to 36 Months
ABSTRACT: This longitudinal study aimed to explore the early presence and
developmental pattern of laterality in reaching kinematics and its relationship to
side use. In order to do so, 3-D kinematic measurements as well as 2-D video
recordings of right-left reaching movements were successively carried out for
17 infants at the ages of 6, 9, 12, and 36 months. Additional investigations of hand
preference were made at 36 months. As four infants were prematurely born, their
outcomes were compared to those of the fullterm participants. While most of the
infants in the early ages showed a rather inconsistent preference in terms of
frequency and distributions of right-left side use, the analyses of reaching
kinematics revealed a more consistent pattern of fewer movements units (MUs) and
straighter right-sided reaching for the majority of infants at all tested ages.
However, reaching kinematics from the preterm infants were generally more
variable and less side consistent. It is proposed that the development of human
handedness originates from an early right arm rather than hand preference in that
representations of asymmetry in bilateral projections (involved in arm movements)
developmentally precede contralateral projections (involved in refined hand/finger
movements). ß 2006 Wiley Periodicals, Inc. Dev Psychobiol 48: 444–459, 2006.
Keywords: laterality; handedness; arm preference; kinematics; infants; motor
development; reaching; preterm
Human handedness is associated with differences in
specialization and anatomical structure between the
hemispheres of the brain, and thought to originate from
evolutionary, genetic, environmental, and experiential
factors. However, even though a large number of studies
and theories during the last decades have been trying to
identify and describe the handedness phenomenon and its
origin (see e.g., Corballis, 2003; Hopkins & Rönnqvist,
Received 5 May 2006; Accepted 20 May 2006
Correspondence to: L. Rönnqvist
Contract grant sponsor: Swedish Research Council
Contract grant number: 421-2001-4589
Contract grant sponsor: Norrbacka-Eugenia foundation
Contract grant number: 239/02
Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/dev.20160
ß 2006 Wiley Periodicals, Inc.
1998; Previc, 1991; Provins, 1997; Toga & Thompson,
2003, for reviews), we still lack substantive understanding
of the underlying processes for how hand preference
actually develops during the first years of life. An answer
to this question would provide valuable clues as to how the
brain develops and becomes organized the way it is.
To date, few longitudinal studies have been made with
a focus on side differences related to the developmental
organization and changes of early arm/hand movements.
This is especially true for more specific computer-aided
investigations of kinematics involved in both right and left
reaching activity of young infants as well as of infants
born at-risk for developmental delays. Thus, the main
purpose of the present study was to longitudinally
investigate reaching kinematics in young infants (both
fullterm and moderately preterm) in order to determine to
what extent right-left side differences in kinematic
parameters are present early, consistent, or develop during
Developmental Psychobiology. DOI 10.1002/dev
the infants’ first years to become more evident and stable.
The infants were first investigated at 6 months, an age
when the first fast developmental phase of reaching has
ended and most infants perform relatively stable prehension movements (von Hofsten, 1991), and additionally at
9 and 12 months of age. The final investigation was carried
out when the infants had reached 36 months of age, when a
stable hand preference can be expected (Corbetta &
Thelen, 1999; McManus et al., 1988).
In one of the few longitudinal studies carried out by
means of kinematic registrations of both right and left arm
movements (Corbetta & Thelen, 1996, 1999), it was found
that lateral biases in both reaching and spontaneous arm
movements were unstable and fluctuated in four infants
studied throughout the first year. In these investigations,
differences regarding right-left hand activity and the
average speed performed by each hand were analyzed
and further computed by means of a laterality index.
Although no stable pattern of side preference was found
by means of kinematic parameters, either for the
spontaneous arm movements or for the reaching performances, all infants showed a short period of a right-sided
preference at some point during the first year. These
temporary right-sided biases were later found to be
matched with the infants’ right-handed preference at 3
years of age (Corbetta & Thelen, 1999). In another study,
based on 2-D video scorings of reaching movements
performed by infants from 20 to 32 weeks of age, a shorter
movement time, a more direct hand trajectory, and fewer
corrections of the right hand in comparison to the left were
found on a group level (Morange-Majoux, Peze, & Bloch,
2000).
Most studies so far, however, have been made by
simply observing the frequency of young infants’ right
and left hand use during different tasks in cross-sectional
age groups, and with a lack of congruent outcomes. For
instance, in one study investigating infants from 3 to
8 weeks, the majority (14 of 15) showed a consistent leftsided bias for number of reaches toward targets presented
at different positions from the body midline (McDonnell,
Anderson, & Abraham, 1983). In another, a stable rightsided preference for reaching during the first 6 months of
life was found (Michel & Harkins, 1986). Moreover,
infants with a hand-use preference (independent of side)
for reaching and grasping an object at 7, 9, and 11 months
have been found to display a corresponding side bias for
unimanual manipulation activities (Hinojosa, Sheu, &
Michel, 2003). For closely spaced time intervals around
6 months of age, others have reported large changes in the
classification of hand preference (McCormick & Maurer,
1988). The same picture of fluctuating asymmetries was
found in a longitudinal study from 6 to 12 months (Carlson
& Harris, 1985). Fluctuating arm preferences have also
been noticed in terms of disappearing lateral preferences
Side Differences in Infant Reaching Kinematics
445
during time periods when infants repeatedly use both arms
to attain a target (Fagard & Pezé, 1997). However, beyond
the first year both cross-sectional and longitudinal
outcomes point to the majority of healthy infants attaining
a relatively consistent right-hand preference across a
range of tasks (Archer, Campbell, & Segalowitz, 1988;
Gottfried & Bathurst, 1983).
This inconsistency in findings during the first year of
life may be a consequence of the difficulty in assessing
hand preference within this early time period, in part due
to the varying level of infants’ manual ability over the first
year of age. Furthermore, it is not clear how well such
information describes actual hand-use preference as
measured by frequency or pattern of use (Steenhuis &
Bryden, 1999). The lack of consistency between studies
and fluctuations within longitudinal studies may also be a
consequence of the different assessments and/or ageadjusted criteria used. In addition, a lack of sufficient
sensitivity in the measurement techniques used, and the
choice of parameters to be analyzed could be a part of the
explanation. It has also been suggested that individual
inconsistency and fluctuation in arm and hand preference
during the first year may simply be a result of a poor arm
control that might interfere with an actual preferred lateral
bias during the infants’ first year (Corbetta & Thelen,
1999). A related explanation is that there is an interaction
between hand use and early postural and locomotor
reorganizations during this particular time period of infant
development (Corbetta & Bojczyk, 2002; Corbetta,
Williams, & Snapp-Childs, 2006).
Several investigations of infants’ reaching (Berthier,
Clifton, McCall, & Robin, 1999; Fetters & Todd, 1987;
von Hofsten, 1979, 1991) and grasping movements
(Kuhtz-Buschbeck, Boczek-Funcke, Illert, Joehnk, &
Stolze, 1999; von Hofsten & Rönnqvist, 1988) have shown
that kinematic recordings are sensitive for investigating
age-related changes in healthy, typically developing infants
and children. Findings include that the sequential structuring of the reaching trajectory becomes more systematic,
faster, and smoother over time, and that infants’ ability to
anticipate the finger grip in relation to the object size
becomes evident around 9 months of age. Kinematic
parameters have also been found to be sensitive for
identifying deviations in reaching movements of preterm
infants and children with neurological dysfunctions (e.g.,
Chang, Wu, Wu, & Su, 2005; van der Heide, Fock, Otten,
Stremmelaar, & Hadders-Algra, 2005). However, none
of these studies have been concerned with right-left
side differences related to the kinematic parameters
investigated, nor presenting data of the infants’ frequency
of right-left arm/hand use.
To this date, being perhaps the only study to have
compared in detail the 3-D kinematics of both arms during
reaching in 6-month-old infants, Hopkins and Rönnqvist
446
Rönnqvist and Domellöf
(2002) found an expression of a lateral bias that consisted
of less segmented and smoother right arm reaching
movements (in terms of fewer MUs) in comparison to left
arm. In conjunction with the fact that no hand preference
for grasping or contacting the object was found, this raises
an interesting point about the nature of the early
development of handedness. One important fact is the
proximodistal nature of neural development. In mammals,
the ventromedial pathways develop before the direct
corticospinal system (Kuypers, 1981, 1985; Martin,
2005). These pathways contain the vestibulospinal tract
that projects bilaterally to the spinal cord and controls the
proximal muscles of the arm. Thus, the finding of an early
lateral bias in right reaching movements rather than a
hand preference for grasping suggests that the initial
manifestations of lateral structuring in infants’ reaching
should be regarded as primarily indicative of an
(proximal) arm rather than a (distal) hand preference
(Hopkins & Rönnqvist, 1998, 2002; Rönnqvist, 2003). It
is well known that in the great majority of human adults,
the left hemisphere controls predominantly the movements of the right hand. The developmental question then
is when a lateralized bias in the functioning of the
ventromedial system (i.e., in control of the arm movements) initiates a bias in the corticospinal system (i.e., in
control of the more distal hand movements)?
The aim of the present study was to longitudinally
explore the kinematic characteristics of both right- and
left-sided reaching movements made by young infants. To
do so, we studied 17 infants over the ages 6, 9, 12, and
36 months. Four of the infants were moderately preterm
and 13 were born within a normal gestational age. At
the first three ages, kinematics of right and left successful
reaching movements (i.e., only reaching ending up with
grasping the object) were recorded and a number of
kinematic parameters analyzed. In addition to the
kinematics, the frequency and the distribution of each
infant’s hand use were calculated. Taken together, these
measurements allowed for an investigation of the
relationship between right-left differences in reaching
kinematics and a developmental change in the amount of
right or left hand use/preference to be made.
Based on the theory that arm/hand preference is a
biologically rooted proximal to distal developmental
process, and on the previous finding by Hopkins and
Rönnqvist (2002) of fewer MUs in the right than the
left arm during reaching movements in 6-month-old
infants, we predicted that the spatiotemporal reaching
pattern of the right arm/hand would differ from the left.
Thus, we expected to find less segmented and straighter
right-sided reaching patterns compared to the left in the
majority of infants when successively tested during the
second half of their first year of life. In addition, to further
our understanding of the developmental changes we
Developmental Psychobiology. DOI 10.1002/dev
continued to explore side differences in the same infants
at 36 months. At this age, both the kinematics of their
reaching movements with regard to side differences in
kinematic outcome parameters as well as their arm/hand
preference on three items were investigated. In keeping
with previous reports of established hand preference in
children of this age, we expected to find a stable arm/hand
preference in the majority of the infants, especially for
more demanding tasks.
As the present study includes a sample of prematurely
born infants, an additional objective was to explore
whether dissimilarities in patterns of laterality during
reaching exist depending on birth condition. Preterm
children are a population known to be at-risk for
developing cerebral palsy (CP) and other motor and/or
cognitive problems. Moreover, it is not uncommon that
problems such as CP in preterm children with no apparent
insult to the brain are not detected until later ages
(Myklebust & Gottlieb, 1997). It is further known that
there is an overrepresentation of left- and nonrighthandedness in ex-preterm children (e.g., Marlow, Roberts,
& Cooke, 1989; O’Callaghan, Burn, Mohay, Rogers, &
Tudehope, 1993). Thus, longitudinal studies by means of
kinematic measurements involving both fullterm and
preterm infants could be important in providing more
knowledge about the salient features that might be causing
deviant motor behavior, including deviations in siderelated behavior.
Based on previous findings and reviewed studies, the
following questions were addressed:
(1) Does a lateral preference exist in terms of the
frequency of arm/hand use during successful reaching-grasping in young infants?
(2) How do young infants’ right and left reaching
movements differ in terms of spatio-temporal organization and structuring?
(3) If a side difference exists already at 6 months of age in
terms of less segmented and smoother reaches by the
right arm, how consistent is this side difference over
the second half of the first year?
(4) How are the kinematic characteristics of right-left
reaching movements related to the individual infants’
frequency and distribution of actual arm/hand use?
(5) How is hand preference for different arm/hand
activities at 36 months of age related to the kinematic
characteristics of right and left reaches during the
second half of the first year?
Finally, an additional question was posed in relation to
the inclusion of both fullterm and preterm infants in the
present study:
(6) Do kinematic characteristics and side differences in
reaching movements diverge depending on birth
condition?
Developmental Psychobiology. DOI 10.1002/dev
METHODS
Participants
Seventeen infants (14 boys and 3 girls) participated in the study.
Thirteen of these infants were fullterm (mean gestational
age ¼ 40.2 weeks, range 38–42) and four were moderately
preterm (mean ¼ 35.1 weeks, range 33–36). All infants were
healthy, with no known sensory, motor, or neurological
impairments. The study was approved by the Ethical Review
Board of the Swedish Council for Research in Humanities and
Social Sciences, and all parents gave their informed consent. The
infants were observed longitudinally at the age of 6 (M ¼ 6.2),
9 (M ¼ 8.9), 12 (M ¼ 12.1), and 36 months (M ¼ 35.7). At the
first three observations (6, 9, and 12 months), the four preterm
infants were observed at their corrected age (1 week). One
family moved within the time period of this study, hence data
from 16 children were collected at 36 months.
Experimental Set-Up and Procedure
Prior to each recording session the infants were familiarized with
the laboratory setting. Before the recordings started, a 10 mm3
reflective marker was attached to the right and the left wrist. At
the ages 6, 9, and 12 months, the reaching targets used consisted
of a set of six differently colored, easy graspable spheres (4 cm in
diameter), each with a small bell inside. All spherical objects
were mounted on a base construction (5 cm long, 2 cm in
diameter) equipped with reflective tape (1 cm wide) to enable
identification of the exact time of hand-object contact. Each
spherical object could be attached to a specially constructed
displayer by means of a magnet fixated to the base, and was
presented at the infant’s shoulder height in three possible
positions: right, left, or midline. The distance between the object
centers of these three positions was 12 cm (36 cm in total).
To make the reaching and grasping task more challenging
when the infants were tested at 36 months of age, the targets
consisted of six differently colored pegs (1.2 cm3, 10 cm long),
placed in a pegboard at right, left, or midline position (12 cm
between position centers). The pegboard was securely fastened
to a table in front of the infant. On top of each peg, a reflective
half-spherical marker (1 cm3) was fastened to enable identification of the exact time of hand-object contact. The seating
conditions were age adjusted by means of using three different,
specially designed infant chairs (when tested at 6 and 9 months of
age the same chair was used). The distance between the infant
and the placement of the object was individually adjusted
enabling the object to be reachable at an extended arm length
when the infant was appropriately seated in the chair.
When the infant was judged to be in an alert and optimal state
for testing, the experimenter attached the object on to the displayer
(at 6, 9, and 12 months) or to the pegboard (at 36 months) in front
of the infant. The object to be presented was hidden from the infant
until it was positioned. At least four presentations in each of the
three positions (right, left, midline) were given, until the infant no
longer was interested in reaching and grasping. The presentation
order in relation to the three positions was randomized but
counterbalanced in terms of number of object presentations per
position (block of 12 trials; 4 trials 3 positions). If the infant’s
Side Differences in Infant Reaching Kinematics
447
mood and interest allowed, a second block of 12 trials was
presented directly following the first when tested in the second
half of the first year (thus, to optimize the number of reliable
recordings). In addition to the kinematic recording session when
tested at the age of 36 months, the hand preference of each infant
was also assessed in terms of frequency of hand use for three
different items: precision throwing (picking up a ball and throwing
it at a specific goal), drawing (picking up a pencil and drawing on a
paper), and hammering (picking up a hammer and nailing in eight
plastic spikes to a child toy hammering plate). Five trials of each
item were performed and hand use for each trial identified. The
object to be used (ball, pencil, hammer) was always presented in
midline.
Kinematic Recordings
Kinematic data were recorded by means of a six-camera
ProReflex system (Qualisys, Inc., Gothenburg, Sweden). Each
camera monitored the reflective wrist markers at a frame rate of
240/s. Recordings were triggered by pressing a hand-held external
trigger, always occurring when the infant’s hand(s) contacted the
object. The pretrigger function allowed the cameras to continuously fill their buffer memory with data until the recording was
triggered. The frames from the predefined pretrigger time (2 s)
were then added to the captured frames for the measurement.
Thus, recording onset started 2 s prior to hand-object contact,
ensuring that the entire reaching movement was registered. Only
kinematic data from successful reaching movements (ending up
with grasping of the object) were collected.
Although the starting position and the choice of hand was
unrestricted when tested at the ages of 6, 9, and 12 months, the
infant’s hands had to be at rest when the object was placed in the
position to be reached and grasped for. When tested at 36 months,
the infant’s hands were initially situated in a right-left start
position (approximately 24 cm between each hand, with the finger
tips 18 cm from the target object) indicated by colored tape on the
table. Prior to each trial, the experimenter both verbally informed
the infant about, as well as actually pointed at, the hand to be used.
At all tested ages the total recording time for each trial was set to
4 s. The 2-D data from each camera were stored on a computer,
with the third dimension being reconstructed at a later stage using
a direct linear transform algorithm (MacReflex Software).
All measurement sessions were also simultaneously video
recorded. These recordings served three functions. Firstly, if
questions concerning the reaching movement arose during
the analysis of the ProReflex data, they made it possible to
double check. Secondly, they were used in combination with the
kinematic recordings (velocity profile and spatial displacement
of the wrist marker) to identify the precise onset of a reaching
movement in relation to the hand positions at start. Thirdly,
they enabled the scorings of the amount of both successful
(i.e., grasping the object) and nonsuccessful (i.e., not grasping
the object) reaches.
Data Scorings
First, the scorings based on the video recordings and the 3-D
kinematics from each testing session at the ages of 6, 9, and
448
Rönnqvist and Domellöf
12 months were made and the number of successful and
nonsuccessful unimanual right, unimanual left, and bimanual
reaches and grasps identified. Following a bilateral reach, the
hand first contacting the object was identified. A bilateral reach
was coded when both hands started to move within maximally
500 ms of each other toward the object, with one hand contacting
or grasping the object and the other hand within <5 cm
distance from the object. All other arm movements culminating
in object contact/grasp were treated as unilateral reaches.
When tested at the age of 36 months, the infants were able to
follow instructions about which hand to use during the
kinematic recording sessions and therefore these scorings were
redundant. Scorings of arm/hand preference on the three items
(throwing, drawing, hammering) tested at 36 months were done
by means of direct observation and then noting which hand the
child employed for picking up and using the object. Only two
children used different hands for pick up and use, and this
only occurred during trials 1 and 3, respectively for one item
(drawing).
The analyses of kinematic parameters from the first three
testing sessions (at the ages 6, 9, and 12 months) are based on
588 reliable recordings of successful reaching and grasping trials
(128 at 6, 228 at 9, and 232 at 12 months) out of a total of
824 trials. An additional 325 nonsuccessful reaching trials were
identified from the video recordings, that is, trials when the
infants’ reaches were incomplete, too short, and/or when they
missed the object and consequently did not end up in grasping it.
The majority of these nonsuccessful reaching trials were
performed at the age of 6 months (195, 37.2% with the right,
48.6% with left, and 14.2% bimanual), 96 at 9 months (42.4%
with right, 46.4% with left, and 11.2% bimanual), and 34 at
12 months (51.4% with right, 46.3% with left, and 2.3%
bimanual). At 36 months, the number of reliable kinematic
recordings of successful reaching and grasping trials was 166 out
of a total of 192. No nonsuccessful reaches were identified at
this age. In total 132 trials (106 at 6, 9, and 12 months, and 26 at
36 months) had to be excluded from further analysis due to
marker(s) being out of the view of the cameras, the reach onset
too early to capture in relation to the predefined pretrigger time
of the measurement system, and in a few cases when an infant
was reaching/grasping without visually focusing on the object
(i.e., looking in another direction than at the object).
Kinematic Outcome Parameters and Criteria
All kinematic-derived parameters were smoothed by means of a
second-order 10 Hz dual pass Butterworth filter, and analyzed in
MATLAB (The MathWorks, Inc., Boston, MA). For every reach,
onset time, the time of hand-object touch, and the time of hand
offset of the reach phase (time between first hand-object touch
and the time to establishing the grasp) were identified. These
measures were derived from the 3-D spatial plots (Fig. 1A) and
the 3-D tangential velocity and acceleration profiles (Fig. 1B
and C).
The variables describing absolute values of the infants’ rightleft reaches were: total duration (TD), cumulative (total 3-D)
distance of the reaching trajectory (CD), the time between the
first hand-object touch and the offset time of the hand (TTO),
Developmental Psychobiology. DOI 10.1002/dev
FIGURE 1 (A) 3-D image of a right-sided reaching movement
by a 6-month-old boy, with corresponding (B) velocity and (C)
acceleration profiles. The lines in (B) mark the movement onset,
peak velocity, object touch, and movement offset. The lines in
(C) mark the onset, peak deceleration, touch, and offset.
peak velocity (PV), time-to-peak velocity (TPV), the placement
of the peak velocity by means of percentage of the reach duration
(PPV), and the velocity at hand-object touch (VT). From the
same markers, the number of movements units (MUs) was
computed according to an algorithm devised by von Hofsten
(1991)1, examples of which the typical progression, from a
multisegmented velocity profile at 6 months to a more adultlike,
bell-shaped profile at 36 months are shown in Figure 2A–D. A
final parameter concerned the straightness of a reaching
movement, computed as the ratio between the actual distance
1
A movement unit consists of one acceleration phase and one
deceleration phase. The beginning of these respective phases are defined
as an accumulated increase or decrease in velocity of at least 20 mm/s and
an acceleration or deceleration exceeding 5 mm/s2.
Developmental Psychobiology. DOI 10.1002/dev
Side Differences in Infant Reaching Kinematics
449
Analysis
The frequency and distributions of successful right, left, or
bimanual reaching movements in relation to the three object
positions were analyzed by means of a 3 (age: 6, 9, 12 months) 3 (object position: right, left, midline) 3 (hand-use: right, left,
bimanual) ANOVAs, with repeated measures for the last two
factors. All kinematic parameters from when tested at 6, 9, and
12 months of age were subjected to separate 3 (age) 2 (birth
condition: fullterm, preterm) 2 (side: right, left) ANOVAs.
Note: in this analysis, the trials consisting of bimanual reaches
were included by dividing them into right or left hand use with
regard to which of the hands that first touched the object. Side
could not be treated as a repeated measure for these data due to
the fact that the individual number of reliable recordings of
reaches for one or the other side were too few (< 3) for
calculating individual means, or nonexisting (as was the case for
two infants at 6, and one at 12 months). At 36 months, separate
one-way ANOVAs for testing side differences were employed
for all kinematic parameters. For all analyses, where appropriate,
significant main and interaction effects were further analyzed
using the Scheffé post hoc test. The majority of analyses were
performed on an individual level, based on the infant’s mean
values. It should be noted that we are aware that the use of
parametric ANOVA analysis is not optimal as the data on the
level of individual infants showed nonnormal distributions. Yet,
these ANOVAs offered the only solution to analyzing the
combined effect of multiple factors on the kinematic outcome
parameters. A Spearman correlation was conducted to investigate the relationship between the right-left frequencies of
successful reaches and the outcome from kinematic parameters
based on individual means. Any other test employed is indicated
in the subsequent text. A preset alpha level of p < .05 was used
for all analyses.
RESULTS
Side Distribution of Successful Reaching
Table 1 presents the frequency and distributions of hand
use during successful reaching trials included in the
analyses made based on the outcomes from the video
scorings.
FIGURE 2 Examples of four tangential velocity profiles, with
movement units indicated, from the right-sided reaching movements of a boy aged (A) 6 months, (B) 9 months, (C) 12 months,
and (D) 36 months. Note: The circles on the respective reaching
trajectory mark the start/stop of a movement unit, and the small
vertical lines indicate the velocity peaks within respective MU.
The number of MUs are five, two, one, and one, for (A), (B), (C),
and (D), respectively.
the wrist marker was transported and the shortest distance
connecting the hand at movement onset to the center of
the object. Accordingly, the closer this ratio is to 1.00, the
straighter the reach.
Right-Left Side Differences. A significant main effect of
side was found, F(2,90) ¼ 26.2, p < .0001, and a significant interaction between age and side, F(2,90) ¼ 7.6,
p < .0001 (Fig. 3).
As can be seen from Figure 3, there was no evident side
difference regarding the distribution of hand use found
at 6 months, although more unimanual left than both
unimanual right and bimanual reaching were performed.
However, the post hoc test showed that at 9 months of
age the infants used significantly more unimanual than
bimanual reaching (p < .01), but no significant right-left
difference was found. At 12 months, the infants showed
450
Developmental Psychobiology. DOI 10.1002/dev
Rönnqvist and Domellöf
Table 1. Total Number of Successful Reach-Grasp and
Distribution of Relative Arm/Hand Use (Unimanual Right,
Unimanual Left, Bimanual) for Each Infant Up To
12 Months
Successful Reach-Grasp
Arm/Hand Use (%)
Infants
ALa
ISJa
JEa
KLa
AD
AN
ED
EL
EM
IS
JI
JO
KR
MI
NI
OL
WI
Total (n)
Right
Left
Bimanual
52
43
26
27
58
48
43
57
51
42
65
46
55
62
60
46
43
76.9
11.8
66.6
40.1
57.1
59.5
12.5
20.8
60.5
34.5
46.9
46.6
60.4
62.2
41.2
45.9
55.8
23.1
52.9
33.4
48.6
28.6
28.6
68.7
66.6
32.6
51.7
51.0
53.3
34.8
28.8
35.3
29.8
44.2
0
35.3
0
13.3
14.3
11.9
21.8
14.6
6.9
13.8
2.1
0
4.6
8.8
23.5
24.3
0
A
Right Hand
Left Hand
35
30
25
% 20
15
10
5
0
6 months
Left
B
Mid
Right
35
30
25
9 months
% 20
15
10
5
0
Left
C
Mid
Right
35
30
25
12 months
% 20
15
10
5
0
Left
Note: n, number.
a
Prematurely born infants.
significantly more right reaching than left (p < .05), and
significantly more unimanual (independent of side) than
bimanual reaching (p < .01). On the whole, the majority
of infants showed an increasing number of unimanual
right reaching and a decreasing number of bimanual
reaching over the first three ages tested (i.e., over the
second half of the first year).
A significant effect of object position, F(2,90) ¼ 3.5,
p < .05, and an interaction between object position and
side of arm/hand use, F(4,180) ¼ 53.1, p < .0001, were
further revealed (Fig. 4A–C). Although a few contral70
Right
Left
Bimanual
60
50
(%)
40
30
20
10
0
6-months
9-months
Mid
Right
Object position
12-months
AGE
FIGURE 3 Distribution of the relative frequencies of
successful arm/hand use (unimanual right, unimanual left,
bimanual) over the first three testing ages (6, 9, and 12 months).
FIGURE 4 Distribution of the relative frequencies of rightleft arm/hand use in relation to the object position at (A)
6 months, (B) 9 months, and (C) 12 months.
ateral reaches (i.e., crossing the midline) were performed
already when tested at the age of 6 and 9 months, the
infants mainly reached and grasped with the ipsilateral
arm/hand, thus, at the same side as the placement of the
object (Fig. 4A and B). When the object was placed in
the midline position, no clear side preference could be
observed at these ages (6 and 9 months). However, when
tested at 12 months of age, significantly more right
reaching-grasping of an object placed to the right than left
reaching-grasping when the object was placed to the
left was found (Fig. 4C). Additionally, at 12 months
there was significantly more right arm/hand crossing the
midline to reach for and grasp an object placed on the left
side than left arm/hand contralateral reaching-grasping.
In line with these side differences, more right reachinggrasping was also found when the object was placed in the
midline when tested at 12 months of age, compared to the
younger ages of 6 and 9 months.
Furthermore, the results from the additional arm/hand
preference tests carried out when the infants were
36 months of age showed that all of the participating
infants displayed a significant and consistent preference
Developmental Psychobiology. DOI 10.1002/dev
for using their right arm/hand in all the three tasks tested.
The distribution of the frequencies of the right-left arm/
hand usage for the three tasks was: 98.7% right and 1.3%
left for precision throwing; 89.7% right and 10.3% left for
drawing; and 94.4% right and 5.6% left for hammering.
Differences Related to Preterm Infants. Regarding the
number of successful reaching-grasping trials during the
first three testing ages (6, 9, and 12 months), no significant
differences between the fullterm and the preterm
infants were found with regard to the total number of
trials (p ¼ .07), or the frequency of right and left hand
use distribution (p ¼ .12). Furthermore, no significant
differences between fullterm and preterm infants for any
of the three tasks tested at 36 months of age could be
found.
Kinematic Findings
Figure 5 illustrates the typical developmental pattern in
the kinematics for the movement distance of the wrist
marker trajectories over time and the corresponding
velocity profiles from the right- and left-sided reaching
movements made by one infant at the four different test
occasions (6, 9, 12, and 36 months).
Table 2 presents the means and standard deviations for
all kinematic parameters investigated. Note: as no
significant differences were found between 36-monthold fullterm and preterm infants, data from the kinematic
parameters of all infants are pooled together at this age.
Right-Left Side Differences. Analysis of side effects on
kinematic outcome parameters associated with the
infants’ reaching movements at the ages of 6, 9, and
12 months revealed a significant overall effect of side for
number of MUs, F(1,74) ¼ 8.26, p < .01. As judged by
post hoc testing, there were significantly fewer MUs
(p < .05) during right arm reaching movements compared
to left at all of the ages tested during the second half of the
first year (Tab. 2). In addition, a similar side difference by
means of fewer MUs was also found when the infants were
tested at the age of 36 months, F(1,32) ¼ 7.6, p < .01.
A significant overall effect of side was also found
for the straightness of the reaching movements,
F(1,74) ¼ 4.51, p < .05. The post hoc test revealed
significantly (p < .05) straighter right-sided reaching
trajectories in comparison to left when tested at 6 and
12 months of age. However, with the four preterm infants
excluded from this analysis, the significantly straighter
right-sided reaching trajectories are found at 9 and
12 months of age (Tab. 2). When the infants were tested
at 36 months of age, no significant side difference was
found related to the straightness of the reach (Tab. 2). For
the other kinematic parameters analyzed in this study, no
Side Differences in Infant Reaching Kinematics
451
significant overall effects of side were found at 6, 9, and
12 months, nor when tested at 36 months of age.
Differences Related to Preterm Infants. Several differences between fullterm and prematurely born infants were
revealed with regard to side differences in the kinematic
findings. At the age of 12 months, the pattern of fewer
MUs for right-sided reaching, as found for the fullterm
infants, was not evident for the prematurely born infants.
On the contrary, at this age they showed more MUs for the
right reaching movements in comparison to the left
(Tab. 2). In terms of straightness of the reaching, the
prematurely born infants had significantly straighter rightsided reaching trajectories than left at 6 and 9 months, but
not at 12 months, in contrast to 9 and 12 months for the
fullterm (Tab. 2). This result is also in line with the finding
of more MUs in the right reaching movements compared
to the left for the 12-month-old prematurely born but not
for the fullterm infants. At 36 months, no difference
between the fullterm and the prematurely born infants was
found in terms of straightness of the reaching.
Furthermore, even if no overall side differences for the
remaining kinematic parameters were found, significant
right-left differences related to interaction effects between
side and birth condition for a range of additional variables
were present, all associated with the prematurely born
infants (Tab. 2). Based on post hoc tests, the following
significant right-left side differences were found (all
results compared to right-sided reaching and related
only to the prematurely born infants): for TD by means
of a shorter TD for left-sided reaching movements at
6 months; for TTO by means of a shorter TTO for leftsided reaching at 6 months; for PV by means of a lower PV
for left-sided reaching at 9 months; for TPV by means of
earlier TPV for left-sided reaching at 6 months, a later
TPV for left reaching at 9 months, and an earlier TPV for
left-sided reaching at 12 months; for PPV by means of an
earlier PPV for left-sided reaching at 6 months.
In general, differences between preterm and fullterm
infants were evident already from the visual inspections of
the kinematic outcome parameters, where the prematurely
born infants seemed to be less consistent (in regards to
both intra- and inter-variability) than the fullterm infants.
The statistical analyses also confirmed these differences.
It was found that, independent of side, the straightness
of the reaching trajectory differed between the preterm
and the fullterm infants, F(1,74) ¼ 16.68, p < .0001.
Prematurely born infants in general showed less straight
reaching trajectories in comparison to fullterm infants,
particularly when tested at 6 and 9 months of age (Tab. 1).
Additionally, a significant difference was found for the
number of MUs within the reach, F(1,74) ¼ 9.56, p < .01,
characterized by reaches of preterm infants containing
significantly more MUs at 6, 9, and 12 months than
452
Rönnqvist and Domellöf
Developmental Psychobiology. DOI 10.1002/dev
FIGURE 5 Overlay plots of the corresponding profiles for distance from reaching onset to hand
object contact (distance against time) and tangential velocity in the right-sided (bold line) and leftsided (thin line) reaching movements of a boy aged (A) 6 months, (B) 9 months, (C) 12 months, and (D)
36 months.
L
R
L
R
L
R
L
R
L
R
L
R
L
R
La
R
La
R
TD: total duration (s)
1.144 (.54)
1.248 (.55)
151 (77)
155 (53)
131 (114)
121 (86)
355 (163)
352 (144)
415 (322)
381 (398)
43.7 (23.8)
36.3 (27.3)
151 (104)
111 (84)
2.07 (.67)
2.15 (.72)
3.74 (1.76)b
3.22 (.99)
1.021 (.42)
1.024 (.40)
172 (81)
185 (61)
96 (116)
107 (73)
398 (165)
427 (169)
359 (226)
340 (205)
41.7 (23.1)
38.2 (19.2)
131 (99)
121 (88)
1.62 (.77)b
1.47 (.36)
3.18 (1.39)b
2.87 (1.21)
Mean (SD)
Mean (SD)
.910 (.35)
.932 (.35)
213 (127)
189 (88)
109 (102)
108 (74)
471 (234)
446 (235)
345 (214)
393 (277)
39.0 (22.4)
47.4 (27.0)
122 (94)
134 (122)
1.46 (.74)b
1.39 (.35)
3.18 (1.48)b
2.55 (1.33)
Mean (SD)
12
Note: L, left; R, right; SD, standard deviation; n, number; s, seconds; mm, millimeters; ms, milliseconds.
a
Overall effect of side.
b
Significant right-left difference.
MU: movement units (n)
STR: straightness
TPV: time to peak
velocity (ms)
PPV: peak placement of
the reach (%)
VT: velocity at touch (mm/s)
CD: cumulative
distance (mm)
TTO: touch-offset
difference (ms)
PV: peak velocity (mm/s)
Side
Kinematic Parameters
9
1.200 (.52)
1.050 (.31)
167 (89)
196 (62)
55 (47)b
119 (81)
459 (295)
493 (133)
285 (196)b
513 (446)
26.4 (18.8)b
51.4 (29.1)
153 (226)
252 (132)
3.64 (2.49)b
2.98 (1.97)
4.88 (1.81)b
4.14 (1.95)
b
Mean (SD)
6
1.211 (.34)
1.208 (.74)
154 (50)
194 (107)
111 (81)
109 (82)
301 (80)b
419 (145)
535 (263)b
412 (202)
49.0 (20.7)
43.9 (24.4)
118 (110)
80 (79)
1.82 (.18)b
1.60 (.52)
3.89 (1.88)b
3.11 (1.41)
Mean (SD)
9
Age (months)
Age (months)
6
Preterm Born (n ¼ 4)
Fullterm Born (n ¼ 13)
1.100 (.70)
1.186 (.76)
218 (139)
190 (94)
79 (56)
84 (55)
513 (358)
415 (234)
212 (219)b
248 (195)
24.7 (21.4)
28.0 (23.6)
103 (74)
102 (107)
1.47 (.42)
1.53 (.51)
3.13 (1.68)b
3.89 (2.47)
Mean (SD)
12
.729 (.22)
.728 (.20)
246 (35)
253 (46)
106 (06)
114 (05)
681 (170)
690 (172)
376 (138)
396 (159)
52.2 (17.1)
54.3 (19.8)
54 (67)
57 (63)
1.32 (.14)
1.31 (.16)
2.13 (1.08)b
1.74 (.74)
Mean (SD)
36
Age (months)
All (n ¼ 16)
Table 2. Means and Standard Deviations for Kinematic Parameters During Reaching and Grasping as a Function of Birth Condition (Fullterm, Preterm) and Age (6, 9, 12,
and 36 Months)
Developmental Psychobiology. DOI 10.1002/dev
Side Differences in Infant Reaching Kinematics
453
454
Developmental Psychobiology. DOI 10.1002/dev
Rönnqvist and Domellöf
fullterm infants (Tab. 1). Thus, the reaching movements of
preterm infants were less smooth in terms of a more
segmented velocity profile.
A significant difference was also found for the time
spent between hand-object touch and offset of the
reaching (TTO), F(1,74) ¼ 4.08, p < .05. The prematurely
born infants were found to have a shorter TTO time in
comparison to the fullterm infants, but the post hoc test
showed that this was only significant when tested at
12 months. Although not significant, F(1,74) ¼ 2.77,
p ¼ .09, the preterm infants also tended to have a longer
reaching duration in comparison to the fullterm, most
apparent when tested at 12 months of age (Tab. 1). No
other analysis of the investigated kinematic parameters
showed a significant difference between the fullterm and
the preterm infants, nor were there any significant
interactions found between birth condition and testing
age. Furthermore, when the infants were tested at
36 months of age there were no significant differences
found between infants born fullterm and prematurely for
any of the parameters analyzed.
distributions of successful reaching movements when
tested in the early ages. Thus, the reaching was not marked
by an overall side preference at 6 and 9 months, although
an increasing amount of right-sided reaching could be
observed in most infants at 12 months of age. In terms of
the amount of successful reaching for individual infants at
6, 9, and 12 months of age, 10 of the infants showed more
right-sided reaching and 7 more left-sided. However, for
four of these infants the side differences seen were more or
less negligible at all of the first three testing ages.
Looking at the individual infants’ outcomes as
revealed by kinematics, it was found that, in agreement
with the significant overall right-left side difference for
number of MUs, the great majority of infants showed
significantly (t-tests, p < .05 on individual infants outcomes) fewer MUs in their right reaching in comparison to
their left (Tab. 3).
Consistent over the first three ages tested, fewer MUs in
the right reaches in comparison to left reaches were found
in 10 of the 17 infants. Three infants showed a pattern of
fewer MUs at two out of three test occasions (2/3), and
four at one out of three (1/3). Two of the latter four infants
were prematurely born. Adding the outcomes from the
analyses of MUs at 36 months (including 16 of the
children), a consistent right-left difference (i.e., at 4/4 or 3/4
test sessions) in terms of fewer MUs in the right reaches
Side Consistency
The majority of infants did not show any clear side
preference with regard to the frequency and side
Table 3. Total Number of Kinematic Recordings and Distribution of Number of MUs (Right, Left) for Each Infant at 6, 9, 12,
and 36 Months
MUs (n)
Kinematic
Recordings
6 Months
9 Months
12 Months
36 Months
Trial
Side
Side
Side
Side
Infants
Total (n)
Right (M)
Left (M)
Right (M)
Left (M)
Right (M)
Left (M)
Right (M)
Left (M)
a
46
45
32
38
53
34
43
41
54
26
51
58
54
43
40
38
58
2.28
4.10
3.40
1.99
2.70
3.20
2.33
3.20
2.00
3.20
3.00
3.60
3.45
—
4.00
3.20
3.07
3.66
4.80
5.80
3.72
3.71
3.70
4.00
3.80
2.77
3.74
4.00
4.50
—
5.00
4.30
3.80
5.80
3.05
4.69
5.30
3.60
3.21
3.10
3.25
2.83
2.57
2.70
3.10
2.33
3.00
3.80
2.00
2.33
2.41
3.26
4.00
4.33
3.30
4.00
3.50
4.10
3.43
2.60
3.10
2.75
3.25
4.00
3.20
3.12
3.70
2.75
2.45
3.25
5.50
3.20
2.10
2.83
3.50
2.66
1.64
2.50
3.20
1.87
2.84
3.16
2.50
2.30
2.67
2.71
3.23
4.00
1.60
3.43
3.50
—
3.69
2.00
3.20
3.25
3.60
3.25
3.28
2.33
3.00
2.76
—
1.80
2.13
1.67
1.50
1.00
1.79
1.60
1.60
1.97
1.31
1.34
2.00
1.86
1.75
1.35
1.50
—
3.02
2.43
1.50
1.72
2.00
2.24
1.10
1.67
2.66
1.74
1.73
2.50
1.87
3.00
2.00
2.28
AL
ISJa
JEa
KLa
AD
AN
ED
EL
EM
IS
JI
JO
KR
MI
NI
OL
WI
Note: Significant fewer MUs in right reaching movements for respective age tested are highlighted in bold. M, mean.
a
Prematurely born infants.
Developmental Psychobiology. DOI 10.1002/dev
was found for 11 of the infants. Five of the infants showed
no clear side consistency, of whom three were prematurely
born (Tab. 3). None of the infants tested showed
consistently fewer MUs in their left reaching movements.
In agreement with the findings of fewer MUs in the
right reaching, the majority (11) of the infants also showed
a significant (t-tests, p < .05) and consistently straighter
reaching trajectory for their right-sided reaching movements in comparison to their left over all ages tested (6, 9,
12, 36 months). The remaining six infants showed a more
inconsistent right-left side difference in terms of reaching
trajectory straightness. However, this inconsistency was
mainly related to when the infants were tested at 6 months
of age, and when a higher variability of the reaching
trajectory in general was found. None of the infants
showed a consistently straighter reaching trajectory for
left-sided reaching over the tested ages.
In addition, Spearman correlation showed that the
individual frequencies and distributions of successful
right-left reaches over the first three testing ages were not
significantly correlated with the individual infant’s reaching kinematics in terms of number of MUs for the
respective right-left reaches (r ¼ .206, p > .05). Thus,
independent of the frequency of successful right-left
reaching distributions in infants tested during the second
half of the first year (6, 9, and 12 months), the pattern of
less segmented right-sided reaching movements in
comparison to left was evident in the findings.
Age Effects
Independent of birth condition and side, the ANOVAs
performed on all kinematic variables signaled a significant effect with regard to the infants’ testing age for the
following variables: total reaching duration (p < .01),
cumulative distance of the reaching (p < .001), actual PV
(p < .01), straightness of the reaching trajectory
(p < .0001), and number of MUs (p < .001) as illustrated
in Figure 2A–D. It was found that the reaching duration
progressively decreased over the first three ages tested,
from a mean duration of 1.17 s at 6 months to a mean
duration of .98 s at 12 months, with the largest decrement
between 6 and 9 months. In contrast to this finding, the
cumulative distance of the reach trajectory increased over
the ages tested, from an average distance of 168 mm at
6 months to 211 mm at 12 months of age. The most
plausible explanation for this increasing length of the
reach trajectory over age is the parallel increasing growth
in length of the infants’ arms.
The age effect for PV is mainly explained by an
increasing PV between 6 and 12 months. However, as can
be seen in Table 2, the PV, as well as the TPV and the
placement of the PV, were rather variable over the first
Side Differences in Infant Reaching Kinematics
455
three ages tested. This finding was particularly true for the
preterm infants. The most evident overall effects of
age were found for the straightness of the reach and the
number of MUs per reach. It was found that, with an
increasing age, the distal reach trajectory became
progressively both straighter and smoother (less segmented). This is also confirmed by the results from when the
infants were tested at 36 months of age (Tab. 2).
DISCUSSION
The present study aimed at investigating whether there are
right-left side differences in the reaching kinematics of
developing infants, the consistency of such side differences, and the relationship between reaching kinematics
and frequency of arm/hand use. By carrying out
successive quantitative analyses of infants’ reaching
movements at the ages 6, 9, 12, and 36 months, we
expected to find side differences that may not otherwise be visible and/or detected by simply focusing on the
frequency of the infants’ choice of right or left arm/hand
for reaching-grasping. Overall, it was found that the
majority of the infants in the current study showed a
relatively inconsistent side distribution of reachinggrasping over the ages tested; this was especially true in
the early ages of 6 and 9 months. However, the kinematics
in terms of the spatiotemporal structuring of the infants’
reaching trajectories indicated a more consistent and
stable right-left difference by means of straighter and less
segmented right-sided reaching movements in most
infants/children. This finding was true for most of the
fullterm infants, whereas the prematurely born infants
included in this study generally showed a more inconsistent and fluctuating outcome in terms of right-left
reaching kinematics and side consistency.
When investigated at the ages of 6 and 9 months, most
of the infants did not show any evident side preference
with regard to the frequency of right- or left-sided
reaching and grasping. At 12 months, however, a rather
evident right-sided preference for reaching and grasping
was found in most of the infants tested. This finding is in
keeping with the distal progress in anticipatory preparation and adjustment of hand shape when grasping
differently sized object in infants (Fagard, 2000; von
Hofsten & Rönnqvist, 1988; White, Castle, & Held,
1964), which are abilities dependent on direct corticospinal connections providing contralateral control of the
fingers. Hence, hand preference seems to develop in
synchrony with a substantial maturation of the corticospinal system in control of more refined hand/finger
movements. In addition, when investigated at 36 months
of age, all participating children in the present study
additionally showed a clear-cut right-sided bias for
456
Rönnqvist and Domellöf
different complex items involving performances with
both arm and hand (throwing, drawing, hammering as
well as for grasping the ‘‘tool’’).
No stable pattern of side preference was found with
respect to the frequency and side distribution of individual
infants’ reaching and grasping before 12 months of age.
This implicates that measurement by only focusing on the
frequency and distribution of right-left reaching and
grasping in infants less than 1 year of age may not provide
a valid index of hand preference and/or prediction for later
handedness. However, it is notable that although the
infants in the present study were free to choose whatever
hand to reach and grasp the object with, the experimental
set-up was deliberately constructed to optimize reaching
and grasping performance with both the right and the left
sides. This was done in order to increase our ability to
make kinematic recordings of both the right and left
reaching movements in individual infants, allowing side
comparisons to be made. In most studies reviewed, the
target object for the infant to reach and grasp for have been
positioned in a midline position. In the present study,
target objects were presented in a right, left, and a midline
position. Thus, we are aware that this method may have
affected the frequency of right-left reaches found in this
study. Still, the method may have a better ecological
validity than if the objects always are presented in a
midline position, which is not the case in infants’ natural
settings.
Moreover, it is well known from previous studies that
young infants prefer to reach and grasp with the hand at
the same side as the placement of a stationary object (e.g.,
Fagard, 1998), a result that was also confirmed in the
present study. However, the notion that the ability to cross
the midline during reaching-grasping is associated with
the maturation of the corpus callosum, with spontaneous
midline crossing first occurring after 1 year of age
(Bishop, 1990), seems to be overly stressed. In the present
study, reaching across the midline was present (although
not so frequently occurring) already at 6 and 9 months of
age. At 12 months of age, reaching across the midline was
a more frequent observation, and mainly in terms of more
right reaches crossing the midline toward an object to the
left in the majority of infants. The majority of infants also
showed a later midline crossing with the left arm/hand in
comparison to the right, and not as frequent, in line with
findings from both typically developing 12-month-old
infants as well as infants with agenesis of the corpus
callosum (see Sacco, Moutard, & Fagard, this issue).
Taken together, these findings suggest that reaches across
midline might be a more sensitive indicator of early side
preference than reaching and grasping frequency per se.
Turning to the kinematic findings, the results from the
present study verify the findings of Hopkins and
Rönnqvist (2002), and are in line with Morange-Majoux
Developmental Psychobiology. DOI 10.1002/dev
et al. (2000), with regard to significant side differences in
terms of less segmented and straighter right-sided reaching trajectories in comparison to left-sided in 6-month-old
infants. As judged by the present study, we can now add to
these findings that such side differences are also evident
when infants are tested at 9, 12, and 36 months of age.
Furthermore, on an individual basis, these side differences
were found to be consistent over time in most of the infants
investigated. Still, some infants showed a relative
inconsistency in terms of right-left differences regarding
the straightness and segmentation of the reaching movement, although this was mainly the case for the few
preterm infants included in the study. However, none of
the participating infants tested showed an over age
consistent pattern of fewer MUs and/or a straighter
trajectory for left-sided reaching. Furthermore, in line
with previous studies of hand preference in children at
about 3 years of age, all of the children in the present study
showed an evident right-hand preference at 36 months of
age.
In agreement with Corbetta and Thelen (1996, 1999), a
fluctuating and inconsistent velocity profile in infant
reaching movements during the second half of the first
year was confirmed also in the present study. It was found
that PV, as well as TPV and the placement of the PV, were
rather variable and inconsistent over the first three ages
tested. One explanation for the incoherent velocity
profiles in the present study, particularly evident in the
younger ages of 6 and 9 months, is that the reaching
movements are structured into several segments of
velocity-based MUs, whereas the within-reach velocity
peaks sometimes varies with regard to its corresponding
MU placement (exemplified in Fig. 2A). In more mature
reaches, the first transport unit is generally the largest,
containing the highest PV and transporting the hand the
longest distance (e.g., Jeannerod, 1984). In the present
study, the velocity pattern was more variable and
inconsistent when the infants were investigated at the
younger ages (especially at 6 and 9 months). Thus, it was
found that both spatial and/or temporal correction sometimes occurred later in the reaching path, resulting in a
corresponding MU with a higher velocity peak than in the
first unit. Nevertheless, despite the fact that the PV profile
within reaching movements was not found to have a fixed
relationship, the number of velocity-based MUs and the
corresponding right-left side differences turned out to be
more stable.
It is not possible from the current findings of consistent
smoother and straighter right-sided reaching to determine
whether these are expressions of differences in a biologically rooted neural circuitry that give rise to asymmetries in
the structuring of infants’ early arm movements, or the
other way around, as a consequence of early arm use and
activity. Nevertheless, our current findings strongly support
Developmental Psychobiology. DOI 10.1002/dev
the notion that the right-left differences found with regard
to spatiotemporal organization and structuring of early
reaching movements are associated with a biologically
based developmental process. Furthermore, our findings
suggest that this developmental process shows a similar and
rather stable timetable in the vast majority of typically
developing infants. Thus, in line with the suggested
proximal-distal trend in motor development, the neural
systems controlling the proximal arm movements develop
before the systems controlling the distal hand movements
involved in more refined finger movements (i.e., pincer
grip). Hence, the initial manifestations of hemispheric
dominance related to a side preference should be regarded
as primarily the development of a trunk, head, and arm
preference rather than a hemispheric dominance for
handedness.
Arm preference by means of activity seems to appear
several months before term age (McCartney & Hepper,
1999). Thus, in terms of developmental origins, the
present findings of side differences in the spatio-temporal
structuring of infant reaching movements may be
determined by physiological asymmetries that are present
long before the possibility for experience-related movement patterns to develop. According to this prospect,
infants’ arm and hand preference would emerge from
distinctive neural circuits in each hemisphere that are
specialized for controlling different aspects of arm/hand
movements, in keeping with a view of handedness as
innate and genetically rooted (Annett, 1985). In support
for this view is the outcome from the study by McCartney
and Hepper, who investigated fetal arm movements from
12 to 27 weeks gestational age. They found that
throughout all periods of observation, the great majority
of fetuses showed more right than left arm movements.
This early presence of lateralized arm movements was
suggested possibly generating later asymmetries in the
brain, and thought to have a genetic origin. Furthermore, it
has been suggested that newborns’ head position and head
turning preference can predict the arm used in infants’
initial attempts to reach, as well as later hand preference
(e.g., Coryell & Michel, 1978; Michel, 1981). In this view,
head positioning preference to the right or left in the
newborn may induce a lateral asymmetry in looking at and
activating one hand or the other (i.e., a greater visual
experience of one hand leads to it being preferred for later
reaching). Still, before birth, intrauterine position and/or
early postural bias might have an effect on newborns’ head
positioning preference, in line with Previc (1991)’s leftotolith dominance hypothesis.
Even if the development of human hand preference
seems to be biologically rooted, a number of recent studies
have showed that the early developmental process of
structural-functional motor asymmetries can change as a
result of even minor interruptions (e.g., influences of
Side Differences in Infant Reaching Kinematics
457
intrauterine exposure to teratogens and preterm birth),
especially during the early establishment of brain
connectivity. Thus, a polygenetic explanation, which
takes these early intrauterine, perinatal, and postnatal
environmental influences on the early neuronal development into consideration, is probably called for.
A consistent finding is that left- and nonright-handers
are overrepresented in ex-preterm children (e.g., Marlow
et al., 1989; O’Callaghan et al., 1993). This atypical
preference has also been suggested associated with poor
cognitive performances at school age, and consequently
reflecting subtle disturbances in brain organization
(Bracewell & Marlow, 2002). However, the relationship
between nonright handedness in preterm children and
specific deviations in sensory-motor development, as well
as how deviations in the developing nervous system may
be expressed in atypical functional asymmetries, is still
largely unknown. Kinematic analysis has proved to be
promising for investigating, for example, reaching in
children with neurological dysfunctions (e.g., Chang
et al., 2005), or born at-risk for such dysfunctions (e.g.,
Fallang, Saugstad, Grøgaard, & Hadders-Algra, 2003).
Longitudinal kinematic studies of lateralized differences
in early movement patterns of fullterm and preterm infants
could thus add further to the understanding of the
processes that may lead to deviant developmental outcomes in ex-preterm children.
The prematurely born infants in our study displayed a
developmental delay in terms of reaching kinematics,
together with an alternative developmental pattern with
regard to expressions of hand preference. However, at 36
months both the fullterm and preterm children showed a
dominant right-sided preference in terms of hand use and
differences in terms of kinematic characteristics could no
longer be found. With regard to underlying neural
mechanisms, the development of the corticospinal
system has been suggested susceptible to functional
divergence (Martin, 2005), which may also apply to
deviations in side-related behavior. For example, complications following preterm birth may impair the functional
state of the corticospinal system, which could be reflected
in deviant expressions of functional asymmetries. In terms
of motor functioning, brain damage associated with
preterm birth implicates a modulation of functional
neural connections during CNS maturation that may
result in irregularities in motor output. Repeated uncorrelated movements may then reinforce these disordered or
inappropriate neural connections later in development
(Myklebust & Gottlieb, 1997). One explanation for the
temporary deviation from the typical developmental
pattern at 6–12 months found in the present study could
thus be that premature birth (in our study, 33–36 weeks of
gestational age) may only result in very mild complications that, following increased activity and motor
458
Rönnqvist and Domellöf
experiences in a dextrally biased world, progressively are
compensated for. Alternatively, this could be an expression of a general, slower motor development and as a
consequence a later developing arm/hand preference,
thus, not necessarily involving a functional neural
deviation.
Developmental Psychobiology. DOI 10.1002/dev
tion (239/02). We are especially grateful to the parents and their
infants for their participation over the years. We also thank
Thomas Rudolfsson for the MATLAB programming.
REFERENCES
CONCLUSION
Based on our present results and findings from
previous relevant studies, we propose that human hand
preference originates from an early dominant arm
advantage (mainly involving bilateral projections), as it is
not until later in development that the infant gains increased
precision and accuracy of the hand (mainly involving
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arm/hand and/or infants born at-risk for developmental
delays. In addition, incorporating analyses of the kinematics properties of intra-joint dynamics in such studies
would further elucidate the relationship between asymmetries in the movements of the shoulder, arm, and hand.
NOTES
This study was supported by grants from the Swedish Research
Council (421-2001-4589) and by the Norrbacka-Eugenia founda-
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