Journal
Articles
Malfait N, Sanger TD (2006) Does dystonia always include co-contraction? A study of unconstrained reaching in children with
primary and secondary dystonia. Exp Brain Res.
Abstract Article
in PDF format (1 MB)
Dystonia is a movement disorder in which involuntary or intermittent
muscle contractions cause twisting and repetitive movements, abnormal
postures, or both. Excessive co-contraction and abnormalities in the time
course of reciprocal inhibition between antagonist groups of muscles are
considered to be cardinal features of some types of dystonia and reduced
speed of movement is often attributed to involuntary activation of
antagonist muscles about a joint. In the present study we describe muscle
activity during unconstrained multi-joint reaching movements. Children
diagnosed with arm dystonia due to cerebral palsy (CP) or primary
dystonia (n = 7, 4-16 years, 4 with CP, 3 primary) and similar age
healthy subjects pointed alternately to two targets as fast as possible.
The children with dystonia showed decreased speed, greater variability,
and pauses at targets compared with controls. Decreased speed was mostly
due to difficulty in reversing reaching direction, and increased
variability was associated with large fluctuations in the duration of
the pauses at targets, rather than with variations in the
flexion/extension velocity profiles. Surface electromyographic (EMG)
activities were examined to assess if the abnormalities observed in the
children with dystonia could be explained in terms of increased levels
of co-contraction. Unexpectedly, we found that the children with dystonia
showed lower levels of co-contraction than the controls during movement,
and the pauses at targets were associated with reduced levels of
activation rather than with excessive activity in antagonist groups of
muscles. Therefore reduced speed of movement during unconstrained
reaching may not be due to involuntary activation of the antagonist
muscle, and co-contraction of opposing muscles about a joint is not an
obligatory feature of multi-joint movement in children with dystonia.
Darainy M, Malfait N,
Towhidkhah F, Ostry DJ
(2006) Transfer
and durability of acquired patterns of human arm stiffness. Exp Brain
Res 170:227-237.
Abstract Article
in PDF format (308 KB)
We used a robotic device to test the idea that impedance control involves
a process of learning or adaptation that is acquired over time and permits
the voluntary control of the pattern of stiffness at the hand. The tests
were conducted in statics. Subjects were trained over the course of three
successive days to resist the effects of one of three different kinds
of mechanical loads, single axis loads acting in the lateral direction,
single axis loads acting in the forward/backward direction and isotropic
loads that perturbed the limb in eight directions about a circle. We found
that subjects in contact with single axis loads voluntarily modified their
hand stiffness orientation such that changes to the direction of maximum
stiffness mirrored the direction of applied load. In the case of isotropic
loads, a uniform increase in endpoint stiffness was observed. Using a
physiologically realistic model of two-joint arm movement, the experimentally
determined pattern of impedance change could be replicated by assuming
that coactivation of elbow and double joint muscles was independent of
coactivation of muscles at the shoulder. Moreover, using this pattern
of coactivation control we were able to replicate an asymmetric pattern
of rotation of the stiffness ellipse that was observed empirically. The
present findings are consistent with the idea that arm stiffness is controlled
through the use of at least two independent cocontraction commands.
Malfait N, Gribble PL, Ostry
DJ
(2005)
Generalization of motor learning based on multiple field exposures and
local adaptation. J Neurophysiol
93:3327-3338.
Abstract Article
in PDF format (3976 KB)
Previous studies have used transfer of learning over workspace locations
as a means to determine whether subjects code information about dynamics
in extrinsic or intrinsic coordinates. Transfer has been observed when
the torque associated with joint displacement is similar between workspace
locations rather than when the mapping between hand displacement and
force is preserved which is consistent with muscle- or joint based encoding.
In the present study, we address the generality of an intrinsic coding
of dynamics and examine how generalization occurs when the pattern of
torques varies over the workspace. In two initial experiments, we examined
transfer of learning when the direction of a force field was fixed relative
to an external frame of reference. While there were no beneficial effects
of transfer following training at a single location (Experiment 1 and
2), excellent performance was observed at the center of the workspace
following training at two lateral locations (Experiment 2). Experiment
3 and associated simulations assessed the characteristics of this generalization.
In these studies, we examined the patterns of transfer observed following
adaptation to force fields that were composed of two subfields that acted
in opposite directions. The experimental and simulated data are consistent
with the idea that information about dynamics is encoded in intrinsic
coordinates. The nervous system generalizes dynamics learning by interpolating
between sets of control signals, each locally adapted to different patterns
of torques.
Della-Maggiore
V, Malfait N, Ostry DJ, Paus T (2004)
Stimulation of the posterior parietal cortex interferes with arm
trajectory adjustments during the learning of new dynamics. J Neurosci
24:9971-9976.
Abstract Article in PDF format (177
KB)
Substantial neurophysiological evidence points to the posterior parietal
cortex (PPC) as playing a key role in the coordinate transformation necessary
for visually guided reaching. Our goal was to examine the role of PPC in
the context of learning new dynamics of arm movements. We assessed this
possibility by stimulating PPC with transcranial magnetic stimulation (TMS)
while subjects learned to make reaching movements with their right hand
in a velocity-dependent force field. We reasoned that, if PPC is necessary
to adjust the trajectory of the arm as it interacts with a novel mechanical
system, interfering with the functioning of PPC would impair adaptation.
Single pulses of TMS were applied over the left PPC 40 msec after the onset
of movement during adaptation. As a control, another group of subjects was
stimulated over the visual cortex. During early stages of learning, the
magnitude of the error (measured as the deviation of the hand paths) was
similar across groups. By the end of the learning period, however, error
magnitudes decreased to baseline levels for controls but remained significantly
larger for the group stimulated over PPC. Our findings are consistent with
a role of PPC in the adjustment of motor commands necessary for adapting
to a novel mechanical environment.
Darainy M, Malfait N,
Gribble PL, Towhidkhah F, Ostry
DJ (2004) Learning to control arm stiffness under static
conditions. J Neurophysiol 92:3344-3350.
Abstract Article in PDF format
(267 KB)
We used a robotic device to test the idea that impedance control involves
a process of learning or adaptation that is acquired over time and permits
the voluntary control of the pattern of stiffness at the hand. The tests
were conducted in statics. Subjects were trained over the course of three
successive days to resist the effects of one of three different kinds
of mechanical loads, single axis loads acting in the lateral direction,
single axis loads acting in the forward/backward direction and isotropic
loads that perturbed the limb in eight directions about a circle. We found
that subjects in contact with single axis loads voluntarily modified their
hand stiffness orientation such that changes to the direction of maximum
stiffness mirrored the direction of applied load. In the case of isotropic
loads, a uniform increase in endpoint stiffness was observed. Using a
physiologically realistic model of two-joint arm movement, the experimentally
determined pattern of impedance change could be replicated by assuming
that coactivation of elbow and double joint muscles was independent of
coactivation of muscles at the shoulder. Moreover, using this pattern
of coactivation control we were able to replicate an asymmetric pattern
of rotation of the stiffness ellipse that was observed empirically. The
present findings are consistent with the idea that arm stiffness is controlled
through the use of at least two independent cocontraction commands.
Malfait N, Ostry
DJ (2004) Is
interlimb transfer of
force-field adaptation a "cognitive" response to the sudden
introduction of load? J Neurosci 24:8084-8089.
Abstract
Article in PDF format
(342 KB)
Recently, Shadmehr and colleagues (Criscimagna-Hemminger et al. 2003)
reported a pattern of generalization of force-field adaptation between
arms that differs from the pattern that occurs across different configurations
of the same arm. While the intralimb pattern of generalization points
to an intrinsic encoding of dynamics, the interlimb transfer described
by these authors indicates that information about force is represented
in a frame of reference external to the body. In the present study, subjects
adapted to a viscous curl-field in two experimental conditions. In one
condition, the field was introduced suddenly and produced clear deviations
in hand paths; in the second condition, the field was introduced gradually
so that at no point during the adaptation process could subjects observe
or had to correct for a substantial kinematic error. In the first case,
a pattern of interlimb transfer consistent with Criscimagna-Hemminger
et al. was observed, whereas no transfer of learning between limbs occurred
in the second condition. The findings suggest that there is limited transfer
of fine compensatory force adjustment between limbs. Transfer when it
does occur may be largely the results of a "cognitive" strategy that arises
as a result of the sudden introduction of load and associated kinematic
error.
Malfait N, Shiller DM, Ostry
DJ (2002)
Transfer of
motor learning across arm configurations. J Neurosci 22:9656-9660.
Abstract
Article in PDF format
(297 KB)
It has been suggested that learning of new dynamics occurs in intrinsic
coordinates. However, it has also been suggested that elements that encode
hand velocity and hence act in an extrinsic frame of reference play a
role in the acquisition of dynamics. In order to reconcile claims regarding
the coordinate system involved in the representation of dynamics, we have
used a procedure involving the transfer of force-field learning between
two workspace locations. Subjects made point-to-point movements while
holding a two-link manipulandum. Subjects were first trained to make movements
in a single direction at the left of the workspace. They were then tested
for transfer of learning at the right of the workspace. Two groups of
subjects were defined. For subjects in Group J, movements at the left
and right workspace locations were matched in terms of joint displacements.
For subjects in Group H, movements in the two locations had the same hand
displacements. Workspace locations were chosen such that for Group J,
the paths (for training and testing) that were identical in joint space
were orthogonal in hand space. Subjects in Group J showed good transfer
between workspace locations, whereas subjects in Group H showed poor transfer.
The results are in agreement with the idea that new dynamics are encoded
in intrinsic coordinates and that this learning has a limited range of
generalization across joint velocities.
Malfait N, Ramsay JO (2003) The historical functional linear model.
Can J Stat 31:115-128.
Abstract Article
in PDF format (1 MB)
The authors develop a functional linear model in which the values at
time t of a sample of curves yi(t) are explained in a feed-forward sense
by the values of covariate curves xi(s) observed at times s <= t. They
give special attention to the case s in [t - d, t], where the lag parameter
d is estimated from the data. They use the finite element method to estimate
the bivariate parameter regression function B(s, t), which is defined
on the triangular domain s <= t. They apply their model to the problem
of predicting the acceleration of the lower lip during speech on the basis
of electromyographical recordings from a muscle depressing the lip. They
also provide simulation results to guide the calibration of the fitting
process.
Markovits H, Dumas C, Malfait N (1995) Understanding transitivity
of a spatial relationship: a developmental analysis. J Exp Child Psychol 59:124-141.
Abstract Article
in PDF format (916 KB)
Pears and Bryant (1990) found that children as young as 4 years old could
make correct transitive inferences on a task examining their understanding
of the relation "higher than," if the premises were presented in the form
of sets of two block towers. This study extended their investigation by
looking at children's transitive inferences in situations in which the
representation of the premises provided contradictory information depending
on relative or ordinal position of the A and B elements of the three part
series, A > B > C was used. The results showed that performance on problems
similar to those used by Pears and Bryant was very high for 6- and 8-year
olds. However, the 6-year-olds had great difficulty with the more complex
problems, whereas the 8-year-olds did significantly better. These results
are interpreted as indicating the presence of a developmental sequence
of algorithms that enable children to resolve progressively more complex
transitive inference problems.
Markovits H, Venet M, Janveau-Brennan G, Malfait N, Pion N (1996)
Reasoning in young children: fantasy and information retrieval. Child Dev 67:2857-2872
Conference presentations / published abstracts
Malfait N, Henriques DYP, Gribble PL. Local rather than global
remapping during visually guided tracking. Presented at the
36th Annual Meeting of the Society for Neuroscience, Atlanta, GA, 2006
Malfait N, Henriques DYP, Gribble PL. Local adaptation for
local remapping in a tracking task. Presented at the 28th
International Symposium of Computational Neuroscience, Montreal, QC, 2006
Malfait N, Sanger TD. Children with dystonia due to cerebral palsy
do not have abnormal co-contraction of antagonist muscles during unconstrained
reaching. Presented at the 35th Annual Meeting of the Society
for Neuroscience, Washington DC, 2005
Malfait N, Sanger TD. Is abnormal movement in dyskinetic cerebral
palsy associated with co-contraction? Presented at the 15th Annual
Neural Control of Movement Meeting, Key Biscayne, FL, 2005
Malfait N, Ostry DJ. Is interlimb transfer of force-field adaptation
a cognitive response to the sudden introduction of load? Presented
at the 34th Annual Meeting of the Society for Neuroscience, San Diego, CA, 2004
Della-Maggiore V, Malfait N, Ostry DJ, Paus T. Stimulation of
the posterior parietal cortex interferes with arm trajectory adjustments
during the learning of new dynamics. Presented at the 34th Annual
Meeting of the Society for Neuroscience, San Diego, CA, 2004
Darainy M, Malfait N, Gribble PL, Towhidkhah F, Ostry DJ. Coordinate
System of Arm Stiffness Control in Statics. Presented at the
34th Annual Meeting of the Society for Neuroscience, San Diego, CA, 2004
Darainy M, Malfait N, Gribble PL, Towhidkhah F, Ostry DJ. Control
of Human Arm Impedance in Statics. Presented at the 2nd International
Symposium on Measurement, Analysis and Modeling of Human Functions,
Genova, Italy, 2004
Malfait N, Ostry DJ. Is interlimb transfer of force-field adaptation
a cognitive response to the sudden introduction of load? Presented
at the 14th Annual Neural Control of Movement Meeting, Barcelona,
Spain, 2004
Darainy M, Malfait N, Gribble PL, Towhidkhah F, Ostry DJ. Learning
Impedance Control in Statics. Presented at the 14th Annual Neural
Control of Movement Meeting, Barcelona, Spain, 2004
Malfait N, Gribble PL, Ostry DJ. How does the motorsystem encode
dynamic properties of the external world? Presented at the 33rd
Annual Meeting of the Society for Neuroscience, New Orleans, LA, 2003
Malfait N, Ramsay JO. The historical functional linear model: An
approach to develop tests of significance. Presented at
the 68th Annual Meeting of the Psychometric Society, Cagliari, Italy, 2003
Malfait N, Gribble PL, Ostry DJ. How does the motor system encode
dynamic properties of the external world? Presented at the 13th
Annual Neural Control of Movement Meeting, Santa Barbara, CA, 2003
Malfait N, Shiller DM, Ostry DJ. Transfer of motor learning across
arm configurations. Presented at the 32nd Annual Meeting
of the Society for Neuroscience, Orlando, FL, 2002
Malfait N, Shiller DM, Ostry DJ. Decomposition of the transfer
of learning across arm configurations. Presented at the 12th
Annual Neural Control of Movement Meeting, Naples, FL, 2002
Malfait N, Ramsay JO, Froda S. The historical functional linear
model. Presented at the 4th Canadian Conference in Applied
Statistics, Montreal, QC, 2001
Malfait N, Ramsay JO, Froda S. The historical functional linear
model. Presented at the Annual Meeting of the Psychometric Society,
Vancouver, 2000
Malfait N, Ramsay JO, Froda S. Estimation by the finite element
method in a functional linear model. Presented at the International
Conference on Measurement and Multivariate Analysis, Banff, AB, 2000
Malfait N. Estimation par la méthode des éléments finis dans un modèle
linéaire fonctionnel. Presented at the Seminaire du SCAD,
Montreal, QC, 1999
Janveau-Brennan G, Markovits H, Malfait N, Pion N. Effet d'un
contexte fantaisiste dans un tâche de raisonnement. Presented
at the 17e Congrès Annuel de la S.Q.R.P., Québec, QC, 1994
Malfait N, Markovits H, Dumas C. Young children's understanding
of transitivity in a spatial relationship. Presented at the 23rd
Annual Symposium of the Jean Piaget Society, Philadelphia, PA, 1993