Mark F. Bradshaw, Simon J. Watt, Kathleen M. Elliott and Patricia M. Riddell
The effects of a pre-movement delay on the kinematics of prehension in middle childhood
Human Movement Science, In Press, Corrected Proof, Available online 11 September 2004

 

Byblow, W.
Sensorimotor Coordination: Behavioural Modes and Neural Mechanisms
Human Movement Science, Volume 23, Issues 3-4, October 2004, Pages 235-238.

Camachon, Buekers and Montagne
Is the learning of goal-directed displacement effector-independent?
Human Movement Science, Volume 23, Issues 3-4, October 2004, Pages 239-255.

The present experiment was designed to investigate whether the learning
of goal-directed locomotion is effector independent. To answer this
question a bilateral transfer of learning paradigm was used. We wanted
to find out whether learning can be transferred from a trained effector
system to an untrained one. Sixteen participants were asked to proceed
through virtual hallways, while walking on a treadmill or handling of a
joystick, in order to cross a pair of oscillating doors. Participants
received 1050 training trials on the specific effector system before
being transferred to the untrained one. Results indicated a clear
transfer from handling to walking and only a moderate transfer from
walking to handling. This asymmetrical transfer provides partial
support in favor of the effector independent hypothesis. Both the
theoretical implications of this work and the possible mediating effect
of calibration are discussed.

Choi, Hyeg Joo and Mark, Leonard S. 
Scaling affordances for human reach actions
Human Movement Science, In Press, Corrected Proof, Available online 14 October 2004

A methodology developed by Cesari and Newell [Cesari, P., & Newell, K. M. (1999).
The scaling of human grip configuration. Journal of Experimental Psychology: Human
Perception and Performance 25, 927–935; Cesari, P., & Newell, K. M. (2000). The body-scaling
of grip configurations in children aged 6–12 years. Developmental Psychobiology 36, 301–310] was
used to delineate the roles of an object’s weight (W) and distance (D) as well as the actor’s strength (S)
in determining the macroscopic action used to reach for the object.
Participants reached for objects of five different weights placed at 10
distances. The findings of a single discriminant analysis revealed that
when object weight is scaled in terms of each individual’s strength and
reach distance is scaled in terms of each individual’s maximum-seated
reach distance, a single discriminant analysis was able to predict 90%
of the reach modes used by both men and women. The result of the
discriminant analysis was used to construct a body-scaled equation, K = ln D + ln(W/S)/36,
similar in form to the one derived by Cesari and Newell, accurately
predicted the reach action used. Our findings indicate that Cesari and
Newell’s method can identify a complex relationship between geometric
and dynamic constraints that determine the affordances for different
reach actions.

 
Changes in connectivity profiles define functionally distinct regions in human medial frontal cortex.
Johansen-Berg H, Behrens TE, Robson MD, Drobnjak I, Rushworth MF, Brady JM, Smith SM, Higham DJ, Matthews PM.
Oxford Centre for Functional Magnetic Resonance Imaging of the Brain,
University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, United
Kingdom. heidi@fmrib.ox.ac.uk
Proc Natl Acad Sci U S A. 2004 Sep 7;101(36):13335-40. Epub 2004 Aug 30.

A fundamental issue in
neuroscience is the relation between structure and function. However,
gross landmarks do not correspond well to microstructural borders and
cytoarchitecture cannot be visualized in a living brain used for
functional studies. Here, we used diffusion-weighted and functional MRI
to test structure-function relations directly. Distinct neocortical
regions were defined as volumes having similar connectivity profiles
and borders identified where connectivity changed. Without using prior
information, we found an abrupt profile change where the border between
supplementary motor area (SMA) and pre-SMA is expected. Consistent with
this anatomical assignment, putative SMA and pre-SMA connected to motor
and prefrontal regions, respectively. Excellent spatial correlations
were found between volumes defined by using connectivity alone and
volumes activated during tasks designed to involve SMA or pre-SMA
selectively. This finding demonstrates a strong relationship between
structure and function in medial frontal cortex and offers a strategy
for testing such correspondences elsewhere in the brain.

 
Where bottom-up meets top-down: neuronal interactions during perception and imagery.
Mechelli A, Price CJ, Friston KJ, Ishai A.
Wellcome Department of Imaging Neuroscience, Institute of Neurology, 12 Queen
Square, London, WC1N 3BG, UK. andream@fil.ion.ucl.ac.uk
Cereb Cortex. 2004 Nov;14(11):1256-65. Epub 2004 Jun 10.

Functional magnetic resonance imaging (fMRI) studies have identified
category-selective regions in ventral occipito-temporal cortex that
respond preferentially to faces and other objects. The extent to which
these patterns of activation are modulated by bottom-up or top-down
mechanisms is currently unknown. We combined fMRI and dynamic causal
modelling to investigate neuronal interactions between
occipito-temporal, parietal and frontal regions, during visual
perception and visual imagery of faces, houses and chairs. Our results
indicate that, during visual perception, category-selective patterns of
activation in extrastriate cortex are mediated by content-sensitive
forward connections from early visual areas. In contrast, during visual
imagery, category-selective activation is mediated by content-sensitive
backward connections from prefrontal cortex. Additionally, we report
content-unrelated connectivity between parietal cortex and the
category-selective regions, during both perception and imagery. Thus,
our investigation revealed that neuronal interactions between
occipito-temporal, parietal and frontal regions are task- and
stimulus-dependent. Sensory representations of faces and objects are
mediated by bottom-up mechanisms arising in early visual areas and
top-down mechanisms arising in prefrontal cortex, during perception and
imagery respectively. Additionally non-selective, top-down processes,
originating in superior parietal areas, contribute to the generation of
mental images, regardless of their content, and their maintenance in
the 'mind's eye'.

Neilson, Peter D. and Neilson, Megan D. 
A new view on visuomotor channels: The case of the disappearing dynamics
Human Movement Science, Volume 23, Issues 3-4, October 2004, Pages 257-283.

A considerable body of kinematic data supports the proposal that
independent visuomotor channels are involved in the control of the
transport and grip components of reach and grasp. These channels are
seen as having separate perceptual inputs, outputs and internal
processing and are thought by some to correspond to independent
neuroanatomical pathways. The idea that different groups of muscles and
biomechanical structures can be controlled independently is attractive,
but this kinematically-inspired hypothesis fails to take into account
the complexity of the dynamic relationships and their interactions
within the neuromusculoskeletal system. Inertial, viscous, centrifugal,
coriolis, gravitational and reflex cross couplings exist between
efferent drives to muscles and resulting body movements. Rotation at
even a single joint generates a complex set of dynamic reaction forces
and requires coordinated activation of many muscles throughout the body
to maintain posture and balance. In this theoretical paper we present a
new view of independent visuomotor channels in the form of an adaptive
neural controller that can compensate for the above interactions and
decouple the relationships between efferent drives to muscles and
resulting body movements. At the same time, the neural controller
renders all the dynamics (linear and nonlinear), other than time
delays, of the neuromusculoskeletal system, unobservable in the
visuomotor relationships. Using the geometry of nonlinear dynamical
systems we show that, providing certain constraints on the structure of
time delays within the system are satisfied, there exists a neural
controller that can render all the dynamics of the neuromusculoskeletal
system (except for time delays) unobservable in the responses. The
controller simultaneously decouples all the interactive dynamics so
that each of the m independent inputs controls one and only one
degree of freedom of the response. This means that each degree of
freedom in a multi-joint response can be controlled by an independent
component of the visual input, a behaviour that has long been observed
in visual tracking experiments. The controller effectively establishes m
independent visuomotor channels. However, rather than reflecting
separate neuroanatomical pathways, the independent channels result from
a neural controller with convergent and divergent connections to
compensate for the interactive nonlinear dynamics within the
neuromusculoskeletal system. This new view of visuomotor channels has
implications for neural control processes involved in the acquisition
and adaptability of skilled perceptual–motor behaviour in general, as
well as for the design of robotic controllers.

 
Catecholaminergic consolidation of motor cortical neuroplasticity in humans.
Nitsche MA, Grundey J, Liebetanz D, Lang N, Tergau F, Paulus W.
Department of Clinical Neurophysiology, Georg-August-University, Robert Koch Str.
40, 37075 Goettingen, Germany. mnitsch1@gwdg.de
Cereb Cortex. 2004 Nov;14(11):1240-5. Epub 2004 May 13.

Amphetamine, a catecholaminergic re-uptake-blocker, is able to improve neuroplastic
mechanisms in humans. However, so far not much is known about the
underlying physiological mechanisms. Here, we study the impact of
amphetamine on NMDA receptor-dependent long-lasting excitability
modifications in the human motor cortex elicited by weak transcranial
direct current stimulation (tDCS). Amphetamine significantly enhanced
and prolonged increases in anodal, tDCS-induced, long-lasting
excitability. Under amphetamine premedication, anodal tDCS resulted in
an enhancement of excitability which lasted until the morning after
tDCS, compared to approximately 1 h in the placebo condition.
Prolongation of the excitability enhancement was most pronounced for
long-term effects; the duration of short-term excitability enhancement
was only slightly increased. Since the additional application of the
NMDA receptor antagonist dextromethorphane blocked any enhancement of
tDCS-driven excitability under amphetamine, we conclude that
amphetamine consolidates the tDCS-induced neuroplastic effects, but
does not initiate them. The fact that propanolol, a beta-adrenergic
antagonist, diminished the duration of the tDCS-generated after-effects
suggests that adrenergic receptors play a certain role in the
consolidation of NMDA receptor-dependent motor cortical excitability
modifications in humans. This result may enable researchers to optimize
neuroplastic processes in the human brain on the rational basis of
purpose-designed pharmacological interventions.

Shadmehr, R
Generalization as a behavioral window to the neural mechanisms of learning internal models
Human Movement Science, Volume 23, Issue 5, November 2004, Pages 543-568.

In generating motor commands, the brain seems to rely on internal
models that predict physical dynamics of the limb and the external
world. How does the brain compute an internal model? Which neural
structures are involved? We consider a task where a force field is
applied to the hand, altering the physical dynamics of reaching.
Behavioral measures suggest that as the brain adapts to the field, it
maps desired sensory states of the arm into estimates of force. If this
neural computation is performed via a population code, i.e., via a set
of bases, then activity fields of the bases dictate a generalization
function that uses errors experienced in a given state to influence
performance in any other state. The patterns of generalization suggest
that the bases have activity fields that are directionally tuned, but
directional tuning may be bimodal. Limb positions as well as contextual
cues multiplicatively modulate the gain of tuning. These properties are
consistent with the activity fields of cells in the motor cortex and
the cerebellum. We suggest that activity fields of cells in these motor
regions dictate the way we represent internal models of limb dynamics.

 
Fine modulation in network activation during motor execution and motor imagery.
Solodkin A, Hlustik P, Chen EE, Small SL.
Department of Neurology and Brain Research Imaging Center, The University of
Chicago, Chicago, IL 60637, USA. solodkin@uchicago.edu
Cereb Cortex. 2004 Nov;14(11):1246-55. Epub 2004 May 27.

Motor imagery, the 'mental rehearsal of motor acts without overt movements',
involves either a visual representation (visual imagery, VI) or mental
simulation of movement, associated with a kinesthetic feeling (kinetic
imagery, KI). Previous brain imaging work suggests that patterns of
brain activation differ when comparing execution (E) with either type
of imagery but the functional connectivity of the participating
networks has not been studied. Using functional magnetic resonance
imaging (fMRI) and structural equation modeling, this study elucidates
the inter-relationships among the relevant areas for each of the three
motor behaviors. Our results suggest that networks underlying these
behaviors are not identical, despite the extensive overlap between E
and KI. Inputs to M1, which are facilitatory during E, have the
opposite effect during KI, suggesting a physiological mechanism whereby
the system prevents overt movements. Finally, this study highlights the
role of the connection of superior parietal lobule to the supplementary
motor area in both types of motor imagery.

 
The influence of hand posture on corticospinal excitability during motor imagery: a transcranial magnetic stimulation study.
Vargas CD, Olivier E, Craighero L, Fadiga L, Duhamel JR, Sirigu A.
Institute of Cognitive Sciences, UMR 5015, 67 Boulevard Pinel, 69675, Bron Cedex, France.
Cereb Cortex. 2004 Nov;14(11):1200-6. Epub 2004 May 13.

In order to study the interaction between proprioceptive information and
motor imagery, we herein investigate how compatible and incompatible
postural signals influence corticospinal excitability during the mental
simulation of hand movements. Subjects were asked to imagine themselves
joining the tips of the thumb and the little finger while they
maintained one of the two following hand postures: posture A (PA,
compatible), little finger, index and thumb extended, the remaining
fingers flexed; or posture B (PB, incompatible), index and thumb
extended, other fingers flexed. All subjects rated the imagined finger
opposition movements as easier to perform when the hand was kept in PA
than in PB (P < 0.01) and the correlation between the duration of
motor imagery and movement execution was also higher for PA than PB (P
< 0.01). For each posture, motor evoked potentials (MEPs) elicited
by focal transcranial magnetic stimulation (TMS) of the left motor
cortex were recorded from the right opponens pollicis muscle during
both motor imagery (MI) and rest (R) conditions. MEP area varied
according to the hand posture: PA induced a higher increase in
corticospinal excitability, when compared with PB. These results
indicate that the actual limb posture affects the process of motor
imagery. The source of this postural modulation effect is discussed.

 
Synaptic homeostasis and input selectivity follow from a calcium-dependent plasticity model.
Yeung LC, Shouval HZ, Blais BS, Cooper LN.
Institute for Brain and Neural Systems, Department of Physics, Brown University,
Providence, RI 02912, USA. yeung@physics.brown.edu
Proc Natl Acad Sci U S A. 2004 Oct 12;101(41):14943-8. Epub 2004 Oct 04.

Modifications in the strengths of synapses are thought to underlie memory,
learning, and development of cortical circuits. Many cellular mechanisms
of synaptic plasticity have been investigated in which differential
elevations of postsynaptic calcium concentrations play a key role in
determining the direction and magnitude of synaptic changes. We have
previously described a model of plasticity that uses calcium currents
mediated by N-methyl-D-aspartate receptors as the associative signal
for Hebbian learning. However, this model is not completely stable.
Here, we propose a mechanism of stabilization through homeostatic
regulation of intracellular calcium levels. With this model, synapses
are stable and exhibit properties such as those observed in
metaplasticity and synaptic scaling. In addition, the model displays
synaptic competition, allowing structures to emerge in the synaptic
space that reflect the statistical properties of the inputs. Therefore,
the combination of a fast calcium-dependent learning and a slow
stabilization mechanism can account for both the formation of selective
receptive fields and the maintenance of neural circuits in a state of
equilibrium.