Biomedical subjects

Y Burnod

Publications and source records attributed to Y Burnod.

At least 19 recordsLinked to original sources

PubMed2002

Longitudinal study of motor recovery after stroke: recruitment and focusing of brain activation.

BACKGROUND AND PURPOSE: The goal of this study was to characterize cortical reorganization after stroke and its relation with the site of the stroke-induced lesion and degree of motor recovery using functional MRI (fMRI). METHODS: Fourteen stroke patients with an affected upper limb were studied longitudinally. Three fMRI sessions were performed over a period of 1 to 6 months after stroke. Upper limb recovery, Wallerian degeneration of the pyramidal tract, and responses to transcranial magnetic stimulation were assessed. RESULTS: Two main patterns of cortical reorganization were found. Pattern 1 was focusing, in which, after initial recruitment of additional ipsilateral and contralateral areas, activation gradually developed toward a pattern of activation restricted to the contralateral sensorimotor cortex in 9 patients. Five patients were found to have pattern 2, persistent recruitment, in which there was an initial and sustained recruitment of ipsilateral activity. Occurrence of recruitment or focusing seemed to depend mainly on whether the primary motor cortex (M1) was lesioned; persistent recruitment was observed in 3 of 4 patients with M1 injury, and focusing was seen in 8 of 10 patients with spared M1. These patterns had no relation to the degree of recovery; in particular, focusing did not imply recovery. However, there was a clear relation between the degree of recovery and the degree of Wallerian degeneration. CONCLUSIONS: These results suggest that ipsilateral recruitment after stroke corresponds to a compensatory corticocortical process related to the lesion of the contralateral M1 and that the process of compensatory recruitment will persist if M1 is lesioned; otherwise, it will be transient.

Adaptation, Physiological↗

PubMed2001

Temporal order and spatial memory in schizophrenia: a parametric study.

Spatial working memory has been shown to be impaired in schizophrenia. In contrast, memory for temporal order has been poorly studied in patients with schizophrenia. The aim of this study was to compare and to further characterize spatial working memory and sequence reproduction deficits in patients with schizophrenia under stable medication by manipulating cues (pattern versus sequence), delay, set-size and response type in various recall and recognition tasks. This allowed us to dissociate processes as encoding, retention and retrieval and to compare the performance of patients with schizophrenia to the performance of patients with prefrontal lesions, who have been previously tested in the same tasks. Our results show that increase of the set-size and of the delay decreased performance of both groups, and that these factors had larger detrimental effects in patients with schizophrenia than in controls. Furthermore, comparison between tasks revealed retention and retrieval deficits in schizophrenia. Finally, patients with schizophrenia showed impairments not only in recall but also in sequence recognition tasks with delay. This is in contrast to patients with prefrontal lesions, who have previously been shown to have intact recognition of sequences after a delay. These results suggest that the working memory deficit in schizophrenia cannot be restricted to a prefrontal dysfunction.

Adult↗

PubMed2001

Development of a global motor rating scale for young children (0-4 years) including eye-hand grip coordination.

A comparative study of the eight motor rating scales available in Western countries demonstrated methodological differences in the choice of items and standardization. We have developed a global motor rating scale that includes items which measure postural-motor, locomotor (PML) and eye-hand grip coordination (EHGC), and which allows the assessment of an average of motor function level (MFL), PML and EHGC development. Scores obtained were used to define the acquisition of motor age based on the skills completed. The items were selected on the basis of the average age at which the function developed in two populations of healthy full-term French infants, followed from birth to 4 months (n = 60) and from 4 months to 4 years (n = 63). Recent French developmental standards (mean age and standard deviation) of acquisition allow the identification of neuro-psychomotor deviations from normal motor behaviour. This includes both static and dynamic motor coordination sequences. Inter-examiner correlations (n = 3) for 15 randomly selected children indicated a coefficient of 0.90. The scale revealed a sequence in the organization of learned postural-motor, locomotor and eye-hand gripping skills which can contribute to the understanding of brain areas implicated in this maturation process.

Child Development↗

PubMed2001

Recoding arm position to learn visuomotor transformations.

There is strong experimental evidence that guiding the arm toward a visual target involves an initial vectorial transformation from direction in visual space to direction in motor space. Constraints on this transformation are imposed (i) by the neural codes for incoming information: the desired movement direction is thought to be signalled by populations of broadly tuned neurons and arm position by populations of monotonically tuned neurons; and (ii) by the properties of outgoing information: the actual movement direction results from the collective action of broadly tuned neurons whose preferred directions rotate with the position of the arm. A neural network model is presented that computes the visuomotor mapping, given these constraints. Appropriate operations are learned by the network in an unsupervised fashion through repeated action- perception cycles by recoding the arm-related proprioceptive information. The resulting solution has two interesting properties: (i) the required transformation is executed accurately over a large part of the reaching space, although few positions are actually learned; and (ii) properties of single neurons and populations in the network closely resemble those of neurons and populations in parietal and motor cortical regions. This model thus suggests a realistic scenario for the calculation of coordinate transformations and initial motor command for arm reaching movements.

Arm↗

PubMed2001

Script knowledge after severe traumatic brain injury.

Severe diffuse traumatic brain injury (TBI) may impair the performance of daily-life complex activities. The aim of the present study was to assess whether these difficulties are related to a representational impairment of action knowledge. Two tasks requiring the manipulation of scripts were used. The first (script reconstitution) required subjects to sort cards describing actions belonging to 4 different scripts, presented in a random order. The second (script generation) required subjects to generate actions belonging to a given script. The results showed that TBI patients had preserved access to goal representation and action knowledge. However, they demonstrated (1) significant impairments when they had to deal with simultaneous competing sources of information and (2) a lack of inhibitory control on routine overlearned skills. Patients' performance was significantly correlated with behavioral modifications in everyday life. These data suggest that action impairment in severe TBI patients cannot be attributed to an impairment of action knowledge per se. As previously suggested by Schwartz et al., a restriction of limited-capacity processing resources may account for the observed deficits.

Activities of Daily Living↗

PubMed2000

Functional MRI in double cortex: functionality of heterotopia.

A 12-year-old boy with epilepsy and subcortical laminar heterotopia (band heterotopia) underwent a functional MRI protocol to study voluntary motor activity in the hand. Finger tapping produced an activation of a contralateral limited and focused frontal cortical area both in the subcortical band heterotopia and the overlying cortex. Despite its epileptogenic activity, subcortical laminar heterotopia seems to be responsible for part of the functional activity of the brain. This has to be pointed out for epilepsy surgery resecting cortical dysplasia.

Action Potentials↗

PubMed1999

Planning dysfunction in schizophrenia: impairment of potentials preceding fixed/free and single/sequence of self-initiated finger movements.

To test the hypothesis of a planning dysfunction in schizophrenia using a precise temporal definition, the readiness potential (RP), a negative cortical wave preceding self-initiated movements and reflecting motor preparation processes, was studied in patients under stable medication and in controls. The supplementary motor area (SMA), known to be involved in the generation of the RP, has also been implicated in movement selection (fixed versus free) and complexity (single versus sequence). This is the first study using RP for the assessment of the influence of these factors on motor preparation in schizophrenics. Our results show that schizophrenics' RP amplitude is significantly lower than in controls at central and contralateral electrodes. However, RP amplitude increases with task difficulty in both groups, offering important new insight into classical SMA hypoactivation in schizophrenics performing motor tasks. Topographic analysis shows that RP amplitude is, for both groups, significantly higher in sequence than in single movements at fronto-central sites and higher for free than for fixed movements at centro-parietal sites. Finally, RP onset occurs significantly later in schizophrenics than in controls. These results support the view of a motor-preparation and decision-making dysfunction in schizophrenia. They are interpreted within the framework of a fronto-striatal disorder in this disease.

Adult↗

PubMed1999

Parieto-frontal coding of reaching: an integrated framework.

In the last few years, anatomical and physiological studies have provided new insights into the organization of the parieto-frontal network underlying visually guided arm-reaching movements in at least three domains. (1) Network architecture. It has been shown that the different classes of neurons encoding information relevant to reaching are not confined within individual cortical areas, but are common to different areas, which are generally linked by reciprocal association connections. (2) Representation of information. There is evidence suggesting that reach-related populations of neurons do not encode relevant parameters within pure sensory or motor "reference frames", but rather combine them within hybrid dimensions. (3) Visuomotor transformation. It has been proposed that the computation of motor commands for reaching occurs as a simultaneous recruitment of discrete populations of neurons sharing similar properties in different cortical areas, rather than as a serial process from vision to movement, engaging different areas at different times. The goal of this paper was to link experimental (neurophysiological and neuroanatomical) and computational aspects within an integrated framework to illustrate how different neuronal populations in the parieto-frontal network operate a collective and distributed computation for reaching. In this framework, all dynamic (tuning, combinatorial, computational) properties of units are determined by their location relative to three main functional axes of the network, the visual-to-somatic, position-direction, and sensory-motor axis. The visual-to-somatic axis is defined by gradients of activity symmetrical to the central sulcus and distributed over both frontal and parietal cortices. At least four sets of reach-related signals (retinal, gaze, arm position/movement direction, muscle output) are represented along this axis. This architecture defines informational domains where neurons combine different inputs. The position-direction axis is identified by the regular distribution of information over large populations of neurons processing both positional and directional signals (concerning the arm, gaze, visual stimuli, etc.) Therefore, the activity of gaze- and arm-related neurons can represent virtual three-dimensional (3D) pathways for gaze shifts or hand movement. Virtual 3D pathways are thus defined by a combination of directional and positional information. The sensory-motor axis is defined by neurons displaying different temporal relationships with the different reach-related signals, such as target presentation, preparation for intended arm movement, onset of movements, etc. These properties reflect the computation performed by local networks, which are formed by two types of processing units: matching and condition units. Matching units relate different neural representations of virtual 3D pathways for gaze or hand, and can predict motor commands and their sensory consequences. Depending on the units involved, different matching operations can be learned in the network, resulting in the acquisition of different visuo-motor transformations, such as those underlying reaching to foveated targets, reaching to extrafoveal targets, and visual tracking of hand movement trajectory. Condition units link these matching operations to reinforcement contingencies and therefore can shape the collective neural recruitment along the three axes of the network. This will result in a progressive match of retinal, gaze, arm, and muscle signals suitable for moving the hand toward the target.

Animals↗

PubMed1999

Bayesian inference in populations of cortical neurons: a model of motion integration and segmentation in area MT.

A major issue in cortical physiology and computational neuroscience is understanding the interaction between extrinsic signals from feedforward connections and intracortical signals from lateral connections. We propose here a computational model for motion perception based on the assumption that the local cortical circuits in the medio-temporal area (area MT) implement a Bayesian inference principle. This approach establishes a functional balance between feedforward and lateral, excitatory and inhibitory, inputs. The model reproduces most of the known properties of the neurons in area MT in response to moving stimuli. It accounts for important motion perception phenomena including motion transparency, spatial and temporal integration/segmentation. While integrating several properties of previously proposed models, it makes specific testable predictions concerning, in particular, temporal properties of neurons and the architecture of lateral connections in area MT. In addition, the proposed mechanism is consistent with the known properties of local cortical circuits in area V1. This suggests that Bayesian inference may be a general feature of information processing in cortical neuron populations.

Animals↗

PubMed1998

Early motor influences on visuomotor transformations for reaching: a positive image of optic ataxia.

Coding of reaching in the cerebral cortex is based on the operation of distributed populations of parietal and frontal neurons, whose main functional characteristics reside in their combinatorial power, i.e., in the capacity for combining different information related to the spatial aspects of reaching. The tangential distribution of reach-related neurons endowed with different functional properties changes gradually in the cortex and defines, in the parieto-frontal network, trends of functional properties. These visual-to-somatic gradients imply the existence of cortical regions of functional overlaps, i.e., of combinatorial domains, where the integration of different reach-related signals occurs. Studies of early coding of reaching in the mesial parietal areas show how somatomotor information, such as that related to arm posture and movement, influences neuronal activity in the very early stages of the visuomotor transformation underlying the composition of the motor command and is not added "downstream" in the frontal cortex. This influence is probably due to re-entrant signals traveling through fronto-parietal-association connections. Together with the gradient architecture of the network and the reciprocity of cortico-cortical connections, this implies that coding of reaching cannot be regarded as a top-down, serial sequence of coordinate transformation, each performed by a given cortical area, but as a recursive process, where different signals are progressively matched and further elaborated locally, due to intrinsic cortical connections. This model of reaching is also supported by psychophysical studies stressing the parallel processing of the different relevant parameters and the "hybrid" nature of the reference frame where they are combined. The theoretical frame presented here can also offer a background for a new interpretation of a well-known visuomotor disorder, due to superior parietal lesions, i.e., optic ataxia. More than a disconnection syndrome, this can now be interpreted as the consequence of the breakdown of the operations occurring in the combinatorial domains of the superior parietal segment of the parieto-frontal network.

Animals↗

PubMed1997

Bistable behaviour in a neocortical neurone model.

Intracellular recordings have shown that neocortical pyramidal neurones have an intrinsic capacity for regenerative firing. The cellular mechanism of this firing was investigated by computer simulations of a model neurone endowed with standard action potential and persistent sodium (gNaP) conductances. The firing mode of the neurone was determined as a function of leakage and NaP maximal conductances (gl and gNaP). The neurone had two stable states of activity (bistable) over wide range of gl and gNaP, one at the resting potential and the other in a regenerative firing mode, that could be triggered by a transient input. This model points to a cellular mechanism that may contribute to the generation and maintenance of long-lasting sustained neuronal discharges in the cerebral cortex.

Action Potentials↗

PubMed1996

Functional MR imaging of the human sensorimotor cortex during haptic discrimination.

This study attempted to determine whether haptic discriminations of shape (haptic task) activate the same tissue in the central cortical region of normal human subjects as do finger movements (opposition task). Opposition and haptic tasks both activated the central sulcus, as expected from previous imaging studies. The haptic task activated about 50% of the cortical territory activated by the opposition task. The results suggest that exploratory digital movements performed to collect precise somatosensory information and automatic movements performed during finger positioning activate partially overlapping parts of the sensorimotor cortex.

Brain Mapping↗

PubMed1996

Dynamical computational properties of local cortical networks for visual and motor processing: a bayesian framework.

A major unsolved question concerns the interaction between the coding of information in the cortex and the collective neural operations (such as perceptual grouping, mental rotation) that can be performed on this information. A key property of the local networks in the cerebral cortex is to combine thalamocortical or feedforward information with horizontal cortico-cortical connections. Among different types of neural networks compatible with the known functional and architectural properties of the cortex, we show that there exist interesting bayesian solutions resulting in an optimal collective decision made by the neuronal population. We suggest that thalamo-cortical and cortico-cortical synaptic plasticity can be differentially modulated to optimize this collective bayesian decision process. We take two examples of cortical dynamics, one for perceptual grouping in MT, and the other one for mental rotation in M1. We show that a neural implementation of the bayesian principle is both computationally efficient to perform these tasks and consistent with the experimental data on the related neuronal activities. A major implication is that a similar collective decision mechanism should exist in different cortical regions due to the similarity of the cortical functional architecture.

Algorithms↗

PubMed1995

Modelling the acquisition of goal-directed behaviors by populations of neurons.

Recent neurophysiological studies have revealed the patterns of neuronal activity during the acquisition of goal-directed behaviors, both in single cells, and in large populations of neurons. We propose a model which helps three sets of experimental results in the monkey to be understood: (1) activity of single cells vary greatly and only population activities are causally related to behavior. The model shows how a population of stochastic neurons, whose behaviors vary widely, can learn a skilled conditioned movement with only local activity-dependent synaptic changes. (2) typical changes in neuronal activity occur when the rules governing the behavior are changed, i.e. when the relationship between cues and actions to reach a goal changes over time. There are two types of neuronal patterns during changes in reward contingency: a monotonic increasing pattern and a non-monotonic pattern which follows the change in the way the reward is obtained. Units in the model display these two types of change, which correspond to synaptic modifications related to the encoding of the behavioral significance of sensory and motor events. (3) These two patterns of neuronal activity define two populations whose anatomical distributions in the frontal lobe overlap with a gradient organized in the rostro-caudal direction. The model consists of two artificial neural networks, defined by the same set of equations, but which differ in the values of two parameters (P and Q). P defines the adaptive properties of processing units and Q describes the coding of information. The model suggests that a balance in the relative strengths of these parameters distributed along a rostro-caudal gradient can explain the distribution of neuronal types in the frontal lobe of the monkey.

Animals↗

PubMed1995

Learning a new visuomotor transformation: error correction and generalization.

The use of an aiming tool requires learning a new transformation between visual and proprioceptive information and motor command. We have examined this question by quantifying the kinematics of the movement during the transitory phase of adaptation to a rotational bias (60 degrees counterclockwise, then clockwise) added to a standard mouse-cursor device in the plane of the screen. Control-aiming movements were almost linear with a bell-shaped velocity profile. The bias induced an equivalent initial directional error which was usually corrected within 20 trials. The learning trajectories were combinations of spirals and fast or slow straight movements. The posture of the hand was slightly (less than 10 degrees) modified by the bias. These features suggest three corrective processes: on-line continuous correction based on evaluation of the relative cursor-to-target position, discrete correction based on assessment of the discrepancy angle between the cursor-to-target direction and the effective cursor direction, and memorization of trial-to-trial correction. These results are interpreted in the light of neurophysiological data and neural net modeling, which suggest that the visuomotor transformation performed by cortical areas for reaching is effected by projecting the visual information on a reference frame that rotates with the arm. The initial directional error reappeared when the direction of the target was changed and increased with degree of change. The limited generalization suggests that bias correction is stored in relation to the coding of the target direction and that movement towards a new direction is computed as a projection of the previously learned bias on the new visual direction.

Feedback↗

PubMed1994

Neural network models of cortical functions based on the computational properties of the cerebral cortex.

We describe a biologically plausible modelling framework based on the architectural and processing characteristics of the cerebral cortex. Its key feature is a multicellular processing unit (cortical column) reflecting the modular nature of cortical organization and function. In this framework, we describe a neural network model organization and function. In this framework, we describe a neural network model of the neuronal circuits of the cerebral cortex that learn different functions associated with different parts of the cortex: 1) visual integration for invariant pattern recognition, performed by a cooperation between temporal and parietal areas; 2) visual-to-motor transformation for 3D arm reaching movements, performed by parietal and motor areas; and 3) temporal integration and storage of sensorimotor programs, performed by networks linking the prefrontal cortex to associative sensory and motor areas. The architecture of the network is inspired from the features of the architecture of cortical pathways involved in these functions. We propose two rules which describe neural processing and plasticity in the network. The first rule (adaptive tuning if gating) is an analog of operant conditioning and permits to learn to anticipate an action. The second rule (adaptive timing) is based on a bistable state of activity and permits to learn temporally separate events forming a behavioral sequence.

Cerebral Cortex↗

PubMed1992

Visuomotor transformations underlying arm movements toward visual targets: a neural network model of cerebral cortical operations.

We propose a biologically realistic neural network that computes coordinate transformations for the command of arm reaching movements in 3-D space. This model is consistent with anatomical and physiological data on the cortical areas involved in the command of these movements. Studies of the neuronal activity in the motor (Georgopoulos et al., 1986; Schwartz et al., 1988; Caminiti et al., 1990a) and premotor (Caminiti et al., 1990b, 1991) cortices of behaving monkeys have shown that the activity of individual arm-related neurons is broadly tuned around a preferred direction of movements in 3-D space. Recent data demonstrate that in both frontal areas (Caminiti et al., 1990a,b, 1991) these cell preferred directions rotate with the initial position of the arm. Furthermore, the rotation of the population of preferred directions precisely corresponds to the rotation of the arm in space. The neural network model computes the motor command by combining the visual information about movement trajectory with the kinesthetic information concerning the orientation of the arm in space. The appropriate combination, learned by the network from spontaneous movement, can be approximated by a bilinear operation that can be interpreted as a projection of the visual information on a reference frame that rotates with the arm. This bilinear combination implies that neural circuits converging on a single neuron in the motor and premotor cortices can learn and generalize the appropriate command in a 2-D subspace but not in the whole 3-D space. However, the uniform distribution of cell preferred directions in these frontal areas can explain the computation of the correct solution by a population of cortical neurons. The model is consistent with the existing neurophysiological data and predicts how visual and somatic information can be combined in the different processing steps of the visuomotor transformation subserving visual reaching.

Animals↗

PubMed1991

Organizational levels of the cerebral cortex: an integrated model.

We propose a theoretical model of the cerebral cortex which is based on its cellular components and integrates its different levels of organization: (1) cells have general adaptive and memorization properties; (2) cortical columns are repetitive interneuronal circuits which determine an adaptive processing specific to the cerebral cortex; (3) cortical maps effect selective combinations which are very efficient to learn basic behavioural adaptations such as invariant recognition of forms, visually-guided hand movements, or execution of structured motor programs; (4) the network between cortical areas has a global architecture which integrates successive learning experiences into coherent functions such as the human language.

Cerebral Cortex↗