Biomedical subjects

Terence D Sanger

Publications and source records attributed to Terence D Sanger.

16 recordsLinked to original sources

PubMed2007

Does dystonia always include co-contraction? A study of unconstrained reaching in children with primary and secondary dystonia.

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 tar-gets 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.

Adolescent↗

PubMed2006

Bayesian filtering of myoelectric signals.

Surface electromyography is used in research, to estimate the activity of muscle, in prosthetic design, to provide a control signal, and in biofeedback, to provide subjects with a visual or auditory indication of muscle contraction. Unfortunately, successful applications are limited by the variability in the signal and the consequent poor quality of estimates. I propose to use a nonlinear recursive filter based on Bayesian estimation. The desired filtered signal is modeled as a combined diffusion and jump process and the measured electromyographic (EMG) signal is modeled as a random process with a density in the exponential family and rate given by the desired signal. The rate is estimated on-line by calculating the full conditional density given all past measurements from a single electrode. The Bayesian estimate gives the filtered signal that best describes the observed EMG signal. This estimate yields results with very low short-time variability but also with the capability of very rapid response to change. The estimate approximates isometric joint torque with lower error and higher signal-to-noise ratio than current linear methods. Use of the nonlinear filter significantly reduces noise compared with current algorithms, and it may therefore permit more effective use of the EMG signal for prosthetic control, biofeedback, and neurophysiology research.

Adult↗

PubMed2006

Arm trajectories in dyskinetic cerebral palsy have increased random variability.

Dyskinetic cerebral palsy results from injury to the basal ganglia early in life. Symptoms can include hyperkinetic or dystonic arm movements that impair function. It is not known whether these movements comprise a small number of specific abnormal motor patterns or whether they are random and variable. We hypothesize that injury to the basal ganglia leads to impaired filtering and removal of undesired neural signals and that lack of appropriate removal of noisy or irrelevant neural signals leads to random and variable arm movements. To test this hypothesis, we quantified the variability in arm trajectories while seven children with dyskinetic cerebral palsy between the ages of 4 and 13 years old made repeated outward reaching movements. We compared the results with those of 21 healthy children between the ages of 5 and 16 years. The best-fit trajectory to the set of reaching movements for each child was taken as the predictable component of movement. We calculated the ratio of the power in the best-fit trajectory to the total variance. This measure is the signal-to-noise ratio, and it quantifies the extent to which trajectories are predictable. We found that children with dyskinetic cerebral palsy had a significantly reduced signal-to-noise ratio compared with healthy children at similar ages. This result shows that there is increased movement variability, and it is consistent with the hypothesis that inadequate removal of noisy signals could be a cause of the movement disorder in dyskinetic cerebral palsy.

Adolescent↗

PubMed2006

Definition and classification of negative motor signs in childhood.

In this report we describe the outcome of a consensus meeting that occurred at the National Institutes of Health in Bethesda, Maryland, March 12 through 14, 2005. The meeting brought together 39 specialists from multiple clinical and research disciplines including developmental pediatrics, neurology, neurosurgery, orthopedic surgery, physical therapy, occupational therapy, physical medicine and rehabilitation, neurophysiology, muscle physiology, motor control, and biomechanics. The purpose of the meeting was to establish terminology and definitions for 4 aspects of motor disorders that occur in children: weakness, reduced selective motor control, ataxia, and deficits of praxis. The purpose of the definitions is to assist communication between clinicians, select homogeneous groups of children for clinical research trials, facilitate the development of rating scales to assess improvement or deterioration with time, and eventually to better match individual children with specific therapies. "Weakness" is defined as the inability to generate normal voluntary force in a muscle or normal voluntary torque about a joint. "Reduced selective motor control" is defined as the impaired ability to isolate the activation of muscles in a selected pattern in response to demands of a voluntary posture or movement. "Ataxia" is defined as an inability to generate a normal or expected voluntary movement trajectory that cannot be attributed to weakness or involuntary muscle activity about the affected joints. "Apraxia" is defined as an impairment in the ability to accomplish previously learned and performed complex motor actions that is not explained by ataxia, reduced selective motor control, weakness, or involuntary motor activity. "Developmental dyspraxia" is defined as a failure to have ever acquired the ability to perform age-appropriate complex motor actions that is not explained by the presence of inadequate demonstration or practice, ataxia, reduced selective motor control, weakness, or involuntary motor activity.

Apraxias↗

PubMed2005

Hypertonia in children: how and when to treat.

Hypertonia in children can be caused by many different diseases. The most common etiology is cerebral palsy. Spasticity and dystonia are the most common types of hypertonia. There are few options for treatment, and usually treatment has an incomplete effect. Therefore, it is necessary to prioritize goals in order to improve overall functional outcome. The use of any intervention will require the ability to verify the magnitude and importance of the outcome in order to ensure that therapy is efficacious. In general, a complex, flexible, and multifaceted approach will be necessary to improve the motor abilities of children with hypertonia.

Journal Article↗

PubMed2005

Reaching movements in childhood dystonia contain signal-dependent noise.

When reaching, children with dystonia exhibit movements that are slower and more variable than normal children. We hypothesize that in dystonia there is an increase in signal-dependent noise so that there is increased variability with increasing speed. This hypothesis predicts that slower movement in children with dystonia is at least partly due to a compensatory strategy to reduce variability by decreasing speed. To test this hypothesis, we measured the speed of arm movement while children attempted to contact buttons of different sizes. We tested 23 control children and 15 children between the ages of 4 and 16 years with dystonia owing to either cerebral palsy, idiopathic dystonia not due to the DYT1 (torsin A) mutation, or other identified causes. A consistent inverse relationship between movement time and button size was seen for both the control children and the children with dystonia. The variance of movement speed increased with the average speed for all subjects. Children with dystonia moved significantly more slowly at all button sizes, and their movement speed was more sensitive to changes in button size. Therefore, part of the reduction in speed in dystonia is due to relatively greater difficulty in contacting small targets. This finding is consistent with the hypothesis of increased signal-dependent noise in children with dystonia, and we present a simple computational model that provides a possible explanation for the origin of this noise.

Adolescent↗

PubMed2004

Severe resting clonus caused by thyrotoxicosis in a 16-year-old girl with hereditary spastic paraparesis: a case report.

A 16-year-old girl with a history of hereditary spastic paraparesis developed the subacute onset of severe clonus and new upper-extremity spasticity due to thyrotoxicosis. This case demonstrates the ability of hyperthyroid states to worsen symptoms of spasticity in a child with an underlying spastic disorder. It also demonstrates the importance of investigating for common systemic diseases as a cause of movement disorders even in children with underlying known genetic disorders.

Adolescent↗

PubMed2004

Toward a definition of childhood dystonia.

PURPOSE OF REVIEW: The purpose of this review is to summarize recent progress toward providing a consistent, sensitive, specific, and useful definition of dystonia as it presents in childhood. RECENT FINDINGS: An NIH-funded consensus group published a definition of childhood dystonia in January of 2003. Recent work has attempted to identify quantitative methods for diagnosis and measurement of childhood dystonia. Techniques include biomechanical, kinematic, and surface EMG measurements that show promise for providing specific and sensitive measures of childhood dystonia. SUMMARY: The results of current research efforts will be useful for verifying and modifying definitions of dystonia to provide consistent and measurable terms for including children in research trials and selecting appropriate interventions for clinical treatment.

Child↗

PubMed2004

Failure of motor learning for large initial errors.

For certain complex motor tasks, humans may experience the frustration of a lack of improvement despite repeated practice. We investigate a computational basis for failure of motor learning when there is no prior information about the system to be controlled and when it is not practical to perform a thorough random exploration of the set of possible commands. In this case, if the desired movement has never yet been performed, then it may not be possible to learn the correct motor commands since there will be no appropriate training examples. We derive the mathematical basis for this phenomenon when the controller can be modeled as a linear combination of nonlinear basis functions trained using a gradient descent learning rule on the observed commands and their results. We show that there are two failure modes for which continued training examples will never lead to improvement in performance. We suggest that this may provide a model for the lack of improvement in human skills that can occur despite repeated practice of a complex task.

Computer Simulation↗

PubMed2003

Abnormal coupling of knee and hip moments during maximal exertions in persons with cerebral palsy.

The motions of lower-limb extension, adduction, and internal rotation are frequently coupled in persons with cerebral palsy (CP) and are commonly referred to as an extension synergy. However, the underlying joint moments that give rise to these coupled motions are not well understood. We hypothesized that maximal voluntary exertions in a direction of one component of a synergy (e.g., hip extension) would result in the concurrent presence of other components of the synergy in subjects with CP but not in control subjects. To test this hypothesis, we measured three-dimensional moments about the hip and knee as nine subjects with spastic CP and six control subjects performed maximal isometric exertions of the hip and knee flexors and extensors. During maximal hip extension exertions, control subjects simultaneously generated a knee flexion moment, whereas CP subjects generated a knee extension moment (P < 0.05) and a larger hip internal rotation moment than did controls (P < 0.05). During maximal knee extension exertions, control subjects generated a hip flexion moment, whereas CP subjects generated a hip extension moment (P < 0.05). The patterns of joint moments generated by CP subjects are consistent with an extension synergy and may underlie the coupled motion patterns of the lower extremity in such persons.

Adolescent↗

PubMed2003

Neural population codes.

In many regions of the brain, information is represented by patterns of activity occurring over populations of neurons. Understanding the encoding of information in neural population activity is important both for grasping the fundamental computations underlying brain function, and for interpreting signals that may be useful for the control of prosthetic devices. We concentrate on the representation of information in neurons with Poisson spike statistics, in which information is contained in the average spike firing rate. We analyze the properties of population codes in terms of the tuning functions that describe individual neuron behavior. The discussion centers on three computational questions: first, what information is encoded in a population; second, how does the brain compute using populations; and third, when is a population optimal? To answer these questions, we discuss several methods for decoding population activity in an experimental setting. We also discuss how computation can be performed within the brain in networks of interconnected populations. Finally, we examine questions of optimal design of population codes that may help to explain their particular form and the set of variables that are best represented. We show that for population codes based on neurons that have a Poisson distribution of spike probabilities, the behavior and computational properties of the code can be understood in terms of the tuning properties of individual cells.

Action Potentials↗

PubMed2003

Pediatric movement disorders.

PURPOSE OF REVIEW: Pediatric movement disorders are a heterogeneous group of symptoms that occur in the context of a large number of different neurological diseases. Accurate diagnosis and quantification of these disorders is essential for determining outcome, appropriate treatment, and criteria for inclusion in research trials. The purpose of this review is to summarize recent advances in diagnosis and treatment for childhood movement disorders. RECENT FINDINGS: The ultimate goal is to discover new treatments that can lead to measurable improvement in functional outcome for affected children. In order to accomplish this goal, we must have consistent definitions and accurate measurements to determine the diagnosis and severity for each child in a clinic or research trial. Recent progress in defining childhood movement disorders has led to consensus definitions of different types of hypertonia. There has also been progress in the development of outcome measures that relate to meaningful functional performance in a variety of skill areas. Most exciting is the prospect of new treatments, and we survey the current non-medical, medical, and surgical therapies for childhood motor disorders. SUMMARY: Although pediatric movement disorders are a complex and often poorly understood group of symptoms, recent work has shown that there is a possibility of defining, measuring, and ultimately treating these debilitating diseases.

Anticonvulsants↗

PubMed2003

Pathophysiology of pediatric movement disorders.

Pediatric movement disorders constitute a relatively small cluster of symptoms that can be associated with many different underlying diseases. To provide effective treatment, it is essential to understand the relationship between etiology and clinical expression. This article reviews the recent literature on several common pediatric movement disorders, including spasticity, dystonia, chorea, myoclonus, bradykinesia, and tics, and it discusses current models of physiology that may help link the cellular pathology of specific diseases to the expression of clinical symptoms.

Ataxia↗

PubMed2003

Classification and definition of disorders causing hypertonia in childhood.

OBJECTIVE: This report describes the consensus outcome of an interdisciplinary workshop that was held at the National Institutes of Health in April 2001. The purpose of the workshop and this article are to define the terms "spasticity," "dystonia," and "rigidity" as they are used to describe clinical features of hypertonia in children. The definitions presented here are designed to allow differentiation of clinical features even when more than 1 is present simultaneously. METHODS: A consensus agreement was obtained on the best current definitions and their application in clinical situations. RESULTS: "Spasticity" is defined as hypertonia in which 1 or both of the following signs are present: 1) resistance to externally imposed movement increases with increasing speed of stretch and varies with the direction of joint movement, and/or 2) resistance to externally imposed movement rises rapidly above a threshold speed or joint angle. "Dystonia" is defined as a movement disorder in which involuntary sustained or intermittent muscle contractions cause twisting and repetitive movements, abnormal postures, or both. "Rigidity" is defined as hypertonia in which all of the following are true: 1) the resistance to externally imposed joint movement is present at very low speeds of movement, does not depend on imposed speed, and does not exhibit a speed or angle threshold; 2) simultaneous co-contraction of agonists and antagonists may occur, and this is reflected in an immediate resistance to a reversal of the direction of movement about a joint; 3) the limb does not tend to return toward a particular fixed posture or extreme joint angle; and 4) voluntary activity in distant muscle groups does not lead to involuntary movements about the rigid joints, although rigidity may worsen. CONCLUSION: We have provided a set of definitions for the purpose of identifying different components of childhood hypertonia. We encourage the development of clinical rating scales that are based on these definitions, and we encourage research to relate the degree of hypertonia to the degree of functional ability, change over time, and societal participation in children with motor disorders.

Child↗

PubMed2002

Nonlinear sensory cortex response to simultaneous tactile stimuli in writer's cramp.

Writer's cramp is a task-specific dystonia that leads to involuntary hand postures during writing. Abnormalities of sensory processing may play a pathophysiological role in this disorder. Electrophysiology studies in a monkey model of focal dystonia have revealed de-differentiation of sensory maps and the existence of single cells in hand regions of area 3b with enlarged receptive fields that extend to the surfaces of more than one digit. These changes may lead to abnormal processing of simultaneous sensory inputs. To study abnormal processing of simultaneous sensory information in adult humans with writer's cramp, we used functional magnetic resonance imaging to compare the response in primary sensory cortex with simultaneous tactile stimulation of the index and middle finger, with the response to stimulation of each finger alone. We tested five patients with writer's cramp and seven unaffected (normal) subjects. In the normal subjects, a linear combination of the activation patterns for individual finger stimulation predicts the pattern of activity for combined stimulation with 12% error. In writer's cramp patients, the linear combination predicted the combined stimulation pattern with 30% error. Results indicate a nonlinear interaction between the sensory cortical response to individual finger stimulation in writer's cramp. This altered interaction may contribute to the motor abnormalities.

Brain↗

PubMed2002

Decoding neural spike trains: calculating the probability that a spike train and an external signal are related.

Experimental and clinical applications of extracellular recordings of spiking cell activity frequently are used to relate the activity of a cell to externally measurable signals such as surface potentials, sensory stimuli, or movement measurements. When the external signal is time-varying, correlation methods have traditionally been used to quantify the degree of relation with the neural firing. However, in some circumstances correlation methods can give misleading results. A new algorithm is described that estimates the extent to which a spike train is related to a continuous time-varying signal. The technique calculates the probability of generating a spike train with Poisson statistics if the time-varying signal determines the Poisson rate. This is accomplished by successive division of the signal and the spike train into halves and recursive calculation of the probability of each half-signal. The performance of the new algorithm is compared with the performance of correlation methods on simulated data.

Action Potentials↗