Peter D. Neilson

Internal models and intermittency: A theoretical account of human tracking behavior

This paper concerns the use of tracking studies to test a theoretical account of the information processing performed by the human CNS during control of movement. The theory provides a bridge between studies of reaction time and continuous tracking. It is proposed that the human CNS includes neuronal circuitry to compute inverse internal models of the multiple input, multiple output, dynamic, nonlinear relationships between outgoing motor commands and their resulting perceptual consequences. The inverse internal models are employed during movement execution to transform preplanned trajectories of desired perceptual consequences into appropriate outgoing motor commands to achieve them. A finite interval of time is required by the CNS to preplan the desired perceptual consequences of a movement and it does not commence planning a new movement until planning of the old one has been completed. This behavior introduces intermittency into the planning of movements. In this paper we show that the gain and phase frequency response characteristics of the human operator in a visual pursuit tracking task can be derived theoretically from these assumptions. By incorporating the effects of internal model inaccuracy and of speed-accuracy trade-off in performance, it is shown that various aspects of experimentally measured tracking behavior can be accounted for.

Stochastic prediction in pursuit tracking: An experimental test of adaptive model theory

In this paper we test the proposition that in pursuit tracking, subjects compute stochastic (statistical) models of the temporal variations in position of the target and use these models to forecast target position for at least a response time interval into the future. A computer simulation of a human operator employing stochastic model prediction of target position is used to generate a synthetic pursuit tracking response signal. Actual pursuit tracking response signals are measured from 10 normal subjects using the same stimulus signal. Cross correlation and spectral analysis are employed to compute gain and phase frequency response characteristics for both synthetic and actual tracking data. The similarity of the gain and phase curves for synthetic and actual data provides compelling evidence in support of the proposition.

Self-regulation of spasm and spasticity in cerebral palsy.

Four young adult cerebral palsied subjects with a mixture of spasticity and athetosis attended an experimental reflex training program for three one-hour sessions each week over an 18 month period. During each session on-line measures of contraction level and tonic stretch reflex sensitivity from the biceps brachii muscle were shown to the subject on meter displays. Subjects were asked to attempt to control the displays. They were given goals such as: (1) reduce both contraction level and reflex sensitivity displays to zero and (2) increase the contraction level display to 10% of maximum while keeping the reflex sensitivity display at a minimum. Achievement of goals was automatically sensed and used to activate a cassette tape of the subjects favourite music. Contraction level and reflex sensitivity scores were averaged over one-minute intervals to provide a record of long term progress. Elbow-angle and IEMG data were recorded on FM tape for off-line analysis. All four subjects learned to suppress involuntary muscle activity and resting tonic stretch reflex responses. They also learned to produce a two or three-fold variation in action tonic stretch reflex sensitivity while sustaining 10% maximum voluntary contraction. In other words, subjects learned to self-regulate spasm and spasticity at the elbow and to regulate tonic stretch reflex sensitivity independently of contraction level. A visual tracking task requiring voluntary movement about the elbow was employed to assess improvement in functional control of elbow movement. One athetotic subject improved tracking accuracy as a consequence of reducing the amount of involuntary arm movement while the other three subjects showed negligible improvement in functional control.

Mechanisms of muscle growth related to muscle contracture in cerebral palsy.

Over the past 15 years studies in experimental animals have shed considerable light on the mechanisms of longitudinal muscle growth. These studies reveal a crucial role for the functioning of muscle in determining its structure and thereby illuminate the problem of muscle contracture in cerebral palsy

The problem of redundancy in movement control: The adaptive model theory approach

SummaryThe problem of redundancy in movement control is encountered when one attempts to answer the question: How does the central nervous system (CNS) determine the pattern of neural activity required in some 5,000,000 descending motor fibres to control only 100–150 biomechanical degrees of freedom of movement? Mathematically this is equivalent to solving a set of simultaneous equations with many more unknowns than equations. This system of equations is redundant because it has an infinite number of possible solutions. The problem is solved by the neuronal circuitry hypothesized in Adaptive Model Theory (AMT). According to AMT, the CNS includes neuronal circuitry able to compute and maintain adaptively the accuracy of internal models of the reciprocal multivariable relationships between outgoing motor commands and their resulting sensory consequences. To identify these input-output relationships by means of regression analysis, correlations between the input signals have to be taken into account. For example, if the inputs are perfectly correlated, the model reduces to a virtual one-input system. In general, the number of inputs modelled equals the number of degrees of freedom encoded by the signals; that is, the number of independently varying (orthogonal) signals. The adaptive modelling circuitry proposed in AMT automatically tunes itself to extract independently varying sensory and motor signals before computing the dynamic relationships between them. Inverse models are employed during response execution to translate movements preplanned as desired trajectories of these high-level sensory-feature signals into appropriately co-ordinated motor commands to send to the muscles. Since movement is preplanned in terms of a number of orthogonalized sensory-feature signals equal to the number of degrees of freedom in the desired response, the problem of redundancy is solved and the correlation or co-ordination between motor-command signals is automatically introduced by the adaptive models.

Speech motor control and stuttering: a computational model of adaptive sensory-motor processing

Abstract A theoretical account of stuttering is presented in which an inadequacy of neuronal resources for sensory-motor information processing is seen as the basis of the disorder. It is proposed that stutterers are deficient in the processing resources normally responsible for determining and adaptively maintaining the internal models which subserve speech production. A general description of such computational processes is detailed in the form of circuitry for an adaptive controller which can calibrate itself to control any variable, nonlinear, dynamic, multiple input, multiple output system.

An overview of adaptive model theory: solving the problems of redundancy, resources, and nonlinear interactions in human movement control

Adaptive model theory (AMT) is a computational theory that addresses the difficult control problem posed by the musculoskeletal system in interaction with the environment. It proposes that the nervous system creates motor maps and task-dependent synergies to solve the problems of redundancy and limited central resources. These lead to the adaptive formation of task-dependent feedback/feedforward controllers able to generate stable, noninteractive control and render nonlinear interactions unobservable in sensory-motor relationships. AMT offers a unified account of how the nervous system might achieve these solutions by forming internal models. This is presented as the design of a simulator consisting of neural adaptive filters based on cerebellar circuitry. It incorporates a new network module that adaptively models (in real time) nonlinear relationships between inputs with changing and uncertain spectral and amplitude probability density functions as is the case for sensory and motor signals.

Dependence of stretch reflexes on amplitude and bandwidth of stretch in human wrist muscle

Abstract The tonic stretch reflex was investigated using small-amplitude displacements (<4.2°) of the wrist while subjects maintained average contraction levels of 25% of maximum in flexor carpi radialis. The wrist displacements were designed to preclude voluntary following but at the same time were confined to the frequency range most relevant to voluntary movements. They included a broad-frequency band (0–12 Hz) signal as well as sets of narrow-band signals spanning the range from 0 to 10 Hz. The maximum frequency was set so as to remain within the linear encoding bandwidth of the reflex system and thereby minimize distortion. The effects of frequency bandwidth and amplitude of the displacement perturbations were tested in separate experiments. The coherence square, gain and phase between the EMG and angular displacement were calculated in order to characterize the stretch reflex under these conditions. It was found that the phase of the reflex response was dependent on both bandwidth and amplitude. For narrow-band displacements, the phase advance was about 30° greater over the frequency range tested than for broad-band displacements, suggesting that the reflex response may be influenced by the predictability of the perturbation. At the smallest amplitude of 0.3°, the peak phase advance was about 20° greater than at the largest amplitude of 4.2°. The gain was also higher and rose more steeply with frequency at smaller amplitudes. In the frequency range up to 12 Hz, the tonic stretch reflex responds most effectively to smaller-amplitude, more regular, higher-frequency inputs and this is consistent with a role for the reflex in counteracting small-amplitude oscillations, tremors and errors of voluntary movement.

The nature of the loss of strength and dexterity in the upper limb following stroke

Abstract People who had suffered a stroke within the previous two years were tested for strength, dexterity and the ability to generate fast movements of their affected elbow and compared to age-matched controls. Strength was measured via the joint torque generated during a maximal isometric contraction of the elbow flexors and extensors. Dexterity was assessed separately from strength by a tracking task that required skilled interaction of elbow joint flexors and extensors at two speeds — slow and fast. This task was set up so that very little strength was required to perform the task. Additionally, the ability to generate fast movements was measured by requiring subjects to flex and extend their elbow as fast as they could. Performance of the tracking task deteriorated at the faster speed in all subjects, however, this effect was more pronounced following stroke. In the stroke subjects, strength was poorly correlated with dexterity. Generally, stroke subjects could and did move their elbows in flexion and extension faster than was required to follow the targets. Poor performance of the stroke subjects on the tracking task was, therefore, the result of poor muscle control — a separate entity from strength. The results of this study indicate that weakness and loss of dexterity following stroke are separate problems, both of which may contribute to the slowness of movement seen in the clinic.

REDUCTION OF SPASTICITY IN cérébral PALSY USING FEEDBACK OF THE TONIC STRE1CH REFLEX; A CONTROLLED STUDY

Subjects with cérébral palsy aged six to 19 years undertook a training programme to reduce spasticity and contracture of the triceps surae muscle using feedback of the gain of the tonic stretch reflex. Stretch reflex gain was significantly reduced (by 50 per cent on average) in all eight test subjects, whereas there was no significant change in control subjects, matched for age and level of spasticity, who had no reflex training. Contracture of the triceps sutae muscle was not altered. Since the reduction in spasticity can be expected to slow the recurrence of muscle shortening, it is suggested that for correction of muscle contracture, muscle‐lengthening procedures to promote muscle growth should be combined with reflex training to maintain the growth achieved.