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PubMed2000

Validity of weight distribution and sway measurements of the Balance Performance Monitor.

BACKGROUND AND PURPOSE: The Balance Performance Monitor (BPM) is a device which provides auditory and visual feedback on weight distribution and the magnitude of lateral and anterior-posterior sway during quiet standing. This study investigated the validity of the measurements provided by the BPM using a Kistler force plate (KFP) as the gold standard. METHOD: Percentage weight distribution between the BPM foot plates was validated using both a series of calibration weights and the vertical component of ground reaction force, measured by the KFP, during normal standing in 18 young normal subjects. The lateral and anterior-posterior sway indices from the BPM were validated against the standard deviation of the position of the centre of pressure, again obtained using the KFP, during normal standing with eyes open and eyes closed and standing with feet together with eyes open. Concurrent validity of the percentage weight distribution measurements was assessed by calculating the 'limits of agreement' between the corresponding measurements from the BPM and KFP and the 95% confidence intervals for these 'limits'. Differences in the units of measurement obtained from the BPM and KFP resulted in the concurrent validity of the sway indices being assessed using correlation and regression. RESULTS: Excellent agreement was found between the percentage weight distribution values provided by the BPM and the KFP, which showed that the BPM may read only 3% of body weight above or below that given by the KFP. High correlations (r = 0.61-0.99) were found between both the lateral and anterior-posterior sway indices from the BPM and the motion of the centre of pressure from the KFP in the respective direction. Despite this, further analysis of regression equations and the 95% prediction intervals showed poor concurrent validity of the BPM sway indices in relation to KFP measurements. This was thought to be due to the different methods by which the sway indices and the motion of the centre of pressure were calculated. CONCLUSIONS: The BPM may be used to provide a valid measure of the symmetry aspect, but not necessarily the steadiness aspect of postural control.

Adult↗

PubMed2001

Influence of standardized mobilization on the posteroanterior stiffness of the lumbar spine in asymptomatic subjects.

BACKGROUND AND PURPOSE: Spinal mobilization is commonly used to relieve pain and assist recovery of mobility in individuals with low back pain. Fundamental to this concept is the belief that spinal mobilization will influence the mechanical properties of the symptomatic motion segment. The objective of the present study was to examine the segmental effects of a standardized mobilization procedure on the posteroanterior (PA) stiffness of the lumbar spine. METHOD: Audio and visual feedback was used to train a physiotherapist to perform PA mobilization at a consistent load and frequency. After training, twenty-four subjects without low back pain were recruited for the intervention phase of the study. The spinal posteroanterior mobilization (SPAM) apparatus was used to measure the PA stiffness of the lumbar spine at three measurement sites (L1, L3 and L5). The trained physiotherapist then applied the standardized PA mobilization technique via the L3 spinous process for two minutes. Following mobilization, PA stiffness was measured three times at the three locations. RESULTS: The physiotherapist was able to apply a standardized mobilization with a mean force of 146 N (standard deviation (SD) 8 N) at a frequency of 1.5 Hz. The first trial on each assessment demonstrated a pre-condition effect. Two minutes' PA mobilization resulted in no significant change in the PA stiffness of the lumbar spine at the level to which the mobilization was applied, or at the L1 and L5 segments. The 95% confidence intervals (CI) of the difference in PA stiffness before and after testing included zero at each measurement site. CONCLUSIONS: Clinicians should pre-condition the spine when assessing PA stiffness both before and after interventions. A standardized mobilization of 150 N at 1.5 Hz for two minutes had no segmental effect on spinal PA stiffness. Subsequent studies need to consider other mechanisms that may contribute to the changes that occur after PA spinal mobilization.

Adult↗

PubMed1997

Anticipatory control of manipulative forces in Parkinson's disease.

In a previous study we found that subjects with Parkinson's disease (PD) had an impaired capability to initiate and sequence successive movement phases during lifts of small objects using the precision grip, and that they had regular oscillations in the force rates. The present study examined whether these subjects could use anticipatory control, in which the force output is scaled prior to liftoff, based on the object's physical properties. Subjects lifted an instrumented test object between the tips of the thumb and index finger while the employed grip force, load force (vertical lifting force), and corresponding time derivatives were recorded. In the first experiment, the object's weight was varied to assess its influence on the isometric force output. Subjects with PD scaled the isometric force increase according to the object's weight. In another experiment, the weight changed in proportion to the volume to determine whether subjects could make associative transformations between visual size information and the weight of the object. Subjects with PD still scaled the forces toward the expected weight, proportional to the volume of the object. Finally, programmed adjustments in force to sudden self-induced load changes were examined while subjects dropped a disk with one hand into a plate attached to the bottom of the grip instrument, held with the other hand. Subjects with PD had preparatory increases in the grip force prior to the disk contact, which matched the change in load, though may have been more dependent on visual feedback. We conclude that subjects with PD are capable of using anticipatory control to parameterize the isometric force output during a familiar lifting task.

Conditioning, Psychological↗

PubMed2001

Age differences in noise and variability of isometric force production.

This study examined whether age-related improvements observed in the motor performance of children result from a reduction of noise in the output of the sensori-motor system. Children ages 6, 8, and 10 years and young adults (N = 48, 12 per group) performed continuous, constant isometric force contractions with the index finger at four different force levels with and without visual feedback. The results revealed that: (a) performance improved with increases in age, (b) the force output signal exhibited increased irregularity and a more broadband frequency profile with increases in age under conditions with feedback, and (c) there were no age differences in the irregularity of the force signal and smaller age differences in the frequency profiles under conditions without feedback. It is proposed that the age-related enhancements in performance throughout childhood are primarily due to a more appropriate mapping of the organization of the sensori-motor system to the task constraints rather than to reduction of system noise.

Adolescent↗

PubMed1996

Organizing sensory information for postural control in altered sensory environments.

Healthy human subjects can maintain adequate balance despite distorted somatosensory or visual feedback or vestibular feedback distorted by a peripheral vestibular disorder. Although it is not precisely known how this sensorimotor integration task is achieved, the nervous system coordinates information from multiple sensory systems to produce motor commands differently in different sensory environments. These different ways of coordinating sensory information and motor commands can be thought of as "sensorimotor states". The way the nervous system distributes the monitoring of postural sway among states is analysed in this paper as a logical structure of transitions between states. The form of the transition structure is specified and distinguished from a finite state machine. The hypothesis that the nervous system could use a transition structure to maintain balance is tested by developing transition structures which are consistent with a set of experimental observations of postural control in healthy subjects and three groups of patients with peripheral vestibular disease.

Humans↗

PubMed1998

The time course of changes during motor sequence learning: a whole-brain fMRI study.

There is a discrepancy between the results of imaging studies in which subjects learn motor sequences. Some experiments have shown decreases in the activation of some areas as learning increased, whereas others have reported learning-related increases as learning progressed. We have exploited fMRI to measure changes in blood oxygen leve-dependent (BOLD) signal throughout the course of learning. T2*-weighted echo-planar images were acquired over the whole brain for 40 min while the subjects learned a sequence eight moves long by trial and error. The movements were visually paced every 3.2 s and visual feedback was provided to the subjects. A baseline period followed each activation period. The effect due to the experimental conditions was modeled using a square-wave function, time locked to their occurrence. Changes over time in the difference between activation and baseline signal were modeled using a set of polynomial basis functions. This allowed us to take into account linear as well as nonlinear changes over time. Low-frequency changes over time common to both activation and baseline conditions (and thus not learning related) were modeled and removed. Linear and nonlinear changes of BOLD signal over time were found in prefrontal, premotor, and parietal cortex and in neostriatal and cerebellar areas. Single-unit recordings in nonhuman primates during the learning of motor tasks have clearly shown increased activity early in learning, followed by a decrease as learning progressed. Both phenomena can be observed at the population level in the present study.

Adult↗

PubMed1983

Interstitial microwave hyperthermia for brain tumors. Results of a phase-1 clinical trial.

The technical feasibility and clinical safety of interstitial microwave hyperthermia was evaluated in six patients with glioblastoma and malignant astrocytoma. Prior to entry into the study, each patient had received surgery, radiation and nitrosourea chemotherapy. All patients were implanted at open craniotomy with a flexible microwave radiator/sensor (o.d. 1.5 mm) and transcutaneously connected to a 245- MHz microwave generator. Intraoperative thermal field plots and cooling curves were obtained with the aid of non-perturbing probes (o.d. 1.2 mm) perpendicularly driven into the tumor at fixed radial distances from the central antenna. In comparison to similar measurements carried out in normal feline brains, human gliomas were unable to efficiently dissipate heat as demonstrated by doubling of the effective diameter of the thermal field to 4 cm and by prolongation of the decay time in all cooling curves. Patients were also implanted with subarachnoid ICP monitors over the contralateral hemisphere. Two postoperative treatments were given at 45 degrees C for 60 min on the night of surgery and 48 hr later. No patient was aware of power on/power off, there were no permanent neurologic sequelae and there were no significant changes in the ICP. Power was manually controlled with visual feedback in the first three patients and automatically controlled by a computer-based system in the final three patients. Four of the six patients have lived 18 months after implantation and two of these have negative CT scans at 18 and 27 months since recurrence. It appears that interstitial microwave hyperthermia is both feasible and safe within the intracranial cavity and that combined interstitial irradiation and hyperthermia deserves clinical study.

Adult↗

PubMed1993

The use of auto-correlation function to quantify periodicity in smooth pursuit.

Smooth pursuit eye movement was recorded with a DC amplifier during horizontal sinusoidal target movement at 0.3, 0.6, 0.8, 1.0 and 1.2 Hz. Eye movement was digitalized at 100 Hz and 12 bits accuracy. The digitalized eye movements were analyzed by the auto-correlation function, and the auto-correlation coefficient was used to quantify periodicity. In 13 healthy volunteers, the auto-correlation coefficient was nearly equal to 1.0 at 0.3 Hz but gradually decreased as the target frequency increased. In 8 patients with various neurological disorders, the auto-correlation coefficient was significantly smaller at all target frequencies than in the healthy volunteers. Less periodicity, as indicated by the decreased auto-correlation coefficient, suggests a disorder in the control visual feedback mechanism in smooth pursuit.

Adult↗

PubMed1991

Saccade control in a simulated robot camera-head system: neural net architectures for efficient learning of inverse kinematics.

The high speed of saccades means that they cannot be guided by visual feedback, so that any saccadic control system must know in advance the correct output signals to fixate a particular retinal position. To investigate neural-net architectures for learning this inverse-kinematics problem we simulated a 4 deg-of-freedom robot camera-head system, in which the head could pan and tilt and the cameras pan and verge. The main findings were: (1) Linear nets, multilayer perceptrons (MLPs) trained by backpropagation, and cerebellar model arithmetic computers (CMACs) all learnt rapidly to 5-10% accuracy when given perfect error feedback. (2) For additional accuracy (down to 2%) two-layer nets learnt much faster than a single MLP or CMAC: the best combination tried was to have a CMAC learn the errors of a trained linear net. (3) Imperfect error signals were provided by a crude controller whose output was simply proportional to retinal input in the relevant axis, thereby providing a mechanism for (a) controlling the camera-head system when the feedforward neural net controller was wrong or inoperative, and (b) converting sensory error signals into motor error signals as required in supervised learning. It proved possible to train neural-net controllers using these imperfect error signals over a range of learning rates and crude-controller gains. These results suggest that appropriate neural-net architectures can provide practical, accurate and robust adaptive control for saccadic movements. In addition, the arrangement of a crude controller teaching a sophisticated one may be similar to that used by the primate saccadic system, with brainstem circuitry teaching the cerebellum.

Brain Stem↗

PubMed1995

Discharge patterns of tonically firing human motoneurones.

We have attempted to reconcile the different patterns of distribution of interspike intervals that are found in motoneurones made to discharge by intracellular injection of constant current in reduced animal preparations and by voluntary control in human subjects. We recorded long spike trains from single motor units in three human muscles made to discharge at constant mean frequencies with the help of auditory and visual feedback. The distribution of interspike intervals in each spike train was analysed quantitatively. We found that the different pattern of discharge of the human motor units could be accounted for when due allowance was made for the variability of the drive to the human motoneurone which arose because of the feedback process used to maintain the target frequency. A model testing this hypothesis gave results that were qualitatively consistent with the human data.

Acoustic Stimulation↗

PubMed1992

Influences of hand movements on eye movements in tracking tasks in man.

We investigated horizontal smooth pursuit eye movements and hand movements in tracking tasks in order to find out whether hand movements influence eye movements and if so, in what ways. Externally controlled target movements were tracked either by the eyes alone or by the eyes and right hand together. Because a possible influence might depend on the stimulus, we used two classes of target movements: sinusoidal target movements (predictable target movements) and pseudo-random target movements (unpredictable target movements). Our data show that the eye movements contained only a few small saccades when sinusoidal target movements with frequencies higher than about 1 Hz were tracked by eyes and hand together. More and larger saccades were made when the same target movements were tracked by the eyes alone. The difference in smoothness of eye movements was highly significant between the two tracking conditions. Such a difference was not found during the tracking of a pseudo-random target motion. This suggests that the influence of hand movements is related to the predictability of the stimulus. In contrast to the gain of the smooth pursuit eye movements and the maximum of the cross-correlation function, the gain of the composite eye movements did not depend on the tracking condition. The delay of the eye movements with respect to the (sinusoidal) target movements also showed no dependence on the tracking condition. Visual feedback from the tracking hand was found not to play a role in the difference in eye movements for the two tracking conditions.

Feedback↗

PubMed1994

A three-dimensional analysis of vergence movements at various levels of elevation.

Earlier studies have shown that eye positions, recorded in subjects scanning a distant visual scene with the head in a stable position, have only two degrees of freedom (Listing's law). Due to cyclovergence, this law is modified in near-vision. Two previous quantitative studies have documented that the sign of the torsional vergence component depends systematically on elevation: when fixating a nearby target, the eyes show intorsion in up gaze, extorsion in down gaze and no cyclotorsion at some intermediate elevation level (to be denoted as the null elevation). Both studies found a linear cyclovergence/elevation relation, but disagreed on the amount of cyclotorsion. A further uncertainty is how this phenomenon develops dynamically when the binocular fixation point shifts from a far to a near position. Therefore, we have investigated the dynamic coupling between the horizontal and torsional components of vergence in human subjects who were instructed to refixate a light target after it stepped in depth. The target steps were presented at various vertical and horizontal directions relative to the straight-ahead axis of the cyclopean eye. We found that the quantitative relations among horizontal vergence, torsional vergence and elevation were intermediate between those found in the two earlier near-vision studies and that they correspond reasonably to the predictions of a model by Mok and co-workers. The cyclotorsion vergence component had about the same latency and dynamics as the horizontal component. When refixations were studied at different elevations, the torsional vergence component changed from incyclotorsion in up gaze to excyclotorsion in down gaze. In agreement with expectations derived from two quantitative models, the null elevation of cyclovergence was near the binocular primary position. Furthermore, we found no consistent additional dependence on the horizontal direction of the refixation trajectory relative to the midsaggital plane. Other experiments showed that the cyclotorsional changes accompanying convergence were not critically dependent upon the visual conditions. Quantitatively similar similar cyclotorsional components were found even in convergent refixations executed in full darkness towards the location of a remembered (flashed) target. We conclude that visual feedback is unlikely to be very important in controlling cyclovergence in these various conditions.

Adult↗

PubMed1994

Binocular vision and the on-line control of human prehension.

The contribution of binocular visual feedback to the kinematics of human prehension was studied in two related experiments. In both experiments, the field of view of each eye was independently controlled by means of goggles fitted with liquid-crystal shutters. While wearing these goggles, which permitted either a binocular or a monocular view of the world, subjects were required to reach out and grasp a target object, which varied in size and position from trial to trial. In experiment 1, two viewing conditions were used. In one condition, binocular vision was available throughout the entire trial; in the second condition, the initial binocular view was replaced by a monocular view after the reaching movement had been initiated. When only monocular feedback was available, subjects showed a prolonged deceleration phase, although the time they spent in contact with the object was the same in both conditions. In experiment 2, monocular vision was available throughout a given trial in one condition and was replaced by binocular vision upon movement initiation in the second condition. Subjects in this experiment also displayed a prolonged deceleration phase in the monocular feedback condition relative to their performance in the binocular feedback condition. Unlike experiment 1, however, allowing only monocular feedback resulted in an increase in the amount of time subjects spent in contact with the object. Moreover, the object contact phases under the two conditions of experiment 2 were much longer than those observed in experiment 1, in which subjects received initial binocular views of the object.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1993

Egocentric references and human spatial orientation in microgravity. II. Body-centred coordinates in the task of drawing ellipses with prescribed orientation.

This article describes the results of the "ellipses" experiment conducted during the second French-Soviet spaceflight (project Aragatz). The realization of oriented motor tasks, on the basis of internal body representation and without visual feedback, was chosen as a paradigm for studying the determinants of spatial orientation under weightlessness. The process of drawing ellipses in the air, using arm movements with axes parallel or perpendicular to the longitudinal body axis, was studied under normal gravity and in weightlessness, and recorded using a video computer motion-analyzing system (Kinesigraph). On Earth, the experiments were performed in standing and lying positions, and in flight, in the erect position with the feet fixed to the floor. In general, performance of the task in microgravity was not disturbed. Under conditions of spaceflight, the longitudinal ellipse was inclined forward in accordance with the inclination of the whole body relative to the fixed feet. On Earth, the angle between the long axes of longitudinal and transverse ellipses deviated from 90 degrees by 20-30 degrees. The same deviation persisted under microgravity conditions. The distinctive features of ellipses traced by individual subjects were also preserved. It is concluded that an egocentric reference system ensures normal performance of sensorimotor tasks in the absence of a gravitational reference.

Adaptation, Psychological↗

PubMed1992

Effects of practice on final position reproduction.

Three subjects practiced fast, accurate 36 degrees elbow flexion movements to a 2.5 degrees target for 14 sessions of 100 trials (total, 1400 trials). Subjects then returned for a 15th experimental session in which they were asked to perform 15 movements under identical conditions to the practice condition. They were then tested under three experimental conditions without visual feedback: (1) identical to the practice conditions, (2) with small shifts in starting position (+/- 3 degrees of the practiced starting position), that were insufficient for subjective discrimination and, therefore, subjects were instructed to repeat the practiced movements; and (3) with a large shift in starting position (range, +/- 15 degrees of the practiced starting position), under the instruction to move to the same target. Experimental conditions 2 and 3 demonstrated that shifts in starting position were partially correlated with shifts in final position. These results are interpreted from the point of view of the equilibrium-point hypothesis of motor control.

Adult↗

PubMed1991

Changes in perceived finger force produced by muscular contractions under isometric and anisometric conditions.

We compared matching of finger forces under isometric conditions with matching of forces produced against a spring load (anisometric conditions) in twenty normal subjects. The instruction was to generate the same force in both hands holding a grip between thumb and index finger in each hand. Visual feedback indicating the target force and the actual force applied were presented for one (reference) hand only. The forces produced in each hand were measured continuously during matching trials. A special device provided the opportunity to change from isometric to anisometric force production. Matching was required under symmetric conditions, in which force was generated in both hands either isometrically or anisometrically, as well as under asymmetric conditions in which isometric force has to be matched to anisometric force or the reverse. Under symmetric conditions matching error was consistently smaller in anisometric than in isometric force production. However, the striking feature was a severe mismatch between hands when forces had to be produced differently. For most subjects, a force generated against the spring load in the reference hand was greatly overestimated by the matching hand working isometrically. For the reverse condition consistent underestimations were observed. This effect cannot be attributed to left/right differences or a simple confusion of subjects in the asymmetric tasks. Some of the factors confounded with the conditions of force production were ruled out as an explanation by additional experimental controls. The mismatch neither depends on signals related to different finger positions associated with target forces nor is it alleviated when differently produced forces are matched sequentially.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1984

The contribution of coordinated eye and head movements in hand pointing accuracy.

The accuracy of pointing movements of the hand, directed at visual targets 10 degrees to 40 degrees from the midline, was measured in normal human subjects. No visual feedback from the moving hand was available to the subjects. The head could be either maintained stationary (head-fixed condition) or free to move (head-free condition) during the pointing movements. It was found that the error in pointing was reduced for all targets in the head-free condition. This reduction was more important for the more eccentric target (40 degrees). Improvement in accuracy was observed without any significant change in either the latency or the duration of eye, head or hand movements. In the head-free condition, it was found that the head was displaced in the direction of the target by an amount representing no more than 2/3 of the target amplitude. The improvement in accuracy was not influenced by the amplitude of the head movement. A model is proposed which shows how coordinated eye and head movements could improve the encoding of target position.

Adaptation, Physiological↗

PubMed1983

Non-linear effects in visual suppression of vestibular nystagmus.

Visual-vestibular interaction in the control of eye movement was investigated in six subjects during exposure to a low frequency (0.05 Hz) angular oscillation about the longitudinal axis of the body at four levels of peak head velocity: 30, 60, 90 and 120 degrees/s. Eye movements were recorded whilst the subject was presented with a head-fixed visual display consisting of either a single central target or a pair of targets placed at +/- 20 degrees in the periphery. For the lower stimulus levels (30 and 60 degrees/s) the degree of suppression was reasonably constant and the vestibular nystagmus was never completely suppressed. However, during oscillation at higher velocity levels (90 and 120 degrees/s) the relationship between eye velocity and head velocity became non-linear, the degree of suppression being much less during the high velocity periods of the waveform than when the head velocity was low. The changes in suppression may be interpreted as a decrease in gain of visual feedback as a function of increasing image velocity error on the retina.

Eye Movements↗