Localization of a seen finger is based exclusively on proprioception and on vision of the finger.

In a previous study we investigated how the CNS combines simultaneous visual and proprioceptive information about the position of the finger. We found that localization of the index finger of a seen hand was more precise (a smaller variance) than could reasonably be expected from the precision of localization on the basis of vision only and proprioception only. This suggests that, in localizing the tip of the index finger of a seen hand, the CNS may make use of more information than proprioceptive information and visual information about the fingertip. In the present study we investigate whether this additional information stems from additional sources of sensory information. In experiment 1 we tested whether seeing an entire arm instead of only the fingertip gives rise to a more precise proprioceptive and/or visual localization of that fingertip. In experiment 2 we checked whether the presence of a structured visual environment leads to a more precise proprioceptive localization of the index finger of an unseen hand. In experiment 3 we investigated whether looking in the direction of the index finger of an unseen hand improves proprioceptive localization of that finger. We found no significant effect in any of the experiments. The results refute the hypothesis that the investigated effects can explain the previously reported very precise localization of a seen hand. This suggests that localization of a seen finger is based exclusively on proprioception and on vision of the finger. The results suggest that these sensory signals may contain more information than is described by the magnitude of their variances.

Exocentric pointing in three-dimensional space.

A study is reported of an exocentric pointing task in all three dimensions, in near space, with only two visible luminous objects--a pointer and a target. The task of the subject was to aim a pointer at a target. The results clearly show that visual space is not isotropic, since every set direction appeared to consist of two independent components--one in the projection onto a frontoparallel plane (tilt), the other in depth (slant). The tilt component shows a general trend across subjects, an oblique effect, and can be judged monocularly. The slant component is symmetrical in the mid-sagittal plane, requires the use of binocular information, and shows considerable differences between subjects. These differences seem to depend on the amount of binocular information used by each subject. There was a remarkably high level of consistency in the exocentric pointing, despite the absence of environmental cues. The within-subject consistency in the settings of the pointer corresponds to a consistency of about 1 min of arc in disparity of its tip, even though the pointer and target are separated by more than 5 deg.

Autonomous development of decorrelation filters in neural networks with recurrent inhibition.

We perform a quantitative analysis of information processing in a simple neural network model with recurrent inhibition. We postulate that both excitatory and inhibitory synapses continually adapt according to the following Hebbian-type rules: for excitatory synapses correlated pre- and post-synaptic activity induces enhanced excitation; for inhibitory synapses it induces enhanced inhibition. Following synaptic equilibration in unsupervised learning processes, the model is found to perform a novel type of principal-component analysis which involves filtering and decorrelation. In the light of these results we discuss the possible role of the granule-/Golgi-cell subnetwork in the cerebellum.

The precision of proprioceptive position sense.

The purpose of this study was to determine the precision of proprioceptive localization of the hand in humans. We derived spatial probability distributions which describe the precision of localization on the basis of three different sources of information: proprioceptive information about the left hand, proprioceptive information about the right hand, and visual information. In the experiment subjects were seated at a table and had to perform three different position-matching tasks. In each task, the position of a target and the position of an indicator were available in a different combination of two of these three sources of information. From the spatial distributions of indicated positions in these three conditions, we derived spatial probability distributions for proprioceptive localization of the two hands and for visual localization. For proprioception we found that localization in the radial direction with respect to the shoulder is more precise than localization in the azimuthal direction. The distributions for proprioceptive localization also suggest that hand positions closer to the shoulder are localized more precisely than positions further away. These patterns can be understood from the geometry of the arm. In addition, the variability in the indicated positions suggests that the shoulder and elbow angles are known to the central nervous system with a precision of 0.6-1.1 degrees. This is a considerably better precision than the values reported in studies on perception of these angles. This implies that joint angles, or quantities equivalent to them, are represented in the central nervous system more precisely than they are consciously perceived. For visual localization we found that localization in the azimuthal direction with respect to the cyclopean eye is more precise than localization in the radial direction. The precision of the perception of visual direction is of the order of 0.2-0.6 degrees.

How humans combine simultaneous proprioceptive and visual position information.

To enable us to study how humans combine simultaneously present visual and proprioceptive position information, we had subjects perform a matching task. Seated at a table, they placed their left hand under the table concealing it from their gaze. They then had to match the proprioceptively perceived position of the left hand using only proprioceptive, only visual or both proprioceptive and visual information. We analysed the variance of the indicated positions in the various conditions. We compared the results with the predictions of a model in which simultaneously present visual and proprioceptive position information about the same object is integrated in the most effective way. The results are in disagreement with the model: the variance of the condition with both visual and proprioceptive information is smaller than expected from the variances of the other conditions. This means that the available information was integrated in a highly effective way. Furthermore, the results suggest that additional information was used. This information might have been visual information about body parts other than the fingertip or it might have been visual information about the environment.

Motor function in a patient with bilateral lesions of the globus pallidus.

This study describes the long-term deficits of a patient who, after a toxic encephalopathy, sustained extensive bilateral damage to both segments of the globus pallidus (GP) and the right substantia nigra (SN). There were no signs of lesions of the pyramidal tracts or of other motor structures. The most obvious deficits were an abnormal gait with an exaggerated knee extension and a tendency to fall slowly, especially when pushed backward. In contrast, Romberg's test on an unstable platform was normal, as were long-latency leg reflexes induced by perturbations. Inadequate anticipatory and compensatory postural responses, in particular across the hip and knee joints, and slow movements seemed responsible for the falls. Muscle tone was normal but reflex studies showed signs of abnormal facilitation and inhibition at various levels of the neuraxis. We conclude that the GP and SN lesions caused defective input to premotor cortical and brain stem target zones. Dysfunctioning of these zones leads to improper control of the descending ventromedial motor system responsible for locomotion, postural control, and reflex status. The deficits in upper extremity motor performance included delayed and slow movements, inaccurate amplitudes of ballistic responses, a lack of predictive control, and deficits in bimanual coordination. Sensory feedback, proprioceptive more than visual, played a powerful compensating role in rapid aiming movements. Regional blood flow (studied using 15(O)2) was reduced in multiple frontal cortical regions, among which are the hand areas of the supplementary and premotor cortex. We hypothesize that this reflected impaired functioning of these areas, caused by defective bilateral output from GP and SN, and resulting in the motor deficits of the arm and hand.

Misdirections in slow, goal-directed arm movements are not primarily visually based.

In a previous study we found that the initial direction of slow, goal-directed arm movements deviates consistently from the direction of the actual straight line between the starting position and the target position. We now investigate whether these deviations are caused by imperfections or peculiarities in the processing of vision-related spatial information, such as retinal information, and eye- and head-position information. This could lead to incorrect localization of the target relative to the starting position. Subjects were seated in front of a horizontal surface and had to move their arm slowly and accurately in the direction of target positions. We varied the amount of vision-related spatial information. In experiment 1, subjects were presented with visual targets and could see their moving arm. In experiment 2, the subjects were again presented with visual targets, but now they could not see their moving arm. In experiment 3, the subjects were blindfolded and had to move their arm towards tactile targets. In all three experiments we found comparable consistent deviations in the initial movement direction. We also instructed congenitally and early-blind subjects to move their arm towards tactile targets. Their performance showed deviations congruous with those found in the sighted subjects, and possibly somewhat larger. We conclude that the deviations in the initial movement direction of slow, goal-directed arm movements are not primarily visually based. The deviations are generated after all spatial information has been integrated.

Visuomotor performance of normal and clumsy children. II: Arm-tracking with and without visual feedback.

Tracking performance was investigated in normal and clumsy children in two age-groups, six to seven and 10 to 11 years. Target signals moving unpredictably along a straight line had to be tracked, both with and without visual feedback. Performance was described in three ways: (1) performance in the low-frequency range (LF); (2) the delay between target signal and tracking movement (DL); and (3) a measure of tracking quality or over-all similarity in the shape of target signal and tracking movement (Q). Clumsy children in both age-groups had a lower tracking quality (Q) and longer delay (DL) than the normal children. Disturbances in the regulation of attention seemed to affect tracking performance, particularly of the six- to seven-year-old clumsy children. There was no significant difference between normal and clumsy children in the effect of visual feedback on tracking performance. This suggests that clumsiness is not linked to disturbance of integration of visual feedback information and motor processes.

A model for neural control of gradation of muscle force.

A mathematical muscle model is presented that relates neural control signals linearly to muscle force without violating important known physiological constraints, such as the size-principle (Henneman and Mendell 1981) and non-linear twitch summation (Burke et al. 1976). This linearity implies that the neural control signals (defined as a weighted sum of activities in a nerve bundle) can be interpreted as the internal representation of total muscle force. The model allows for different relative contributions from the two force-grading mechanisms, i.e. the recruitment of motor units and the modulation of their firing frequency. It can therefore be applied to a variety of (distal and proximal) muscles. Furthermore, it permits simple mechanisms for controlling muscle force, e.g. in superposed motor tasks. The model confirms our intuitive notion that a weighted sum of activities in a nerve bundle can directly represent an external controlled variable, which in this case is exerted muscle force.

Misdirections in slow goal-directed arm movements and pointer-setting tasks.

Information about the direction of the virtual line between two positions in space (directional information) is used in many decision-making and motor tasks. We investigated how accurately directional information is processed by the brain. Subjects performed two types of task. In both tasks they sat at a table. In the first task they had to move their hand slowly and accurately from an initial position 40 cm in from of them to visually presented targets at a distance of 30 cm from the initial position (movement task). We analysed the initial movement direction. In the second task subjects had to position pointers in the direction of the targets as accurately as they could (perceptive task). We found that in the movement task the subjects started the movements to most targets in a direction that deviated consistently from the direction of the straight line between initial position and target position. The maximum deviation ranged from 5-10 degrees for the various subjects. The mean standard deviation was 4 degrees. In the perceptive task the subjects positioned the pointer in similarly deviating directions. Furthermore, we found that the maximum deviation in the pointer direction depended on the length of the pointer: the smaller the pointer, the larger the consistent deviations in the pointer direction. The shortest pointer showed deviations comparable to the deviations found in the movement task. These findings suggest that the deviations in the two tasks stem from the same source.

Visuomotor performance of normal and clumsy children. I: Fast goal-directed arm-movements with and without visual feedback.

The mechanisms underlying accuracy in fast goal-directed arm-movements were investigated in normal and clumsy children in two age-groups, six to seven and 10 to 11 years. Clumsy children in both age-groups had a longer movement time than normal children; this difference increased slightly when there was visual feedback. For both normal and clumsy children, the relative variability of the total distance moved was smaller than that of the distance moved during acceleration, indicating a variability reduction mechanism in the course of a movement. In the six- to seven-year-old group, the relative variability of the distance moved during acceleration and of the total distance was larger for clumsy than for normal children; this did not reach significance in the 10- to 11-year-old group. It is suggested that motor difficulties are linked to inaccuracy in open-loop control processes and to less efficient use of visual feedback.

Arm-tracking performance with and without visual feedback in children and adults: developmental changes.

Tracking performance was investigated in children (aged 6-7 and 10-11) and in adult subjects. Target signals, moving unpredictably along a straight line, were tracked with the preferred arm, alternately with and without visual feedback. Qualitative observations indicate that tracking is based on continuous adjustments of the ongoing response to the continuously changing target position. No step-and-hold strategy could be detected in any of the three age groups. Tracking performance was described with four simple parameters, derived from linear systems analysis: (a) the delay between target signal and tracking movement (DL); (b) performance at the low-frequency range (LF), (c) performance at the high-frequency range (HF); and (d) a measure of tracking quality or overall similarity in the shape of target signal and tracking movement (Q). There was a considerable improvement in tracking performance with age, even after the age of 10-11, which was mainly demonstrated by a decrease in DL and increases in HF and Q. Tracking performance decreased only to a small extent when visual feedback was withdrawn. Age-related differences in the contribution of visual feedback to tracking performance could not be demonstrated.

Mechanisms underlying accuracy in fast goal-directed arm movements in man.

This study investigated how accuracy is attained in fast goal-directed arm movements. Subjects were instructed to make arm extension movements over three different distances in random order, with and without visual feedback. Target width was varied proportionally with distance. Movement time was kept as short as possible, but there were well-defined limits with respect to accuracy. There appeared to be a large relative variability (variation coefficient [VC]) in the initial acceleration. The VC in the distance the hand moved during the acceleration phase was much smaller. This reduction was accompanied by a strong negative correlation between the initial acceleration and the duration of the acceleration phase. Further, the VC in the total distance moved was less than the VC in the distance moved during acceleration. This result indicates asymmetry between the acceleration and the deceleration phase. This is confirmed by the negative correlation between the distance the hand moved during acceleration and the distance it moved during deceleration. Withdrawal of visual feedback had a significant effect on movement accuracy. No differences were found in the parameters of the acceleration phase in the two feedback conditions, however. our results point to the existence of a powerful variability compensating mechanism within the acceleration phase. This mechanism seems to be independent of visual feedback; this suggests that efferent information (efference copies) and/or proprioceptive information is/are responsible for the timing of agonist and antagonist activation. The asymmetry between the acceleration and deceleration phase contributes to a reduction in the relative variability in the total distance moved. The fact that the withdrawal of visual feedback affected movement variability only during the deceleration phase indicates that visual information is used in the adjustment of antagonist activity.

Differences in coordination of elbow flexor muscles in force tasks and in movement tasks.

Motor-unit activity in m. biceps brachii, m. brachialis and m. brachioradialis during isometric contractions has been compared with motor-unit activity during slow voluntary (extension and flexion) movements made against external loads. During these slow movements the recruitment threshold of m. biceps motor units is considerably lower than it is during isometric contractions but the recruitment threshold of both m. brachialis and m. brachioradialis motor units is considerably higher. For all three elbow flexor muscles the motor-unit firing frequency seems to depend on the direction of movement: the firing frequency is higher during flexion movements (3 deg/s) and lower during extension movements (-3 deg/s) than during isometric contractions. The relative contribution of the biceps to the total exerted flexion torque during slow voluntary movements is estimated to increase from 36% to about 48% and that of the brachialis/brachioradialis is estimated to decrease from 57% to about 45% compared to the relative contribution of these muscles during isometric contractions. This difference in the relative contribution of the three major elbow flexor muscles is shown to be caused by differences in the central activation in force tasks and movement tasks.

Adjustments of fast goal-directed movements in response to an unexpected inertial load.

Subjects made fast goal-directed elbow flexion movements against an inertial load. Target distance was 8 or 16 cm, randomly chosen. To exert a force in the direction of the movement subjects had to activate flexors of both shoulder and elbow, but shoulder flexors did not change appreciably in length during the movement. In 20% of the trials the inertial load was increased or decreased without knowledge of the subjects. Until 90-110 ms after the onset of the agonist muscle activity (about 65-85 ms after the start of movement) EMG activity was very similar in all conditions tested. The changes that occurred in the EMG from that moment on were effectively a later cessation of the agonist activity and a later start of the antagonist activity if the load was increased unexpectedly. If the load was reduced unexpectedly, the agonist activity ceased earlier and the antagonist activity began earlier. The latency at which EMGs started to change was the same for muscles around shoulder and elbow, for agonists and antagonists and for both distances. All adjustments had the same latency (37 ms) relative to the point where the angular velocity of the elbow in the unexpectedly loaded movements differed by 0.6 rad/s from the expected value. We discuss why simple reflex- or servo-mechanisms cannot account for the measured EMG changes. We conclude that appropriate adjustments of motor programmes for fast goal-directed arm movements start within 40 ms of the detection of misjudgment of load.

Differences in central control of m. biceps brachii in movement tasks and force tasks.

Motor-unit activity in m. biceps brachii during isometric flexion contractions has been compared with motor-unit activity during a) slow voluntary movements against constant or increasing preloads and b) flexion contractions while movements were imposed by a torque motor. Recruitment levels and firing frequency behaviour of the motor units were found to be very similar when torques were generated during isometric contractions and during the imposed movements. However, these characteristics of the biceps motor units were quite different during the slow voluntary movements. It is suggested that the central activation of the alpha and/or gamma motoneurone pools of m. biceps brachii is different for force tasks and slow movement tasks, even if the same torques are exerted and/or movements are made.

Motor programmes for goal-directed movements are continuously adjusted according to changes in target location.

We have studied fast arm movements in response to double-step stimuli in two-dimensional space. In a previous paper we found that such movements did not start in the direction of the first or the second target, but in a direction between the two targets. The initial movement direction was found to depend in a continuous fashion on the inter-stimulus interval and on the reaction time. Therefore we concluded that the internal representation of a discrete target displacement is a gradually shifting internal target, moving from the first to the second target location. In this paper we investigate whether the arm movements also show a modification of the trajectory during the movement. An inter-stimulus interval of 100 ms was chosen, because then the initial movement direction is the same as in the response to a single-step displacement. We found that on average double-step trajectories deviate significantly from their original trajectory within 60 ms, and in some cases even within 30 ms of the start of the movement. We conclude that a motor programme is centrally modified according to a changed target location. We hypothesize that the generation of the motor programme starts after the target presentation, and that the activation levels for the appropriate muscles are continuously adjusted to move the hand in the direction of the current internal representation of the target.

Activation of human arm muscles during flexion/extension and supination/pronation tasks: a theory on muscle coordination.

Generally the number of muscles acting across a joint exceeds the number of degrees of freedom available to the joint. This redundancy raises a problem regarding the ratio in which these muscles are activated during a particular motor task. In this paper we present a theory to explain the activation patterns of muscles used during voluntary and reflex induced contractions. The basic assumptions underlying the theory are that 1) coordination of muscles is based on synergistic muscle activities, 2) the synergisms involved satisfy certain transformations of muscle spindle signals to muscle activation signals and 3) muscle spindle output is proportional to the ratio of muscle stretch and muscle length in lengthening muscles, and is zero in shortening muscles. The theory is used to predict the recruitment threshold of motor units in six arm muscles during voluntary isometric contractions. All theoretical predictions are in reasonable agreement with the experimentally observed behavior of a large population of motor units within each muscle. However, within a single muscle sometimes motor-unit populations have been found to have different types of recruitment behavior. This deviating behavior is discussed in the light of the theory presented here.

Inhomogeneous activation of motoneurone pools as revealed by co-contraction of antagonistic human arm muscles.

There is now considerable evidence that motoneurone pools in several human arm muscles are activated inhomogeneously during isometric and reflex-induced contractions in different directions (van Zuylen et al. 1988; Gielen et al. 1988; ter Haar Romeny et al. 1984). In this paper we investigate the activation of antagonist muscles (m. brachialis, m. brachioradialis, m. biceps and m. triceps) during co-contraction of the upper arm muscles. The results show that there is a marked difference between the distribution of the activities of synergistic flexor muscles, or even within these muscles, in co-contraction tasks and in flexion tasks. This discrepancy may be attributed to the existence of inhibitory mechanisms between motoneurone pools of antagonist muscles. These mechanisms can also account for different types of recruitment behaviour of motor unit populations in a single muscle.

Differences in the activation of m. biceps brachii in the control of slow isotonic movements and isometric contractions.

We have compared muscle activation in the control of slow isotonic movements and isometric contractions. Specific attention has been given to the contribution of the two force-grading mechanisms, the recruitment of motor units and the modulation of firing frequency in motor units that have already been recruited. The recruitment order of the m. biceps motor units under study was the same during isometric contractions and slow isotonic movements. However, the recruitment thresholds of the m. biceps units were considerably lower for both isotonic flexion and extension movements, even at velocities as low as 2 deg/s, than for isometric contractions. Furthermore, firing frequency at recruitment was found to depend on the motor task: at recruitment the motoneurone starts firing with a higher firing frequency during isotonic flexion movements and a lower firing frequency during isotonic extension movements than during isometric contractions. Two main conclusions can be drawn from these results. First of all, the concept of one single activation parameter (total synaptic drive?) cannot account for the motor-unit behaviour observed during our experiments: the relative contribution of the two force-grading mechanisms is different for different tasks. Secondly, the distribution of activity among flexor motoneurone pools is different for isometric contractions and isotonic movements.