Biomedical subjects

C C Gielen

Publications and source records attributed to C C Gielen.

At least 37 records · Page 2Linked to original sources

PubMed1995

An alternative three-dimensional interpretation of Hering's equal-innervation law for version and vergence eye movements.

In the context of Hering's equal-innervation law, this paper discusses the problem of how the three-dimensional positions of the two eyes, each expressed by a rotation vector, can be separated into contributions of the version and vergence system. As proposed by Van Rijn and Van den Berg [(1993) Vision Research, 33, 691-708], this can be done by taking the sum and difference of the position rotation vectors of each eye. In our alternative procedure the vergence signal is defined as the rotation which transforms the left eye position into the right eye position and the version signal is the common factor in both eye positions that remains after removing the vergence signal. The version and vergence contributions, defined in this way, can be interpreted straightforwardly as rotations. When Van Rijn and Van den Berg applied their definitions to their own data, they obtained the interesting result that the reconstructed version and vergence contributions were effectively limited to two dimensions (2D). The version signal was confined to Listing's plane (no torsion) whereas the vergence signal remained within a horizontal-torsional plane (no vertical vergence). They showed theoretically that a model based on 2D version/2D vergence control will indeed produce the torsional eye positions in near fixations found in their experiments. This model cannot account for a second set of data in the literature [Mok, Ro, Cadera, Crawford & Vilis (1992) Vision Research, 32, 2055-2064]. With our definitions, we found that the simple 2D version/2D vergence control strategy cannot account for the Van Rijn and Van den Berg (1993) data but is nicely compatible with the considerably smaller amount of cyclotorsion in the data collected by Mok et al. (1992). We also show that, in such a system, having 2D vergence control is compatible with minimization of torsional disparity and provides the cyclovergence signals suitable for stabilizing the eyes in the non-Listing positions caused by a vertical saccade in near vision.

Convergence, Ocular↗

PubMed1994

Frequency dependence of the action-perception cycle for postural control in a moving visual environment: relative phase dynamics.

When standing human subjects are exposed to a moving visual environment, the induced postural sway displays varying degrees of coherence with the visual information. In our experiment we varied the frequency of an oscillatory visual display and analysed the temporal relationship between visual motion and sway. We found that subjects maintain sizeable sway amplitudes even as temporal coherence with the display is lost. Postural sway tended to phase lead (for frequencies below 0.2 Hz) or phase lag (above 0.3 Hz). However, we also observed at a fixed frequency, highly variable phase relationships in which a preferred range of phase lags is prevalent, but phase jumps occur that return the system into the preferred range after phase has begun drifting out of the preferred regime. By comparing the results quantitatively with a dynamical model (the sine-circle map), we show that this effect can be understood as a form of relative coordination and arises through an instability of the dynamics of the action-perception cycle. Because such instabilities cannot arise in passively driven systems, we conclude that postural sway in this situation is actively generated as rhythmic movement which is coupled dynamically to the visual motion.

Humans↗

PubMed1994

The relative activation of muscles during isometric contractions and low-velocity movements against a load.

Surface electromyographic (EMG) and motor unit activity were measured in human arm muscles during isometric contractions and during movements against an elastic load. The direction of force applied proximal to the wrist and movement direction of the wrist were varied in a horizontal plane. During isometric contractions the direction in which the largest EMG activity was measured corresponded to the direction in which motor units had the smallest recruitment threshold, for each muscle. The same was found for movements against an elastic load. However, this direction was different for isometric contractions and for movements. Because the magnitude and sign of these changes varied for different muscles, this resulted in a different relative activation of muscles for the two conditions. The amplitude of the surface EMG during contractions against an elastic load was generally significantly larger than that for isometric contractions against the same load. For m. brachioradialis isometric conditions yielded occasionally increased EMG activity. The change in EMG activity could be attributed completely to changes in motor unit recruitment thresholds leading to proportionate changes in the number of recruited motor units. However, the initial firing rate of motor units at recruitment was the same under both conditions and, therefore, did not contribute to changes in amplitude of surface EMG activity.

Arm↗

PubMed1994

The relation between the direction dependence of electromyographic amplitude and motor unit recruitment thresholds during isometric contractions.

The activation of muscles can be studied by measuring the activity of a representative set of single motor units in a muscle or by measuring the surface electromyographic (EMG) activity of a muscle that results from the contribution of a large number of motor units. In this study we have developed a model showing how the direction dependence of the amplitude of the EMG activity during isometric contractions can be understood from the recruitment thresholds of single motor units when force is applied in various directions within a plane. The model predicts that the direction with the largest EMG activity (called the "preferred direction") corresponds to the direction in which the largest number of motor units is recruited. If one assumes homogeneous activation of a population of motor units, this preferred direction can be shown to be equivalent to the direction in which the recruitment threshold of the motor units is smallest. The experimental data show that, for most muscles in the human arm, the amplitude of surface EMG activity for a constant, isometric force at the wrist was proportional to the cosine of the angle between the muscle's preferred direction and the direction of force. As predicted by the model, the preferred direction coincided with the direction in which the recruitment threshold of motor units was smallest. Musculus deltoideus anterior had a more complicated directional sensitivity for surface EMG activity that could not be well fit by a single cosine function. This effect could be explained by the finding of two subpopulations of motor units within that muscle, each with a different recruitment behavior.

Adult↗

PubMed1994

Temporal stability of the action-perception cycle for postural control in a moving visual environment.

When standing human subjects are exposed to a moving visual environment, the induced postural sway forms a stable temporal relationship with the visual information. We have investigated this relationship experimentally with a new set-up in which a computer generates video images which correspond to the motion of a 3D environment. The suggested mean distance to a sinusoidally moving wall is varied and the temporal relationship to induced sway is analysed (1) in terms of the fluctuations of relative phase between visual and sway motion and (2) in terms of the relaxation time of relative phase as determined from the rate of recovery of the stable relative phase pattern following abrupt changes in the visual motion pattern. The two measures are found to converge to a well-defined temporal stability of the action-perception cycle. Furthermore, we show that this temporal stability is a sensitive measure of the strength of the action-perception coupling. It decreases as the distance of the visual scene from the observer increases. This fact and the increase of mean relative phase are consistent with predictions of a linear second-order system driven by the visual expansion rate. However, the amplitude of visual sway decreases little as visual distance increases, in contradiction to the predictions, and is suggestive of a process that actively generates sway. The visual expansion rate on the optic array is found to decrease strongly with visual distance. This leads to the conclusion that postural control in a moving visual environment cannot be understood simply in terms of minimization of retinal slip, and that dynamic coupling of vision into the postural control system must be taken into account.

Algorithms↗

PubMed1993

Remapping of neural activity in the motor colliculus: a neural network study.

Neurophysiological studies have shown that the deeper layers of the superior colliculus (SC) contain a topographical neural map representing the ocular vectorial displacement required for foveation of the target (motor error). It is known that the location of the active area in this neural map can be updated, not only following changes in retinal error, but also by efference-copy signals representing a change in eye position. Since it can be shown that a two-layer feedforward network cannot perform this task, we have simulated this system by training a three-layered neural network with access to retinal error and efference copy information about eye position. The network was taught to code motor error topographically (as in the collicular motor map) by generating population activity at the appropriate location in its output layer for different combinations of visual and efference copy signals. After the network had learned the required remapping transformation with sufficient precision (error of one deg over an 80 x 80 deg working range), the properties of the trained network were analyzed. From an investigation of the activity patterns of the hidden units in the trained network it appeared that information about target location relative to the head, implicitly present at the level of input signals, is no longer available at the level of the hidden layer. More detailed inspection of the properties of these units revealed that they code motor error. Their movement field is a monotonic function of motor error amplitude, and shows broad direction tuning specific for each unit. Finally, simulations were made with a four layered network with an architecture and access to input signals closely mimicking Robinson's model of the saccadic system. Again, the network was trained to represent motor error topographically in its output layer. The model shows, for the first time, how the computation of the topographical motor error map in the SC from retinal and eye position signals may proceed in two steps, involving a stage where target location is coded in a distributed fashion in craniotopic coordinates and a subsequent supracollicular stage, where radial motor error is represented in a firing-rate code in units with broad tuning characteristics. These two stages in the model show interesting similarities with the characteristics of neuron populations shown neurophysiologically in area 7a and parietal region LIP, respectively.

Brain Mapping↗

PubMed1993

New aspects of human muscle coordination as revealed by motor-unit studies.

Increasing evidence has become available against the view that individual muscles should be regarded as functional units in the control of force by the motor apparatus. The relative contribution of various motoneurone pools (belonging to a single muscle) to muscle force appears to depend on the direction of torque exerted by the corresponding limb. This clearly indicates the presence of more than one control or activation parameter for the total motoneurone pool of a single muscle. Windhorst et al. (1989) have even proposed a combined sensory and neuromuscular partitioning, such that segmental control mechanisms are based on subdivisions of motoneurone pool--muscle complexes. Size-related recruitment occurs within these subdivisions rather than in the motoneurone pool as a whole. Moreover, motoneurones can operate in various modes (different relative contributions of the recruitment mechanism and the firing frequency mechanism to muscle force) related either to movement or to force. This indicates that muscle coordination is organised differently in force tasks and in movement tasks. These properties of motoneurone activation mentioned above are difficult to incorporate in existing models on sensorimotor organisation. Some of these discrepancies will be reviewed in order to highlight future directions for modelling.

Arm↗

PubMed1992

The control of arm pointing movements in three dimensions.

In this study we investigated pointing movements made with an extended arm. Despite the large number of mechanical degrees of freedom, limb orientation adopted during pointing could be described by rotation axes contained on a two-dimensional curved surface. As a result of the curvature, the orientation of a linear plane approximating a small region of the curved surface was dependent on the location of the movements within the full workspace. These results account for earlier suggestions that limb orientation could be described by coplanar rotation vectors and that the orientation of the plane moved with the workspace. Despite the additional complexity, our results indicate that the number of degrees of freedom used to position the extended forearm is reduced from four to two for normal pointing movements. Contributions to orientation of the wrist and hand by supination/pronation of the forearm were minor for changes in shoulder yaw angle. However, supination/pronation added significantly to orientation of the hand for changes in shoulder pitch angle.

Arm↗

PubMed1991

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.

Attention↗

PubMed1991

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.

Age Factors↗

PubMed1990

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.

Arm↗

PubMed1990

Postural responses to simulated moving environments are not invariant for the direction of gaze.

Postural responses were measured for observers instructed to maintain an erect position when viewing a visual scene, which stimulated motion of the observer relative to a corridor with texture on the walls. Both the direction and amplitude of the postural responses appeared to depend on the fixation direction of the observer. This result indicates that postural responses are not invariant with respect to fixation direction. Our results suggest that not one of the frequently suggested features in the optical flow in particular is used to maintain posture but rather that a combination of several optical flow components, that have been suggested in the literature, is relevant.

Fixation, Ocular↗

PubMed1990

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.

Journal Article↗

PubMed1990

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.

Journal Article↗

PubMed1989

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.

Arm↗

PubMed1989

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.

Adult↗

PubMed1989

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.

Adult↗

PubMed1989

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.

Humans↗