Stiffness as a control variable in motor performance

In the experiments described in this article we investigated the control of stiffness of the effector system in relation to parameters of the movement. If the velocity of a movement is chosen, it appears that at the same time the stiffness of the effector system is set to a preferred value. By changing the instruction given to the subject it is possible to change the relationship between velocity and stiffness without changing the kinematic parameters of the movement. Finally, the changes of stiffness and velocity in a learning process are studied. In conformity with what might be expected on intuitive grounds, it is found that velocity increases spontaneously during learning while stiffness decreases. The results are discussed in connection with the generation of motor programmes

A model study of isovolumic and non-isovolumic left ventricular contractions

A model of the left ventricle is proposed. It consists of a thick-walled cylinder-like cavity, the wall of which contains muscle fibres. Orientation and length of the fibres are functions of their position in the wall. During changes of preload and during ejection the dimensions of the cylinder change in a prescribed way. A sliding filament model in series with a real or apparent elastic element is applied to simulate wall tension from which the pressure in the ventricle results. To test the model and to fit the relevant parameters, experiments on isolated perfused rabbit hearts were carried out. The model could simulate isovolumic and non-isovolumic contractions reasonably well in the case when a series elasticity allowing about 6% fibre shortening was present and a preload dependent activation function was applied. Finally it must be concluded that for an accurate estimation of the parameters, not only pressure development and flow must be recorded, but also certain overall ventricular dimensions

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

Interpretation of some properties of AV conduction with the help of analog stimulation

For a better understanding of the phenomenological characteristics of A-V nodal conduction the impulse propagation through a transmission line consisting of units obeying the Bonhoeffer-Van der Poll equations was studied. It appears that impulse propagation through such a transmission line shows similarity with A-V nodal conduction

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

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

Mathematical model of AV conduction in the rat heart

To study A-V nodal conduction of the rat heart the right atrial appendage was stimulated with a variety of pulse sequences. The response was detected with ventricular epicardial electrodes. From the observed relations between P-P and P-R intervals a mathematical model describing A-V nodal conduction has been derived. The model was tested by random atrial stimulation and proved to describe A-V nodal conduction quantitatively

Integration of proprioceptive and visual position-information: An experimentally supported model

To localize one's hand, i.e., to find out its position with respect to the body, humans may use proprioceptive information or visual information or both. It is still not known how the CNS combines simultaneous proprioceptive and visual information. In this study, we investigate in what position in a horizontal plane a hand is localized on the basis of simultaneous proprioceptive and visual information and compare this to the positions in which it is localized on the basis of proprioception only and vision only. Seated at a table, subjects matched target positions on the table top with their unseen left hand under the table. The experiment consisted of three series. In each of these series, the target positions were presented in three conditions: by vision only, by proprioception only, or by both vision and proprioception. In one of the three series, the visual information was veridical. In the other two, it was modified by prisms that displaced the visual field to the left and to the right, respectively. The results show that the mean of the positions indicated in the condition with both vision and proprioception generally lies off the straight line through the means of the other two conditions. In most cases the mean lies on the side predicted by a model describing the integration of multisensory information. According to this model, the visual information and the proprioceptive information are weighted with direction-dependent weights, the weights being related to the direction-dependent precision of the information in such a way that the available information is used very efficiently. Because the proposed model also can explain the unexpectedly small sizes of the variable errors in the localization of a seen hand that were reported earlier, there is strong evidence to support this model. The results imply that the CNS has knowledge about the direction-dependent precision of the proprioceptive and visual information

Changes in recruitment order of motor units in the human biceps muscle

Changes in recruitment threshold of individual motor units of the human biceps (caput longum), a multifunctional muscle, were investigated during different tasks, i.e., isometric flexion of the elbow, isometric supination of the forearm, and isometric exorotation of the humerus of the 110° flexed semiprone horizontal arm. The activity of 17 motor units was recorded by means of fine wire electrodes. Some units were found that could be recruited only by one force, e.g., flexion. In such cases recruitment did not depend on other forces. Most units, however, were recruited when a linear combination of exerted forces exceeded a certain threshold. The contribution of a force to this combination could be different for different motor units. Units with a high threshold for flexion tended to show a lower threshold while simultaneously exerting force in another direction. Units with a low threshold for flexion were more difficult to recruit under this condition. The findings support the view that movements are programmed “directionally”