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Biomedical subjects

J J van der Gon

Publications and source records attributed to J J van der Gon.

13 recordsLinked to original sources
PubMed1996

Vector coding in slow goal-directed arm movements.

It has been found that the estimate of relative target direction is consistently biased. Relative target direction refers to the direction in which a target is located relative to another location in space (e.g., a starting position in the case of goal-directed movements). In this study, we have tested two models that could underlie this biased estimate. The first proposed model is based on a distorted internal representation of locations (i.e., we perceive a target at the "wrong" location). We call this the distorted location model. The second model is based on the idea that the derivation of target direction from spatial information about starting and target position is biased. We call this the biased direction model. These two models lead to different predictions of the deviations that occur when the distance between the starting position and the target position is increased. Since we know from previous studies that the initial direction of slow arm movements reflects the target direction estimate, we tested the two models by analyzing the initial direction of slow arm movements. The results show that the biased direction model can account for the biases we find in the target direction estimate for various target distances, whereas the distorted location model cannot. In two additional experiments, we explored this model further. The results show that the biases depend only on the orientation of the line through starting position and target position relative to the plane through longitudinal head or body axis and starting position. We conclude that the initial part of (slow) goal-directed arm movements is planned on the basis of a (biased) target direction estimate and not on the basis of a wrong internal representation of target location. This supports the hypothesis that we code displacements of our limbs in space as a vector.

Adult↗
PubMed1994

An unsupervised neural network model for the development of reflex co-ordination.

In this paper, we present a model for the development of connections between muscle afferents and motoneurones in the human spinal cord. The model consists of a limb with six muscles, one motoneurone pool, one pooled (Ia-like) afferent for each muscle and a central programme generator. The weights of the connections between the afferents and the motoneurone pools are adapted during centrally induced movements of the limb. The connections between the afferents and the motoneurone pools adapt in a hebbian way, using only local information present at the synapses. This neural network is tested in two examples of a limb with two degrees of freedom and six muscles. Despite the simplifications, the model predicts the pattern of autogenic and heterogenic monosynaptic reflexes quite realistically.

Humans↗
PubMed1988

Postural movements induced by rotations of visual scenes.

The effect of visual information processing of a rotating visual scene on postural responses of a standing subject was studied. Rotation of a scene induced postural sway movements involving rotations in the ankle joint. For modulation frequencies of rotation below 0.3 Hz, a clear correlation was found between the visual stimulus and the postural movements in the lateral direction. The amplitude of postural movements appeared to be independent of the angular velocity of the rotation of the scene if the angular velocity exceeded 5 deg/sec for sinusoidally modulated rotations or 10 deg/sec (rms values) for randomly modulated rotations. To facilitate distinction between the contributions from visual information processing of the incoming visual stimuli and from the motor system, a model is presented that explains some aspects of visuo-motor interaction.

Adult↗
PubMed1984

Relation between location of a motor unit in the human biceps brachii and its critical firing levels for different tasks.

Critical firing levels (CFLs) of single motor units (MUs) in the long head of the human biceps brachii muscle were determined in combinations of two isometric tasks: flexion of the elbow, supination of the lower arm, and exorotation of the humerus, as well as the corresponding antagonistic tasks. The MU activity was recorded by 25-micron bipolar wire electrodes. Four main patterns of MU recruitment, related to the recording location in the muscle, were found: (i) MUs active only when flexing the elbow were located mostly laterally. (ii) MUs active only when supinating were all located medially. (iii) MUs whose CFL depended on a linear combination of flexion and supination forces were all located medially. Some of these MUs could not be recruited during pronation. (iv) Nonlinearly behaving MUs, located centrally. The relative weights of flexion and supination input were constant for all units, whose CFL depended on a linear sum of flexion and supination forces, as well as for the nonlinearly behaving units. Supination and exorotation showed equivalent CFL changes when they were combined with the flexion task. Extension did not change the CFL for supination- or exorotation tasks. No clear difference was found between the ratios of the peak twitch forces in flexion and supination direction for laterally and medially located small muscle areas or single MUs. A simple model of the motoneuron pool organization is proposed to explain our findings.

Action Potentials↗
PubMed1979

Two-dimensional echo-encephalography. 1. Acoustic properties of some tissues and the characteristics of echo patterns obtained using equipment especially adapted for detection of these tissues.

In this paper the results of measurements of ultrasound velocity in, and density of, a number of normal and pathological tissues are given. From the results it can be concluded that intracranial pathological regions will often be detectable with ultrasound techniques, but is is not possible with any degree of certainty to localise precisely the morphological boundaries of these abnormal regions. Because, in the large majority of cases, the echoes from intracranial pathological structural changes are of weak intensity, a simple electronic device is described which, in conjunction with standard echo equipment, has been shown to be of use for better detection of these weak signals.

Brain↗
PubMed1979

Two-dimensional echo-encephalography. 2. Clinical use and reliability of two-dimensional echo-encephalography in neurology and neurosurgery.

More than two hundred patients with intracranial lesions were examined by means of B-scan echo equipment specially adapted for the detectionof weak reflections. An impression of the accuracy of the method is demonstrated by the results obtained in those patients where the diagnosis had been verified at surgery or autopsy. It appeared that B scanning is a quite reliable diagnostic method. It was found that intracranial tumours showed typical echo-patterns. Furthermore, we found that B scanning may be an important diagnostic aid in follow-up examinations of patients. However, the ultrasound technique as used by us has also a restriction: it is unsuited for the estimation of the precise extension of pathological areas and for the estimation of the nature of pathology.

Brain Diseases↗
PubMed1975

Site of initial excitation and current threshold as a function of electrode radius in heart muscle.

End-diastolic current thresholds have been measured in 13 open chested dogs as a function of electrode radius by stimulating the left ventricle with epicardial disc electrodes ranging in a radius from 0-3 mm to 9 mm. Thresholds for cathodal rectangular short stimuli as well as specifically for cathodal make stimulation, proved to be proportional to the electrode radius to the power 1-5. This relationship between radius and threshold can be explained theoretically, assuming that electrical stimulation results in a propagated depolarization front if a critical current density is reached somewhere in the myocardium. The current distribution measured over the electrode and the site of initial depolarization in the tissue are in accordance with this theoretical explanation.

Animals↗