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J B Smeets

Publications and source records attributed to J B Smeets.

29 records · Page 2Linked to original sources
PubMed1996

Modulation of the human vestibuloocular reflex during saccades: probing by high-frequency oscillation and torque pulses of the head.

1. We probed the gain and phase of the vestibuloocular reflex (VOR) during the execution of voluntary gaze saccades, with continuous oscillation or acceleration pulses, applied through a torque helmet. 2. Small-amplitude (< 1 degree), high-frequency (10-14 Hz) head oscillations in the horizontal or vertical plane were superimposed on ongoing horizontal gaze saccades (40-100 degrees). Torque pulses to the head ("with" or "against" gaze) were superimposed on 40 degrees horizontal saccades. Eye and head movements were precisely measured with sensor coils in magnetic fields. 3. Techniques were developed to separate the oscillatory (horizontal or vertical) component from the gaze shift and obtain VOR gain and phase with Fourier techniques from the relation between eye-in-head and head oscillations. These involved either subtraction of exactly matching saccades with and without oscillation (drawback: low yield) or time shifting of successive trials to synchronize the oscillations (drawback: slight time blurring of saccades). 4. The results of these matching and synchronization methods were essentially identical and consistent. Presaccadic gain values of the horizontal VOR (typically about unity) were reduced by, on average, approximately 20 and 50% during horizontal saccades of 40 and 100 degrees, respectively. These percentages may be truncated because of methodological limitations, but even after taking these into account (on the basis of simulation experiments with 2 different, theoretical profiles of suppression) our results do not support a complete saccadic VOR suppression for any substantial fraction of saccadic duration. Qualitatively similar changes were found when the vertical VOR was probed during 100 degrees horizontal saccades. 5. Concomitantly with the reductions in gain, VOR phase was advanced by approximately 20 degrees during the saccade. 6. In the wake of gaze saccades, VOR gain was consistently elevated (to approximately 1.0) above the presaccadic level (approximately 0.9). We submit that this mechanism ensures stable fixation of the newly acquired target at a time when the head is still moving substantially. 7. Although the responses to head torque pulses showed idiosyncratic asymmetries, analysis of the differences in eye and head movements for pulses with and against consistently showed a sharp fall of VOR gain at saccadic onset, following an approximately exponential course with a time constant of approximately 50 ms. This decay may be assumed to reflect VOR gain for a period of approximately 50 ms, after which secondary gaze control mechanisms become dominant. 8. The time course of the gain decay and phase shift of the VOR suggest that suppression of the "integrative (position) loop" of the VOR circuit was more complete than suppression of the direct, "velocity" pathway.

Female↗
PubMed1996

Effects of medroxyprogesterone acetate on appetite, weight, and quality of life in advanced-stage non-hormone-sensitive cancer: a placebo-controlled multicenter study.

PURPOSE: To investigate the effects of medroxyprogesterone acetate (MPA) on appetite, weight, and quality of life (QL) in patients with advanced-stage, incurable, non-hormone-sensitive cancer. PATIENTS AND METHODS: Two hundred six eligible patients were randomized between double-blind MPA 500 mg twice daily or placebo. Appetite (0 to 10 numerical rating scale), weight, and QL (European Organization for Research and Treatment of Cancer Quality of Life Questionnaire [EORTC-QLQ-C30]) were assessed before the start of treatment (t = 0), and 6 weeks (t = 6) and 12 weeks (t = 12) thereafter. RESULTS: One hundred thirty-four patients (68 MPA and 66 placebo) were assessable at t = 6 and 99 patients (53 MPA and 46 placebo) at t = 12. A beneficial effect of MPA on appetite was observed after both 6 weeks (P = .008) and 12 weeks (P = .01) of treatment. After 12 weeks, a mean weight gain of 0.6 +/- 4.4 kg was seen in the MPA, versus an ongoing mean weight loss of 1.4 +/- 4.6 kg in the placebo group. This difference of 2.0 kg was statistically significant (P = .04). During the study, several areas of QL deteriorated in the total group of patients. With the exception of an improvement in appetite and possible also a reduction in nausea and vomiting, no measurable beneficial effects of MPA on QL could be demonstrated. The side effects profile of MPA was favorable: only a trend toward an increase in (usually mild) peripheral edema was observed. CONCLUSION: In weight-losing, advanced-stage non-hormone-sensitive cancer patients, MPA exhibits a mild side effects profile, has a beneficial effect on appetite, and may prevent further weight loss. However, general QL in the present study was not measurably influenced by MPA treatment.

Analysis of Variance↗
PubMed1995

Prediction of a moving target's position in fast goal-directed action.

Subjects made fast goal-directed arm movements towards moving targets. In some cases, the perceived direction of target motion was manipulated by moving the background. By comparing the trajectories towards moving targets with those towards static targets, we determined the position towards which subjects were aiming at movement onset. We showed that this position was an extrapolation in the target's perceived direction from its position at that moment using its perceived direction of motion. If subjects were to continue to extrapolate in the perceived direction of target motion from the position at which they perceive the target at each instant, the error would decrease during the movements. By analysing the differences between subjects' arm movements towards targets moving in different (apparent) directions with a linear second-order model, we show that the reduction in the error that this predicts is not enough to explain how subjects compensate for their initial misjudgments.

Arm↗
PubMed1995

Moving one's finger to a visually specified position: target orientation influences the finger's path.

It has previously been shown that, when subjects are instructed to move their finger slowly from one point to another the finger follows a path that deviates systematically from a straight line connecting the two points. The deviation depends on the angle between this fictive line and a line connecting the subject's finger with his body. In the present study, we examined whether the deviation also depends on the target's orientation. In two experiments, subjects were instructed to move a finger slowly towards five targets. We recorded the finger's movements. In one experiment, the targets were aligned. In the other, they were oriented radially around the starting point. Otherwise, conditions were the same. The difference in target orientation influenced the finger's path. Most importantly, when the targets were oriented radially around the starting point, the finger's path was straight. We conclude that pointing is more than moving the finger to a specified position.

Fingers↗
PubMed1995

Perception and action are based on the same visual information: distinction between position and velocity.

Ss were presented with spiders running from left to right at various velocities over a structured background. Motion of the background influenced the perceived velocity of the spider: Motion of the background in the opposite direction than the spider increased the perceived velocity. The perceived position of the spider was not influenced by background motion. Ss were asked to hit the spiders as quickly as possible. Fast spiders were hit with a higher velocity than slow spiders. The same effect was found if the spiders only differed in apparent velocity, induced by motion of the background. The trajectory of the hit was not influenced by motion of the background. The authors concluded that although velocity is nothing but the change of position in time, velocity and position are processed independently. Furthermore, these two separately processed sources of information are used in both perception and action.

Acceleration↗
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↗
PubMed1994

The difference between the perception of absolute and relative motion: a reaction time study.

We used a reaction-time paradigm to examine the extent to which motion detection depends on relative motion. In the absence of relative motion, the responses could be described by a simple model based on the detection of a fixed change in position. If relative motion was present, the responses could be modelled using characteristics of motion detectors. Comparing reaction times when relative and absolute velocity are equal with ones when relative velocity is twice the absolute velocity reveals that these detectors measure relative motion.

Humans↗
PubMed1991

Dependence of autogenic and heterogenic stretch reflexes on pre-load activity in the human arm.

1. Subjects held their right arm in a horizontal plane. The angle of the elbow was 90 deg. They exerted forces in several directions in the plane of the arm, varying independently the pre-load torques about shoulder and elbow. We measured electromyographic (EMG) activity in several arm muscles in response to force perturbations which extended the shoulder, without changing the elbow angle. 2. The EMG activity in flexors of both shoulder and elbow showed reflex responses at short latency (approximately 25 ms). In all muscles the reflex activity increased with the pre-load activity of that muscle. 3. The short-latency reflex activity of m. brachialis, which was not stretched by the perturbations, was independent of the pre-load activity of the muscles acting over the shoulder. 4. From these results we conclude that the force resulting from the short-latency reflex, assessed from the EMGs, does not counteract the perturbations exactly. Having found that the short-latency reflex is dependent on the pre-load direction, we argue that this dependence makes the short-latency reflex suitable for correcting fast movements for misjudgements of load. 5. At longer latencies (greater than 50 ms) the direction of the force resulting from the reflex, assessed from the EMGs, was almost independent of the direction of the pre-load. In our experiment the force resulting from the long-latency reflex counteracted the perturbations quite well.

Arm↗
PubMed1990

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.

Arm↗
PubMed1985

A randomized comparative trial of combined versus alternating therapy with cytostatic drugs and high-dose medroxyprogesteron acetate in advanced breast cancer.

A prospective multicenter trial was conducted in 155 consecutive patients with Stage IV breast cancer randomly allocated to receive either (1) vincristin (V) 1.2 mg/m2 (maximum dose, 2 mg), Adriamycin (A) (doxorubicin) 40 mg/m2, and cyclophosphamide (C) 500 mg/m2, all intravenously on day 1, every 4 weeks, in combination with medroxyprogesteron acetate (MPA) 600 mg orally on days 1 through 14, 500 mg intramuscularly on days 1 through 28, and twice weekly afterwards (combined chemoendocrine approach) or (2) the same combination chemotherapy (VAC) for three cycles alternating with MPA in the above-mentioned dosage during 8 weeks (alternating chemoendocrine approach). Results show an overall response rate of 73% with 26% complete responses in the combined treatment arm, whereas in the alternating arm, an overall response rate of 76% with 20% complete responses was observed. In patients with more than one metastatic site, response rate was higher in the combination treatment, and only in this arm were complete responses observed in these patients. Although the median duration of response was long in both treatment arms (combination, 19 months versus alternating, 21 months), the median overall survival in both groups was not definitely prolonged (22 versus 24 months, respectively). However, results in subsets of patients suggest that the alternating chemoendocrine approach may be better for estrogen receptor (ER)-negative patients, for patients younger than 51 years of age, and for patients with a disease-free interval of 1 year or less. Patients with these parameters probably belong to the same population. It is concluded that combination of chemotherapy and high-dose MPA may be indicated in ER-positive patients when a clinical response is urgently needed. In ER-negative patients, the alternating use of both treatment modalities deserves further investigation.

Aged↗