Biomedical subjects

J B Smeets

Publications and source records attributed to J B Smeets.

At least 19 recordsLinked to original sources

PubMed2001

The effect of expectations on hitting moving targets: influence of the preceding target's speed.

When hitting a target that is moving, the time for planning the interception is limited. Instead of waiting for all the necessary information about the target's position and speed before starting to move, subjects could use their previous experience with similar targets to make initial guesses and adjust as new information becomes available. In the present study we examined whether the speed of the preceding target influences a hitting movement. Subjects hit moving targets that appeared on a screen about 40 cm in front of them. The targets moved at 6, 12 or 18 cm/s. Both the hand's initial movement direction and the final hitting error depended on the speed of the preceding target. We conclude that people control the way they hit moving targets on the basis of the speed of the preceding target.

Analysis of Variance↗

PubMed2001

Endpoints of arm movements to visual targets.

Reaching out for objects with an unseen arm involves using both visual and kinesthetic information. Neither visual nor kinesthetic information is perfect. Each is subject to both constant and variable errors. To evaluate how such errors influence performance in natural goal-directed movements, we asked subjects to align a real 5-cm cube, which they held in their hand but could not see, with a three-dimensional visual simulation of such a cube. The simulated cube was presented at one of four target locations at the corners of an imaginary tetraeder. Subjects made successive, self-paced movements between these target locations. They could not see anything except the simulated cube throughout the experiment. Initial analysis of the spatial dispersion of movement endpoints demonstrated that the major source of errors under these conditions was visual. Further analysis of the relationship between variability of the starting positions and endpoints showed that the errors were primarily in judging the endpoint, rather than the direction or amplitude of the required movement vector. The findings support endpoint control of human goal-directed movements.

Arm↗

PubMed2001

Independent movements of the digits in grasping.

Reaching out for an object is often considered to consist of the control of two components: transporting the hand to the object's position, and scaling the grip to the object's size. We recently proposed an alternative view. According to this view, grasping consists of controlling the digits, not the hypothetical transport and grip. This alternative view assumes that the opening of the hand emerges from the trajectories of the digits. We therefore studied the movements of the digits in grasping. We asked subjects to grasp disks (diameters ranging from 5 to 8 cm) at marked positions with two digits. The positions were at opposite sides of the disk, at the same distance from the starting position, so that the orientation of the surface was the same for both digits. The subjects grasped the disks either with the index finger and thumb of the dominant hand, with the same digits of the non-dominant hand, or bimanually with both index fingers. Our predictions are: that the well-known relation between object size and grip aperture holds for each digit; that the same relation holds if the object is grasped with two hands instead of with the thumb and finger of one hand; that maximum deviation, variability and duration of the digit movements are related; and that variations in the timing of the maximum deviation of one digit are independent of those in the other digit. In accordance with our predictions, we found that the maximum deviation of both digits increased with 0.75 times the object radius, independent of the hand(s) used. The movements of the thumb were more variable than those of the index finger, which was reflected by a larger deviation earlier in the movement. The timing of the maximum deviation of the two digits was independent. These results on the digits' movements are consistent with our view that grasping can be understood as the largely independent movements of the digits. The results are not in conflict with the hypothesis that the grip is controlled during grasping, but can only be explained by extending that hypothesis post hoc.

Central Nervous System↗

PubMed2001

Smooth eye movements and spatial localisation.

We asked subjects to align a target that flashed as their eyes rotated to the right in pursuit of a moving ring, with a target that flashed as their eyes rotated to the left in pursuit of the ring. Subjects systematically mislocalised the targets in the direction of pursuit. When the ring and flashes were the only structures that were visible, the alignment error was about 4 cm, corresponding to a timing error of about 100 ms. The timing error was independent of the position along the ring's path, but did depend to some extent on pursuit velocity. Adding a textured background reduced the mislocalisation considerably, presumably because it enabled subjects to localise the targets relative to the surrounding. There was almost no mislocalisation if the subject was not pursuing the ring. It is suggested that the mislocalisation arises because incoming retinal signals are combined directly with outgoing oculo-motor commands, with no attempt to account for any of the involved neuronal and muscular delays.

Computer Graphics↗

PubMed2001

How vertical disparities assist judgements of distance.

The ratio of the vertical sizes of corresponding features in the two eyes' retinal images depends both on the associated object's distance and on its horizontal direction relative to the head (eccentricity). It is known that manipulations of vertical size ratio can affect perceived distance, size, depth and shape. We examined how observers use the vertical size ratio to determine the viewing distance. Do they use the horizontal gradient of vertical size ratio, or do they combine the vertical size ratio itself with the eccentricity at which it is found? Distance scaling (as measured by having subjects set an ellipsoid's size and shape to match a tennis ball) was no better when the judged object was 30 degrees to the right of the head (where vertical size ratios vary considerably with distance) than when it was located straight ahead. Distance scaling improved when vertical disparities were presented within larger visual fields, irrespective of where this was relative to the head. Our results support the proposal that subjects use the horizontal gradient of vertical size ratio to estimate the distance of an object that they are looking at.

Distance Perception↗

PubMed2000

Hitting moving objects. The dependency of hand velocity on the speed of the target.

In previous studies, subjects did not hit slowly moving objects as quickly as fast ones, despite being instructed to hit them all as quickly as possible. In those studies the targets moved at constant but unpredictable velocities, and it has been suggested that subjects were unable to adjust the hand's path to suit the velocity of the target. To compensate for this, they adjusted the speed of their hand to that of the target (speed coupling). According to this hypothesis, speed coupling is necessary only when subjects are unable to correctly predict the next target velocity and only if they have to be accurate. We show here that decreasing the uncertainty about the upcoming target's velocity or enlarging the tip of the hitting weapon does not make speed coupling disappear. Moreover, there is a negative correlation between hand velocity and strength of speed coupling, whereas the hypothesis predicts a positive correlation. The hypothesis is therefore rejected. We propose that speed coupling is a result of different speed-accuracy tradeoffs applying to different target velocities.

Hand↗

PubMed2000

The relation between movement parameters and motor learning.

In a recent paper, Flament et al. (1999) studied the process of learning to flex the elbow faster. They concluded from their data that time-related parameters (e.g. movement time) changed faster during learning than magnitude-related parameters (e.g. peak velocity) and discussed this finding in terms of neural substrates responsible for the apparently different learning mechanisms. In this paper, I will argue that finding different time constants does not imply different learning mechanisms and will give a theoretical example of the development of parameters during learning to move faster. Despite the fact that only one learning process is modelled, various kinematic parameters show different time courses of learning. The differences the model predicts are comparable with the experimental results.

Biomechanical Phenomena↗

PubMed2000

Comparing extra-retinal information about distance and direction.

The idea that extra-retinal information about the orientation of the eyes could be used to judge an object's distance has a long history, and has been the issue of considerable debate throughout this century. We here show that the poor performance in comparison with judgements of direction has geometrical rather than physiological reasons, and discuss why previous studies have misled us into believing that information about distance is even poorer than the geometry predicts.

Convergence, Ocular↗

PubMed2000

Motion extrapolation is not responsible for the flash-lag effect.

To achieve perceptual alignment between a flashed target and a moving one, subjects typically require the flashed target to be aligned with a position that the moving target will only reach some time after the flash (the flash-lag effect). We examined how the magnitude of this misalignment changes near an abrupt change in velocity. The magnitude of the misalignment turns out to depend on the target's velocity after, rather than before, the flash. Thus, the misalignment cannot be caused by motion extrapolation. Neither can it be the inevitable consequence of a difference between the time it takes to process flashed and moving stimuli, because the magnitude of the misalignment is influenced by the extent to which subjects can anticipate the flash. We propose that it is the consequence of having to 'sample' the moving target's position in response to the flash.

Humans↗

PubMed2000

Activity of high-dose epirubicin combined with gemcitabine in advanced non-small-cell lung cancer: a multicenter phase I and II study.

The aim of the study was to evaluate efficacy and tolerance of epirubicin and gemcitabine as first-line chemotherapy in patients with advanced non-small-cell lung cancer. A phase I study was performed with the combination of escalating doses of epirubicin intravenously on day 1 and a fixed dose of gemcitabine on days 1 and 8 of a 21 -day cycle. Eighteen patients were included in the phase I part of the study before the maximum tolerated dose was found. Dose-limiting toxicity was febrile neutropenia. The phase II part of the study was continued with epirubicin 100 mg m(-2) on day 1 and gemcitabine 1125 mg m(-2) on days 1 and 8 of a 21-day cycle. Forty-three chemotherapy-naive patients were included. The median age of the patients was 60 years (range 26-75). Most patients (74%) were in stage IV. Granulocytopenia CTC grade 4 occurred in 32.5% and thrombocytopenia grade 4 in 11.6% of cycles. Febrile neutropenia occurred in six patients. Non-haematological toxicity was mainly mucositis CTC grade 2 and 3 in 35% of patients. The tumour response rate was 49% (95% confidence interval (CI) 35-63%). The median survival time for the patients was 42 weeks (95% CI 13-69).

Antineoplastic Combined Chemotherapy Protocols↗

PubMed2000

Early components of the human vestibulo-ocular response to head rotation: latency and gain.

To characterize vestibulo-ocular reflex (VOR) properties in the time window in which contributions by other systems are minimal, eye movements during the first 50-100 ms after the start of transient angular head accelerations ( approximately 1000 degrees /s(2)) imposed by a torque helmet were analyzed in normal human subjects. Orientations of the head and both eyes were recorded with magnetic search coils (resolution, approximately 1 min arc; 1000 samples/s). Typically, the first response to a head perturbation was an anti-compensatory eye movement with zero latency, peak-velocity of several degrees per second, and peak excursion of several tenths of a degree. This was interpreted as a passive mechanical response to linear acceleration of the orbital tissues caused by eccentric rotation of the eye. The response was modeled as a damped oscillation (approximately 13 Hz) of the orbital contents, approaching a constant eye deviation for a sustained linear acceleration. The subsequent compensatory eye movements showed (like the head movements) a linear increase in velocity, which allowed estimates of latency and gain with linear regressions. After appropriate accounting for the preceding passive eye movements, average VOR latency (for pooled eyes, directions, and subjects) was calculated as 8.6 ms. Paired comparisons between the two eyes revealed that the latency for the eye contralateral to the direction of head rotation was, on average, 1.3 ms shorter than for the ipsilateral eye. This highly significant average inter-ocular difference was attributed to the additional internuclear abducens neuron in the pathway to the ipsilateral eye. Average acceleration gain (ratio between slopes of eye and head velocities) over the first 40-50 ms was approximately 1.1. Instantaneous velocity gain, calculated as Veye(t)/Vhead(t-latency), showed a gradual build-up converging toward unity (often after a slight overshoot). Instantaneous acceleration gain also converged toward unity but showed a much steeper build-up and larger oscillations. This behavior of acceleration and velocity gain could be accounted for by modeling the eye movements as the sum of the passive response to the linear acceleration and the active rotational VOR. Due to the latency and the anticompensatory component, gaze stabilization was never complete. The influence of visual targets was limited. The initial VOR was identical with a distant target (continuously visible or interrupted) and in complete darkness. A near visual target caused VOR gain to rise to a higher level, but the time after which the difference between far and near targets emerged varied between individuals.

Abducens Nerve↗

PubMed1999

A new view on grasping.

Reaching out for object is often described as consisting of two components that are based on different visual information. Information about the object's position and orientation guides the hand to the object, while information about the object's shape and size determines how the fingers move relative to the thumb to grasp it. We propose an alternative description, which consists of determining suitable positions on the object - on the basis of its shape, surface roughness, and so on - and then moving one's thumb and fingers more or less independently to these positions. We modeled this description using a minimum-jerk approach, whereby the finger and thumb approach their respective target positions approximately orthogonally to the surface. Our model predicts how experimental variable such as object size, movement speed, fragility, and required accuracy will influence the timing and size of the maximum aperture of the hand. An extensive review of experimental studies on grasping showed that the predicted influences correspond to human behavior.

Arm↗

PubMed1998

Hitting moving targets. Continuous control of the acceleration of the hand on the basis of the target's velocity.

Previous studies on how we hit moving targets have revealed that the direction in which we move our hand is continuously adjusted on the basis of the target's perceived position, with a delay of about 110 ms. In the present study we show that the acceleration of the hand is also under such continuous control. Subjects were instructed to hit moving targets (running spiders) as quickly as possible with a rod. We found that changing the velocity of the target influenced the speed with which the rod was moved. The influence was noticeable about 200 ms after the target's velocity changed. The extent of the influence was consistent with a direct dependence of the acceleration of the hand on the target's velocity. We conclude that the acceleration of the hand is continuously adjusted on the basis of the speed of the target, with a delay of about 200 ms.

Animals↗

PubMed1997

Fast Responses of the Human Hand to Changes in Target Position.

If a target toward which an individual moves his hand suddenly moves, he adjusts the movement of his hand accordingly. Does he use visual information on the target's velocity to anticipate where he will reach the target? These questions were addressed in the present study. Subjects (N = 6 in each of 4 experiments) were instructed to hit a disk with a rod as soon as it appeared on a screen. Trajectories of the hand toward stationary disks were compared with those toward disks that jumped leftward or rightward as soon as the subject's hand started moving toward the screen, and with those in which either the disk or the background started moving leftward or rightward. About 110 ms after the disk was suddenly displaced, the moving hand was diverted in the direction of the perturbation. When the background moved, the disk's perceived position shifted in the direction in which the background was moving, but the disk appeared to be moving in the opposite direction. When hitting such disks, subjects adjusted their movement in accordance with the perceived position, rather than moving their hand in the direction of the perceived motion in anticipation of the disk's future displacement. Thus, subjects did not use the perceived velocity to anticipate where they would reach the target but responded only to the change in position.

arm movement↗

PubMed1997

Holding an object one is looking at: kinesthetic information on the object's distance does not improve visual judgments of its size.

Visual judgements of distance are often inaccurate. Nevertheless, information on distance must be procured if retinal image size is to be used to judge an object's dimensions. In the present study, we examined whether kinesthetic information about an object's distance--based on the posture of the arm and hand when holding it--influences the object's perceived size. Subjects were presented with a computer simulation of a cube. This cube's position was coupled to that of a rod in the subject's hand. Its size was varied between presentations. Subjects had to judge whether the cube they saw was larger than, smaller than, or the same size as a reference. On some presentations, a small difference was introduced between the positions of the rod and of the simulated cube. When the simulated cube was slightly closer than the rod, subjects judged the cube to be larger. When it was farther away, they judged it to be smaller. We show that these changes in perceived size are due to alterations in the cube's distance from the subject rather than to kinesthetic information.

Distance Perception↗

PubMed1996

Size illusion influences how we lift but not how we grasp an object.

Reaching out for an object is often described as consisting of two components that are based on different visual information. Information on the object's position and orientation guides the hand to the object, while information on the object's shape and size determines how the fingers move relative to the thumb to grasp it. We propose an alternative description, which consists of determining suitable positions on the object-on the basis of its shape, surface texture, and so on- and then moving one's thumb and fingers to these positions. This could lead to the same performance without requiring distinct visual information on the object's orientation or size. If so, an illusory change in size need not influence the distance between thumb and fingers when reaching out for an object. However, as the object's size is used to estimate its weight, the illusory change in size should influence the force that is exerted to lift the object. To find out whether this is so, eight subjects were asked to pick up brass disks from a fixed position straight in front of them. The illusory change in size was brought about by presenting five converging lines in two different configurations under the disks. As predicted, the illusion influenced the force used to lift the disks, but not the distance between the subjects' thumbs and fingers when reaching for the disks.

Hand Strength↗

PubMed1996

Goal-directed arm movements change eye-head coordination.

We compared the head movements accompanying gaze shifts while our subjects executed different manual operations, requiring gaze shifts of about 30 degrees. The different tasks yielded different latencies between gaze shifts and hand movements, and different maximum velocities of the hand. These changes in eye-hand coordination had a clear effect on eye-head coordination: the latencies and maximum velocities of head and hand were correlated. The same correlation between movements of the head and hand was also found within a task. Therefore, the changes in eye-head coordination are not caused by changes in the strategy of the subjects. We conclude that head movements and saccades during gaze shifts are not based on the same command: head movements depend both on the actual saccade and on possible future gaze shifts.

Arm↗

PubMed1996

Is judging time-to-contact based on 'tau'?

An investigation was undertaken into whether judgments of time-to-contact between a laterally moving object and a bar are based on the direct perception of an optical variable (tau), or on the ratio between the perceived distance and perceived velocity of the object. A moving background was used to induce changes in the perceived velocities without changing the optical variables that specify time-to-contact. Background motion induced large systematic errors in the estimated time-to-contact. It is concluded that the judgment of time-to-contact is primarily based on the ratio between the perceived distance and the perceived velocity, and not on tau.

Acceleration↗