Biomedical subjects

A C Sittig

Publications and source records attributed to A C Sittig.

11 recordsLinked to original sources

PubMed1996

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.

Adult↗

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

Misdirections in slow, goal-directed arm movements are not primarily visually based.

In a previous study we found that the initial direction of slow, goal-directed arm movements deviates consistently from the direction of the actual straight line between the starting position and the target position. We now investigate whether these deviations are caused by imperfections or peculiarities in the processing of vision-related spatial information, such as retinal information, and eye- and head-position information. This could lead to incorrect localization of the target relative to the starting position. Subjects were seated in front of a horizontal surface and had to move their arm slowly and accurately in the direction of target positions. We varied the amount of vision-related spatial information. In experiment 1, subjects were presented with visual targets and could see their moving arm. In experiment 2, the subjects were again presented with visual targets, but now they could not see their moving arm. In experiment 3, the subjects were blindfolded and had to move their arm towards tactile targets. In all three experiments we found comparable consistent deviations in the initial movement direction. We also instructed congenitally and early-blind subjects to move their arm towards tactile targets. Their performance showed deviations congruous with those found in the sighted subjects, and possibly somewhat larger. We conclude that the deviations in the initial movement direction of slow, goal-directed arm movements are not primarily visually based. The deviations are generated after all spatial information has been integrated.

Adult↗

PubMed1994

Orientation dependent misalignments in a visual alignment task.

Subjects performed a three-dot alignment in the frontoparallel plane. We found systematic deviations in alignment, especially for diagonally oriented stimuli. The biases did not depend on the angular size of the stimuli which was varied between 0.8 and 20 deg. We put forward a tentative explanation based on saccade trajectories. Extending the task to judgements of the straightness of virtual lines consisting of a varying number of dots showed that the biases decrease gradually when the number of dots increases. This suggests that there are two different and competing mechanisms to judge the straightness of virtual lines.

Adult↗

PubMed1993

Mechanisms of velocity constancy.

Human observers can compare the physical velocities of objects (cm/sec) moving at different distances quite well, although the objects' retinal velocities (deg/sec) may vary considerably. This perceptual ability is called velocity constancy. We conducted a number of experiments to investigate what mechanisms observers use to attain this constancy and if pure motion signals can also be matched according to their physical speeds. Subjects were asked to match the velocities of two moving stimuli presented at different viewing distances. The stimuli consisted of sparse random-dot kinematograms or drifting Julesz patterns. The subjects matched the true physical velocities of the stimuli provided that the two visual scenes contained identical size references. Knowledge of the actual viewing distances proved to be irrelevant for evaluating the physical velocities of the stimuli. We conclude that velocity constancy is based upon a relative scaling algorithm.

Algorithms↗

PubMed1991

Misdirections in slow goal-directed arm movements and pointer-setting tasks.

Information about the direction of the virtual line between two positions in space (directional information) is used in many decision-making and motor tasks. We investigated how accurately directional information is processed by the brain. Subjects performed two types of task. In both tasks they sat at a table. In the first task they had to move their hand slowly and accurately from an initial position 40 cm in from of them to visually presented targets at a distance of 30 cm from the initial position (movement task). We analysed the initial movement direction. In the second task subjects had to position pointers in the direction of the targets as accurately as they could (perceptive task). We found that in the movement task the subjects started the movements to most targets in a direction that deviated consistently from the direction of the straight line between initial position and target position. The maximum deviation ranged from 5-10 degrees for the various subjects. The mean standard deviation was 4 degrees. In the perceptive task the subjects positioned the pointer in similarly deviating directions. Furthermore, we found that the maximum deviation in the pointer direction depended on the length of the pointer: the smaller the pointer, the larger the consistent deviations in the pointer direction. The shortest pointer showed deviations comparable to the deviations found in the movement task. These findings suggest that the deviations in the two tasks stem from the same source.

Arm↗

PubMed1990

Reconstruction of the Frank vectorcardiogram from standard electrocardiographic leads: diagnostic comparison of different methods.

Three methods for reconstructing the Frank VCG from the standard 12-lead ECG were studied. The first was based on multivariate regression, the second on a model of the cardio-electrical activity, and the third method used a quasi-orthogonal set of ECG leads. The methods were evaluated on a test set of 90 cases by a numerical distance measure and by the agreement in diagnostic classification of the original and reconstructed VCGs. The original and reconstructed VCGs were presented separately and in random order to three referees. Eighteen of the original VCGs were presented three times to estimate the intra-observer agreement. Kappa statistics were used to quantify the agreement between diagnostic classifications. Separately, one referee was simultaneously presented the original VCG and its three reconstructions for all cases. Each reconstruction VCG was classified as either diagnostically 'same' as the original, 'borderline' or 'different'. The performance of the regression method and the model-based method was comparable. Both methods were preferable to the quasi-orthogonal method. The kappa values for the preferred methods indicated a good to excellent diagnostic agreement between the original and reconstructed VCGs. Only one out of ninety VCGs that were reconstructed with the regression method was classified as 'different' compared with the original VCGs; three VCGs were classified as 'different' with the model-based method. It was also found that estimation of similarity by a distance measure could not replace diagnostic evaluation by skilled observers.

Electrocardiography↗

PubMed1990

The Delphi method to validate diagnostic knowledge in computerized ECG interpretation.

We investigated the applicability of the Delphi method for increasing the agreement among multiple cardiologists on, firstly, their classifications of a set of electrocardiograms and, secondly, their reasons for these classifications. Five cardiologists were requested to judge the computer classifications of a set of thirty ECGs. If a cardiologist disagreed with the computer classification, he had to provide a new classification and a reason for this change. The results of this first round were compiled and anonymously fed back to the cardiologists. In a second round the cardiologists were asked once again to judge the ECGs and to rate the reasons provided in the first round. The level of agreement was estimated by means of the kappa statistic. The Delphi procedure substantially increased the agreement on the classifications among the cardiologists. The final agreement was very high and comparable with the intraobserver agreement. There was also a high level of agreement on the reasons provided by the cardiologists. However, their use in improving the program's performance is hampered by the qualitative nature of many of the reasons. Suggestions are given for a more formalized elicitation of knowledge.

Delphi Technique↗

PubMed1987

The contribution of afferent information on position and velocity to the control of slow and fast human forearm movements.

We applied vibration at various rates to the biceps tendon of a passive, restrained arm in normal human subjects and measured its effect on the perception of forearm position and the perception of forearm velocity. The disturbances of limb position perception and limb velocity perception depended on the vibration rate in distinctly different ways. We thereupon applied vibration at various rates to the biceps tendon during the performances of non-visually-guided slow and fast forearm movements. The vibration-rate-dependence of the disturbance of slow movements matched the vibration-rate-dependence of the disturbance of limb position perception. The vibration-rate-dependence of the disturbance of fast limb movements matched the vibration-rate-dependence of the disturbance of limb velocity perception. It is concluded that afferent position information is dominant in the control of slow movements, whereas mainly afferent velocity information is used in the control of fast movements.

Adult↗

PubMed1985

The attainment of target position during step-tracking movements despite a shift of initial position.

Subjects performed a step-tracking forearm movement at maximum velocity without visual guidance. A considerable shift of initial forearm position, which remained unnoticed by the subject, was induced by vibration of the biceps tendon. Notwithstanding the shift of initial position subjects attained the correct final position, irrespective of whether the vibration was switched off before the movement or continued during the movement. Recordings of biceps and triceps electromyograms show that muscle activities were programmed to produce a movement from the actual initial position to the target position. The findings indicate that correct information on forearm position is available to the central nervous system at a subconscious level even if position perception is disturbed through vibration.

Biomechanical Phenomena↗

PubMed1985

Separate control of arm position and velocity demonstrated by vibration of muscle tendon in man.

The effect of muscle tendon vibration on the performance of some simple motor tasks and on kinesthesia was studied in normal humans. Subjects performed non-visually-guided slow arm movements to match either the position or the velocity of a visual target. In the experiments designed to study kinesthesia subjects indicated the perceived position or velocity of their passively moved arm. Vibration was applied over either the biceps or the triceps tendon. Position and velocity matching were found to be disturbed by vibration in essentially different ways, as were the perception of imposed position and the perception of imposed velocity. However, the vibration induced disturbance of position matching was congruent with the distortion of position perception. The effect of vibration on velocity on the perception of velocity. It is concluded that the afferent information pathways that give rise to the perception of position and velocity respectively can be used separately in the control of slow movements under different conditions.

Adult↗