Biomedical subjects

M Brüwer

Publications and source records attributed to M Brüwer.

24 records · Page 2Linked to original sources

PubMed1991

[Preferred gaze direction and the vergence system at rest].

In two experiments the effects of binocular vergence, head tilt, and prolonged gaze inclination on the preferred vertical gaze direction were studied. Concurrently, dark vergence was measured to test a hypothesis on the effect that binocular vergence has on the preferred vertical direction of gaze. The previously reported influence of observation distance on the preferred gaze inclination was replicated. When the head was tilted backward, the preferred gaze inclination was lower and the effect of observation distance was reduced; when the head was tilted forward, the preferred gaze inclination was higher and the effect of observation distance was increased. After prolonged upward or downward gaze inclination the preferred vertical gaze direction was shifted up or down, respectively, while the effect of observation distance remained constant. Overall the change of the preferred vertical direction of gaze upon binocular fixation of near stimuli was accompanied by a reduced discrepancy between convergence and the resting state of the vergence system; additional implications of the hypothesis on the relation between individual resting vergence and the effect of observation distance on the preferred inclination of gaze could not be substantiated.

Adult↗

PubMed1990

On the cost functions for the control of the human arm movement.

The aim of our investigation is to understand the mechanisms which control the movement of the human arm. The arm is here considered as a redundant system: the shoulder, elbow and wrist joints, which provide three degrees of freedom, combine to move the hand in a horizontal plane, i.e. a two dimensional space. Thus the system has one extra degree of freedom. Earlier investigations of the static situation led to the hypothesis that independent cost functions were attached to each of the three joints and that the configuration chosen for a given target position is that which provides the minimum total cost (Cruse 1986). The aim of the current investigation was to look for measurable values corresponding to the hypothetical cost functions. Experiments using pointers of different lengths attached to the hand showed that the strategy in choosing the joint angles are independent of the limb length. The muscle force necessary to reach a given angle is increased by a spring mounted across a joint. In this situation the angles of the loaded joint are changed for a given target point to give way to the force effect. This leads to the conclusion that the hypothetical cost functions are not independent of the physiological costs necessary to hold the joint at a given angle. The cost functions seem to depend on joint angle and on the force which is necessary to hold the joint in a given position. Cost functions are measured by psychophysical methods. The results show U-shaped curves which can be approximated by parabolas. The position of minimum cost (maximum comfort) for one joint showed no or weak dependency on the angles of the other joints. For each subject these "psychophysical" cost functions are compared with the hypothetical cost functions. The comparison showed reasonable agreement. This supports the assumption that the psychophysically measured "comfort functions" provide a measure for the hypothetical cost functions postulated to explain the targeting movements. Targeting experiments using a four joint arm which has two extra degrees of freedom showed a much larger scatter compared to the three joint arm. Nevertheless, the results still conform to the hypothesis that also in this case the minimum cost principle is applied to solve the redundancy problem. As the cost function for the whole arm shows a large minimum valley, quite a large range of arm positions is possible of about equal total costs.(ABSTRACT TRUNCATED AT 400 WORDS)

Arm↗

PubMed1990

A network model for the control of the movement of a redundant manipulator.

In an earlier investigation (Cruse and Brüwer 1987) an algorithmic model was proposed which describes targeting movements of a human arm when restricted to a horizontal plane. As three joints at shoulder, elbow and wrist are allowed to move, the system is redundant. Two models are discussed here which replace this algorithmic model by a network model. Both networks solve the static problem, i.e. they provide the joint angles which the arm has to adopt in order to reach a given point in the workspace. In the first model the position of this point is given in the form of x--y coordinates, the second model obtains this information by means of a retina-like input layer. The second model is expanded by a simple procedure to describe movements from a start to an end point. The results qualitatively correspond to those obtained from human subjects. The advantages of the network models in comparison to the algorithmic model are discussed.

Arm↗

PubMed1989

[Additive hypotheses of the effect of head and eye movement on dark convergence].

Dark vergence depends on the vertical direction of gaze; it decreases with raised gaze and increases with lowered gaze. The vertical direction of gaze can be varied by means or raising or lowering the eyes or by way of tilting the head forward or backward. The effects of both manipulations on dark vergence are different. According to Heuer (1988) the effects of head tilt and eye inclination on dark vergence are almost, but not exactly, additive. In Exp. 1 the hypothesis of additive effects of gaze direction and eye inclination was tested and could not be rejected. The two additive hypotheses (head tilt and eye inclination vs. gaze inclination and eye inclination) result in different predictions for dark vergence with "compensatory" head and eye inclinations, which leave the direction of gaze in space invariant. In Exp. 2 it was shown that predictions from both hypotheses deviated from the observed values of dark vergence. Thus none of the two additive hypotheses provides exact predictions of dark vergence for all possible combinations of head tilt and eye inclination. For practical purposes the approximation might be sufficient. In particular, although mean dark vergence cannot be predicted exactly, individual differences can be predicted quite accurately.

Adolescent↗

PubMed1988

The effects of sustained vertical gaze deviation on the resting state of the vergence system.

Vergence of the eyes in the dark depends on the vertical direction of gaze. In three experiments this phenomenon was studied using eye inclinations and head tilts of 5 min duration during which monocular or binocular near and distant stimuli were inspected. The effects of eye inclination and head tilt on dark vergence were temporally stable; they exhibited even a tendency to increase with the passage of time. On return to horizontal gaze, positive aftereffects were found, that is dark vergence was biased towards its value with a vertically deviated gaze. The effect of vertical gaze direction turned out to be additive to the effect of binocular near fixation. These findings represent evidence against the hypothesis that the effect of vertical gaze direction on dark vergence is mediated by a feedforward signal that is related to voluntary effort in raising or depressing the eyes.

Adaptation, Ocular↗

PubMed1987

The human arm as a redundant manipulator: the control of path and joint angles.

The movements studied involved moving the tip of a pointer attached to the hand from a given starting point to a given end point in a horizontal plane. Three joints--the shoulder, elbow and wrist--were free to move. Thus the system represented a redundant manipulator. The coordination of the movements of the three joints was recorded and analyzed. The study concerned how the joints are controlled during a movement. The results are used to evaluate several current hypotheses for motor control. Basically, the incremental changes are calculated so as to move the tip of the manipulator along a straight line in the workspace. The values of the individual joints seem to be determined as follows. Starting from the initial values the incremental changes in the three joint angles represent a compromise between two criteria: 1) the amount of the angular change should be about the same in the three joints, and 2) the angular changes should minimize the total cost of the arm position as determined by cost functions defined for each joint as a function of angle. By itself, this mechanism would produce strongly curved trajectories in joint space which could include additional acceleration and deceleration in a joint. These are reduced by the influence of a third criterion which fits with the mass-spring hypothesis. Thus the path is calculated as a compromise between a straight line in workspace and a straight line in joint space. The latter can produce curved paths in the workspace such as were actually found in the experiments. A model calculation shows that these hypotheses can qualitatively describe the experimental findings.

Arm↗