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Biomedical subjects

G Ishai

Publications and source records attributed to G Ishai.

9 recordsLinked to original sources
PubMed1994

Two metric solutions to three-dimensional reconstruction for an eye in pure rotations.

A problem in space perception concerns how a mobile observer acquires information about the structure of objects. Earlier research derived the optic-flow equations for an eye undergoing pure rotations. It was suggested that, by utilizing three points and two views, one can recover the distance of points and the motion parameters. The radius of the eyeball was the metric unit. Yet the common view regards this problem as indeterminate. We derived a unique solution in the discrete case, which required three points and two views. However, when we observed a single bright point, a substantial amount of visual stability existed. We therefore derived a solution in the differential approach for a single point, which is based on a distinction that we made between mathematical and visual points. Both solutions were checked with a computer simulation and were found to be accurate, supporting the space perception in navigation (SPIN) theory.

Computer Simulation↗
PubMed1984

Evaluation of AK prostheses comparing conventional with adaptive knee control devices.

An objective evaluation of an experimental adaptive controlled knee, using kinematic, ground reactions and time/distance data was carried out. The comparison of amputee's walking with the experimental and the normal prosthesis was based on five parameters, representing the main walking pattern criteria. Significant differences were detected, in favour of the adaptive controlled knee, i.e. improvement in hip moment peaks (50%), hip muscular effort (37%), reduced need for compensatory movements, and improved symmetry.

Artificial Limbs↗
PubMed1983

Adaptive microcomputer control of an artificial knee in level walking.

An experimental microcomputer controlled prosthesis is discussed, where knee damping is governed by kinematic signals, picked up from both legs. The control algorithm is based on division of the walking cycle into ten stages; each stage characterized by an appropriate damping level which governs the restraining moment at the knee, as functionally required. Stage identification is carried out by the kinematic inputs. The experimental prosthesis was tested on one amputee, the first author of this paper. The control system was found to be more advantageous than a conventional valve system in significantly reducing hip muscular effort on the prosthetic side. Gait symmetry is also improved.

Computers↗
PubMed1983

Effects of alignment variables on thigh axial torque during swing phase in AK amputee gait.

It is suggested that a major source of discomfort for above-knee amputees during the swing phase of walking, is the thigh axial torque (TAT) transferred at the stump--socket interface. The relation between TAT and variations in its six relevant alignment adjustments, has been investigated. A computerized routine has been established which indicates optimum choice of alignment setting, based on minimal TAT peaks. Feasibility for attenuating swing phase TAT has been demonstrated in three simulated patterns of amputee gait. As a conclusion, it is suggested that a useful clinical tool could be based on the presented alignment optimization procedure and may be expanded to include other factors associated with swing and stance phase comfort and performance.

Amputation, Surgical↗
PubMed1980

Visual stability and space perception in monocular vision: mathematical model.

A deterministic model for monocular space perception is presented. According to the model, retinal luminance changes due to involuntary eye movements are detected and locally analyzed to yield the angular velocity of each image point. The stable three-dimensional spatial coordinates of viewed objects are then reconstructed using a method of infinitesimal transformations. The extraction of the movement (parallax) field from the optical flow is represented by a set of differential equations, the derivation of which is based on the conservation of energy principle. The relation of the model to retinal neurophysiology and to various aspects of visual space perception is discussed.

Depth Perception↗
PubMedDate not supplied

Parameter estimation for a prosthetic ankle.

The mechanical parameters of a model of an energy storage and return ankle prosthesis are estimated for normal level walking by means of an optimization procedure. The walking cycle is divided into six fields, such that the power does not change sign within each field; the transition between successive fields occurs at zero power. The optimal spring stiffness as a function of time, is found by optimizing a quadratic cost function to minimize the difference between the estimated ankle moments and the moments in normal walking. The optimization is subjected to four continuous constraints within each field and to two continuity constraints for the transitions between successive fields. The time-varying spring stiffness and the implications of additional external energy are discussed and are presented as recommendations for the designer.

Ankle Joint↗
PubMedDate not supplied

A system for monitoring the position and time of feet contact during walking.

An analogue time/distance measurement for use in walking studies and based on foot contact is presented. The system utilizes a d.c.current source, feeding resistance wires bonded longitudinally to a walkway, and shorted by conducting strips on the shoe. An example of an application of the system is included: a comparison of the walking performance of an amputee with two different knee systems.

Biomedical Engineering↗