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Biomedical subjects

D Simons

Publications and source records attributed to D Simons.

10 recordsLinked to original sources
PubMed1989

Chordal geometry determines the shape and extent of systolic anterior mitral motion: in vitro studies.

In patients with hypertrophic cardiomyopathy, the mitral valve moves anteriorly and assumes a unique shape, with mitral-septal contact centrally and preserved valve orifice area laterally. This shape is not clearly predicted by the Venturi mechanism, which stresses flow above the valve as opposed to changes intrinsic to the valve. On the other hand, it has been suggested that displacement of the papillary muscles anteriorly and toward one another, as observed in this disease, can promote anterior mitral valve motion and produce this unusual shape. The purpose of this in vitro study was to test the hypotheses that anterior motion of a membrane in a flow field can be generated by altering the distribution or effectiveness of chordal tension tethering the membrane, and that the shape achieved by this membrane depends on the geometry of chordal tension. Accordingly, a horizontal leaflet mounted in a flow chamber was attached by chords at its distal end to a series of upstream screws. Chordal tension could be varied by turning the screws or redirected by shifting the screws anteriorly. Anterior leaflet motion having the same unusual configuration seen in patients was reproduced by decreasing central chordal restraint while tension on the leaflet edges was maintained. Directing chordal tension anteriorly caused greater degrees of anterior motion at earlier stages in the release of chordal restraint; increased flow rate had a similar but less marked effect. These studies suggest that primary geometric alterations in the papillary-mitral apparatus can play an important role in determining the presence and geometry of systolic anterior mitral motion. The nature of these alterations suggests a role for anterior and inward papillary muscle displacement in promoting such motion. The geometric factors embodied in this model can explain many observed features of this motion not adequately explained by the Venturi effect, such as early systolic onset and the importance of a distal residual leaflet. Finally, flow visualization studies emphasize the importance in this process of drag forces caused by interposing the leaflet into the flow stream, and of geometric factors that enhance such forces.

Cardiomyopathy, Hypertrophic↗
PubMed1986

Cognitive processes in Cebus monkeys (Cebus apella) when solving problem-box tasks.

Although monkeys are capable of solving problem-box tasks, they were said to perform without 'understanding the value of unlocking' and to use 'kinesthetic control' only [Kohts, 1928]. However, the cognitive processes are much more complex--at least in Cebus monkeys: the problem-solving process was conducted according to the position of the catch in relation to the edge of the door, i.e. our monkey learnt to use visual control. The main characteristics of the monkey's behavior were similar to those exhibited by children. Problem-box tasks proved to be a suitable method for the comparative analysis of cognitive processes in primates.

Animals↗
PubMed1979

Below-knee orthosis: a wrap-around design for ankle-foot control.

Control of foot and ankle positions with orthotic appliances is generally more difficult to accomplish in children with spastic than flaccid paralytic disorders. Our new design for a custom molded plastic ankle-foot orthosis has proven superior to other designs in most applications. The appliance is vacuum molded from very thin polypropylene plastic and innerfaced with a foam material (Aliplast). The limb is enclosed completely in plastic with an anterior opening secured by several Velcro straps. Although the material is soft and pliable, the circumferential support provides remarkably rigid control of the limb. The orthosis has been successfully applied to children with polio, meningomyelocele and all forms of spastic and athetoid cerebral palsy. In addition to superior limb control, another advantage appears to be reflex inhibition of abnormal motor tone in may spastic patients. Patients have preferred this orthosis to those previously worn probably due to greater comfort. The limitation of adjustability of dorsiflexion can be compensated by heel or sole shoe lifts. Excessive heat created by the "wrap-around" design can be reduced by a thin cotton sock extended above the orthosis.

Adult↗
PubMed1976

Effects of serial lesions of somatosensory cortex and further neodecortication on retention of a rough-smooth discrimination in rats.

Five groups of rats with bilateral lesions of the somatosensory cortex and one of animals sustained only sham operations were tested for retention of a rough-smooth discrimination. Two of the lesion groups had sequential unilateral ablations, in one case with interoperative testing, and three groups had one-stage bilateral lesions. The two groups of animals with serial lesions did not differ from each other or from sham operates in relearning the task. RAts with one-stage lesions and preoperative overtraining also performed well, but the other one-stage groups showed deficits relative to control and serial lesion groups. In the second experiment the sham operated rats from Experiment 1 experiences lesions anterior and posterior to the somatosensory zones. These lesions did not affect retention. Somatosensory cortex then was ablated in one operation and severe performance decrements were seen. Removal of additional neocortex in a sample of animals that had relearned the discrimination after one-stage somatosensory cortex lesions (Exp. 1) also affected retention. In contrast, retention was not impaired on some of the measures in those animals that originally had two-stage ablations. The findings from these two experiments show that some ablation effects can be circumvented with overtraining or serial lesion techniques. The data also indicate that non-somatosensory cortex may play a role in recovery after somatic cortex lesions, but that the substrates underlying recovery might not be the same after one-stage and two-stage ablations.

Animals↗
PubMed1976

["Counting" experiments with ravens using "matching-from-sample" methods. A contribution on problem-solving behavior in higher aniMALS].

The research goal is divided into two parts: 1. Detailed analysis of behavior in selection in a 6-choice matching-from-sample task. 2. Discussion of all observed preferences on the basis of the models of Sutherland and Mackintosh and of Miller, Galanter and Pribram. The experiments were run with 4 one-year-old ravens (Corvus corax); two of them were probably 2 males. The design is similar to Koehler's. In the task the correct responses were equally distributed between all stimuli and positions (random system). Although the task ("counting") was learned, a lot of relevant preferences for stimuli, positions, patterns etc. show that problem-solving in higher animals can be very complex. It could be shown that only a detailed analysis may clarify animal learning processes.

Animals↗
PubMed1975

Effects of serial lesions of somatosensory cortex and further neodecortication on tactile retention in rats.

Four groups of rats with bilateral lesions of somatosensory cortex and one of animals sustaining only sham operations were tested for retention of a difficult tactile discrimination. Two of the lesion groups had serial ablations, in one case with interoperative testing, and two had one-stage lesions. Bilateral ablations of somatosensory cortex severely retarded retention in all lesion groups relative to the control group and serial and one-stage groups did not differ from each other. The sham operated rats then experienced lesions of cortex anterior and posterior to the somatosensory areas. These lesions only marginally affected retention. Somatosensory cortex then was ablated and severe performance decrements were seen. Removal of additional neocortex in animals that previously had relearned the discrimination after somatosensory cortex lesions also resulted in very poor retention. These data demonstrate the importance of the somatosensory cortex in mediating tactile discriminations and suggest that non-somatosensory cortex may play a role in recovery after somatosensory cortical lesions.

Animals↗