104,449 research outputs found
Motion computation and visual orientation in flies
No full textEgelhaaf M, Borst A. Motion computation and visual orientation in flies. Comparative Biochemistry and Physiology, A: Comparative Physiology. 1993;104(4):659-673.Visual orientation greatly relies on the evaluation of the motion patterns received by the eyes when the animal moves around. In a combination of behavioral, neurophysiological and pharmacological analysis and modelling, the mechanisms are established by which the visual system of the fly extracts three types of-basic retinal motion patterns. Coherent retinal large-field motion as is induced during deviations of the animal from its course, image expansion occurring when the animal approaches an obstacle, and relative motion which is induced when a nearby object is passed in front of its background. Separate neuronal networks are specifically tuned to each of these motion patterns and make use of them in three different orientation tasks: in compensatory course stabilization, the control of landing behaviour and the fixation of objects
Mechanisms of dendritic integration underlying gain control in fly motion-sensitive interneurons
No full textBorst A, Egelhaaf M, Haag J. Mechanisms of dendritic integration underlying gain control in fly motion-sensitive interneurons. Journal of Computational Neuroscience. 1995;2(1):5-18
Synapse distribution on VCH, an inhibitory, motion-sensitive interneuron in the fly visual system
No full textGauck V, Egelhaaf M, Borst A. Synapse distribution on VCH, an inhibitory, motion-sensitive interneuron in the fly visual system. JOURNAL OF COMPARATIVE NEUROLOGY. 1997;381(4):489-499
A preparation of the blowfly (*Calliphora erythrocephala*) brain for in vitro electrophysiological and pharmacological studies
No full textBrotz TM, Egelhaaf M, Borst A. A preparation of the blowfly (*Calliphora erythrocephala*) brain for in vitro electrophysiological and pharmacological studies. Journal of Neuroscience Methods. 1995;57(1):37-46
De Soendaneesche Taalcursus.
No full textAlternate pages blank.On cover: Met medewerking van L. Borst en Ch. G. Verwer.Mode of access: Internet
Two-Dimensional Motion Perception in Flies
No full textBorst A, Egelhaaf M, Seung HS. Two-Dimensional Motion Perception in Flies. Neural Computation. 1993;5(6):856-868.We study two-dimensional motion perception in flies using a semicircular visual stimulus. Measurements of both the H1-neuron and the optomotor response are consistent with a simple model supposing spatial integration of the outputs of correlation-type motion detectors. In both experiment and model, there is substantial H1 and horizontal (yaw) optomotor response to purely vertical motion of the stimulus. We conclude that the fly's optomotor response to a two-dimensional pattern, depending on its structure, may deviate considerably from the direction of pattern motion
Some observations on the regularizing field for gradient damage models
No full textGradient enhanced material models can potentially preserve well-posedness of incremental boundary value problems also after the onset of strain softening. Gradient dependent constitutive relations are rooted in the assumption that some scalar or tensor field, which appears in the yield function, has to be enriched by adding a term involving its second-order gradient field. For gradient-dependent plasticity this term is universally accepted to be the equivalent plastic strain. For gradient-dependent damage models different choices have been presented in the literature. They all possess the desired regularization of the solution, but they are not identical as regards the structural response. In this paper the implications of the choice of the regularization field are discussed. As an example a plasticity-like damage material model is formulated. Finally, a well-known one-dimensional problem is solved analytically, and the results are compared with results obtained via other gradient approaches
Discontinuous modelling of strain localisation and failure
No full textThe computational simulation of failure in solids poses many challenges. A proper understanding of how structures respond under loading, both before and past the peak load, is important for safe and economical constructions. This requires numerical models for failure which are both faithful to the physical reality and mathematically well founded. A serious computational issue is that of objectivity with respect to the spatial discretisation of a problem. This requires that upon refinement of the spatial discretisation of a problem, a unique, physically meaningful result is approached. One approach to ensure objectivity with respect to spatial discretisation when simulating failure in solids is to allow displacement discontinuities in the solution. In this work, different techniques, of varying complexity, are developed to simulate displacement discontinuities which are independent of the spatial discretisation using finite elements. The different techniques are then critically evaluated. The first model examined involves adding only the effect of a displacement discontinuity to a finite element as an incompatible strain mode. This allows a traction–separation relationship to be applied at an interface and can be implemented simply in a standard finite element code. It is however shown that this type of model can be cast in an equivalent continuum format, a form which is known to be sensitive to the spatial discretisation. The second approach developed involves the addition of the Heaviside function to the underlying finite element interpolation basis. This method is based on the partition of unity concept, and allows the Heaviside function to be added locally to a finite element mesh to simulate a propagating displacement discontinuity. The approach is formulated for geometrically linear, geometrically nonlinear, quasi-static and dynamic problems. It is shown to be completely independent of the spatial discretisation. The partition of unity-based model is used also to simulate failure using a regularised strain softening model. When a critical level of inelastic deformation is reached, a displacement discontinuity is inserted. This model is better suited to modelling the entire failure process than a continuum or discontinuous model alone. Through numerical examples, it is shown that the inclusion of a displacement discontinuity during the failure process can lead to a different failure mode than for a continuum-only model
Loss-of-function shRNA screens to identify mechanisms of PARP inhibitor resistance in BRCA1-mutated mouse mammary tumors
No full textRiele, H.P.J. te [Promotor]Rottenberg, S. [Promotor]Borst, P. [Copromotor
Calcium accumulation in visual interneurons of the fly: stimulus dependence and relationship to membrane potential
No full textEgelhaaf M, Borst A. Calcium accumulation in visual interneurons of the fly: stimulus dependence and relationship to membrane potential. Journal of neurophysiology. 1995;73(6):2540-2552.1. The large motion-sensitive tangential neurons in the fly third visual neuropil spatially pool the postsynaptic signals of many local elements. The changes in membrane potential and calcium concentration induced in these cells by visual motion are analyzed in vivo by simultaneous optical and intracellular voltage recording techniques. 2. Visual motion in the preferred direction leads to depolarization of the cell and to calcium accumulation mainly in the axon terminal, the soma, and the dendritic tree. During motion in the null direction, the cell hyperpolarizes and virtually no changes in calcium concentration can be observed. 3. Dendritic calcium accumulation is first restricted to those dendritic branches that are close to the sites of direct synaptic input. In other parts of the dendrite the calcium concentration increases more slowly and usually reaches only lower levels. 4. Calcium starts accumulating at the onset of motion. However, the calcium concentration reaches its final steady-state level much later than the corresponding membrane potential changes. Even if these are completely transient at high temporal frequencies of pattern motion, the calcium signal stays high until the stimulus pattern stops moving. 5. The amplitute of the calcium signal depends on the temporal frequency of pattern motion in a similar way as do the corresponding membrane potential changes. However, there exist differences that can be attributed to the different time courses of both signals. 6. Depolarization of the dendritic tree by current injection through a microelectrode leads to similar changes in calcium accumulation as does activation by synaptic input, suggesting that calcium enters the cell via voltage-dependent channels. The possible function of calcium channels for dendritic integration of synaptic input is discussed
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