119,834 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
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
A. Borst. Les Cathares ; W. L. Wakefield. Heresy, Crusade and Inquisition in Southern France
No full textJolivet Jean. A. Borst. Les Cathares ; W. L. Wakefield. Heresy, Crusade and Inquisition in Southern France. In: Revue de l'histoire des religions, tome 190, n°2, 1976. pp. 204-205
A. Borst. Les Cathares ; W. L. Wakefield. Heresy, Crusade and Inquisition in Southern France
No full textJolivet Jean. A. Borst. Les Cathares ; W. L. Wakefield. Heresy, Crusade and Inquisition in Southern France. In: Revue de l'histoire des religions, tome 190, n°2, 1976. pp. 204-205
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
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
Transient and steady-state response properties of movement detectors
No full textEgelhaaf M, Borst A. Transient and steady-state response properties of movement detectors. Journal of the Optical Society of America, A: Optics, Image Science, and Vision. 1989;6(1):116-127.The transient and steady-state responses of movement detectors are studied at various pattern contrasts (i) by intracellularly recording from an identified movement-sensitive interneuron in the fly's brain and (ii) by comparing these results with computer simulations of an array of movement detectors of the correlation type. At the onset of stimulus motion, the membrane potential oscillates with a frequency corresponding to the temporal frequency of the stimulus pattern before it settles at its steady-state level. Both the transient and the steady-state response amplitudes show a characteristic contrast dependence. As is shown by computer modeling, the transient behavior that we found in the experiments reflects an intrinsic property of the general scheme of movement detectors of the correlation type. To account for the contrast dependence, however, this general scheme has to be elaborated by (i) a subtraction stage, which eliminates the background light intensity from the detector input signal, and (ii) saturation characteristics in both branches of each movement-detector subunit
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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