Microsecond differences in the arrival time of a sound at the two ears (interaural time differences, ITDs) are the main cue for localizing low-frequency sounds in space. Traditionally, ITDs are thought to be encoded by an array of coincidence-detector neurons, receiving excitatory inputs from the two ears via axons of variable length ('delay lines'), to create a topographic map of azimuthal auditory space. Compelling evidence for the existence of such a map in the mammalian lTD detector, the medial superior olive (MSO), however, is lacking. Equally puzzling is the role of a--temporally very precise glycine--mediated inhibitory input to MSO neurons. Using in vivo recordings from the MSO of the Mongolian gerbil, we found the responses of ITD-sensitive neurons to be inconsistent with the idea of a topographic map of auditory space. Moreover, local application of glycine and its antagonist strychnine by iontophoresis (through glass pipette electrodes, by means of an electric current) revealed that precisely timed glycine-controlled inhibition is a critical part of the mechanism by which the physiologically relevant range of ITDs is encoded in the MSO. A computer model, simulating the response of a coincidence-detector neuron with bilateral excitatory inputs and a temporally precise contralateral inhibitory input, supports this conclusion.

The dominant cue for localization of low-frequency sounds are microsecond differences in the time-of-arrival of sounds at the two ears [interaural time difference (ITD)]. In mammals, ITD sensitivity is established in the medial superior olive (MSO) by coincidence detection of excitatory inputs from both ears. Hence the relative delay of the binaural inputs is crucial for adjusting ITD sensitivity in MSO cells. How these delays are constructed is, however, still unknown. Specifically, the question of whether inhibitory inputs are involved in timing the net excitation in MSO cells, and if so how, is controversial. These inhibitory inputs derive from the nuclei of the trapezoid body, which have physiological and structural specializations for high-fidelity temporal transmission, raising the possibility that well timed inhibition is involved in tuning ITD sensitivity. Here, we present physiological and pharmacological data from in vivo extracellular MSO recordings in anesthetized gerbils. Reversible blockade of synaptic inhibition by iontophoretic application of the glycine antagonist strychnine increased firing rates and significantly shifted ITD sensitivity of MSO neurons. This indicates that glycinergic inhibition plays a major role in tuning the delays of binaural excitation. We also tonically applied glycine, which lowered firing rates but also shifted ITD sensitivity in a way analogous to strychnine. Hence tonic glycine application experimentally decoupled the effect of inhibition from the timing of its inputs. We conclude that, for proper ITD processing, not only is inhibition necessary, but it must also be precisely timed.

There is a demand for good theoretical understanding of the response of an atomic force microscope cantilever to the extremely nonlinear impacts received while tapping a sample. A model and numerical simulations are presented in this paper which provide a very pleasing comparison with experimental results. The dependence of the cantilever amplitude and phase upon the sample stiffness, adhesion and damping are investigated using these simulations, and it is found that 'topographic' tapping images are not independent of sample properties, nor will it be trivial to measure materials' properties from the tapping data. The simulation can be applied to other probe microscope configurations as well.

The ascending auditory pathway is characterized by parallel processing. At the brain stem level, several structures are involved that are known to serve different well-defined functions. However, the function of one prominent brain stem nucleus, the rodent superior paraolivary nucleus (SPN) and its putative homologue in other mammals, the dorsomedial periolivary nucleus, is unknown. Based on extracellular recordings from anesthetized gerbils, we tested the role of the SPN in sound localization and temporal processing. First, the existence of binaural inputs indicates that the SPN might be involved in sound localization. Although almost half of the neurons exhibited binaural interactions (most of them excited from both sides), effects of interaural time and intensity differences (ITD; IID) were weak and ambiguous. Thus a straightforward function of SPN in sound localization appears to be implausible. Second, inputs from octopus and multipolar/stellate cells of the cochlear nucleus and from principal cells of the medial nucleus of the trapezoid body could relate to precise temporal processing in the SPN. Based on discharge types, two subpopulations of SPN cells were observed: about 60% of the neurons responded to pure tones with sustained discharges, with irregular spike patterns and no phase-locking. Only four neurons showed a regular spike pattern ("chopping"). About 40% of the neurons responded with phasic ON or OFF discharges. Average first spike latency observed in neurons with sustained discharges was significantly shorter than that of ON responders, but had a considerably higher trial-to-trial variation ("jitter"). A subpopulation of ON responders showed a jitter of less than +/-0.1 ms. Most neurons (66%) responded to sinusoidally amplitude-modulated sounds (SAM) with an ongoing response, phase-locked to the stimulus envelope. Again, ON responders showed a significantly higher temporal precision in the phase-locked discharge compared with the sustained responders. High variability was observed among spike-rate-based modulation transfer functions. Histologically, a massive concentration of cytochemical markers for glycinergic input to SPN cells was demonstrated. Application of glycine or its blockade revealed profound effects of glycinergic inhibition on the auditory responses of SPN neurons. The existence of at least two subpopulations of neurons is in line with different subsets of SPN cells that can be distinguished morphologically. One temporally less precise population might modulate the processing of its target structures by providing a rather diffuse inhibition. In contrast, precise ON responders might provide a short, initial inhibitory pulse to its targets.

Enhanced adhesion through local epitaxy of transition-metal nitride coatings on ferritic steel promoted by metal ion etching in a combined cathodic arc/unbalanced magnetron deposition system Phase images acquired while intermittently contacting a sample surface with the tip of an atomic force microscope cantilever are not easy to relate to material properties. We have simulated dynamic force curves and compared simulated with experimental results. For some cantilever-sample combinations, the interaction remains a surface effect, whereas for others, the tip penetrates the sample significantly. Height artifacts in the ''topography'' images, and the role of the sample stiffness, work of adhesion, damping, and topography in the cantilever response manifest themselves to different extents depending on the indentation depth.

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We study several possible treatments of the solid-fluid boundary in lattice Boltzmann simulations of solid-particle suspensions. Our aim is to avoid the complications of the boundary rule pioneered by Ladd [J. Fluid Mech. 271, 285 (1994); 271, 311 {1994)],introduced by treating the solid-Auid interactions on the links between lattice nodes rather than on the lattice nodes themselves. We show that simply treating the interactions in a similar manner on the lattice nodes is not a valid alternative due to the presence of nonrelaxing Quid distributions that do not allow steady flows to be reached. After showing the failure of the so-called "forcing method, " in which the lattice velocity distributions inside the solid particle are forced to represent the local solid body velocity, we introduce a boundary treatment at the lattice nodes. In combination with two further simplifications in the general algorithm, this method produces results comparable to those obtained with Ladd's boundary rule, especially in the computations of bulk transport coefficients of so1id-particle suspensions. When used together with a Quctuating lattice Boltzmann method, it allows for the Quctuation-dissipation theorem to be obeyed exactly at all solidparticle volume fractions. PACS number(s): 02.70. -c, 47.15.Pn, 82.70.Dd, 82.70.Kj I. INTRGDUCTIQNSuspensions of submicrometer sized particles in a liquid are found in industrial processes and products, such as paints, pharmaceuticals, ceramics, and foods, and in nature as biological fluids. There is thus considerable interest in understanding the dynamic properties of such suspensions. The theoretical description of these systems at particle volume fractions P exceeding the dilute limit (say P & 0.05) is considerably complicated by the presence of indirect, or hydrodynamic, interactions between the particles. These interactions result from the velocity fields set up in the suspending liquid by the relative motion of the solid particles [I]. The treatment of these forces is complicated by the fact that they are of manybody and long-ranged nature. Numerical simulations are thus becoming an important method of studying the dynamical properties of suspensions. Most simulation algorithms such as Brownian dynamics [2], Stokesian dynamics [3], or the multipole method [4] are based on the clear time-scale separation that exists between the dynamics of the fiuid and the dynamics of the solid particles. This separation implies that the development of the hydrodynamic interactions is instantaneous and that they, therefore, depend on the positions and velocities of all the particles. For this reason, these algorithms scale as the square or cube of the number of particles. Very recently however, an alternative technique for such simulations has been proposed [5,6]. It is based upon the combination of Newtonian dynamics of the solid particles with a discretized Boltzmann equation (lattice Boltzmann equation, LBE) for the fluid phase. The state of the Quid is updated on a regular lattice while the solid partic...

The tip of an atomic force microscope in intermittent contact with a sample surface was numerically simulated. The model for the tip-sample system was that of the simple harmonic oscillator for the cantilever, Maugis continuum mechanics when the tip was in contact with the sample, and either the van der Waals or capillary force if the tip was out of contact with the sample. Of particular interest were (i) the instantaneous pressures beneath the tip, (ii) the contributions from the tip-sample interaction to the damping of the cantilever, and (iii) the role of capillary forces in determining the cantilever response. We found an estimate for the pressure underneath the tip, that there are multiple sources for cantilever damping, and that the behavior due to the capillary force is complex.The utility of intermittent-contact mode atomic force microscopy (IC AFM) [1] lies in its ability to lower, and possibly eliminate, lateral forces that are applied to the sample by the tip during contact mode. This enables imaging of particulate matter that is not well-bound to the sample surface, and prevents inadvertant modification of some polymers.The tip tapping the sample surface is a very nonlinear event: for most of its oscillation cycle, the cantilever is free from external forces, then as it nears the sample, long-ranged attractive or repulsive forces may modify its behavior, and finally a brief strong repulsive shock is exerted upon it. Under these conditions, it is not possible to find a reasonable analytical solution to the system's equation of motion, and therefore numerical simulations of the tip-sample dynamics are necessary.Our first results concerning numerical IC AFM simulations were recently published [2]. The major conclusions from that paper are that the model -one differential equation with five parameters relating to the tip-sample interaction (modulus, curvature, work of adhesion, interatomic distance, and damping) -replicates the experimental data in most circumstances. The unfortunate consequence is that four of the five parameters must be determined by other means in order to interpret phase-contrast images and then one unknown parameter might be extracted from the data. More pleasing is the result that the cantilever motion is relatively insensitive to the specific tip-sample interaction and that this allows one to approximate constant high set-point amplitude images as the sample topography.Previously, we used van der Waals interactions as the long-range forces; here we also discuss the capillary force. It is our intent in this publication to build upon our previous work and address the following questions. (i) What are the instantaneous pressures beneath the tip? (ii) What are the contributions to the damping of the cantilever from the tipsample interaction? (iii) How do liquid layers on the surfaces influence the cantilever response? The modelThe essential second-order differential equation used as a starting point for the model is a driven, damped harmonic oscillator, with additional terms to describ...