Short Stimulus Pulses Research Articles

Humidity-dependent synaptic characteristics in gelatin-based organic transistors

Organic synaptic transistors based on gelatin gate dielectric exhibit humidity-dependent synaptic characteristics. The device showed excellent field-effect transistor behavior without hysteresis when measured in a vacuum and dry air, but clear pinched hysteresis loops appeared in a humid environment. A large difference in the hysteresis window was observed under a small scanning range of gate voltage in different humid conditions, while the difference vanished under a large scanning range of gate voltage. The excitatory postsynaptic current strongly relies on stimulus pulse width, which is more pronounced in high humidity conditions, suggesting that slow proton transportation through a sequence of hopping may be responsible for the less pronounced dependence of stimulus amplitude with a short stimulus pulse width. The simulation of an artificial neural network for handwritten digit recognition revealed that the ultimate recognition accuracy at high relative humidity is higher than that at low relative humidity due to the large nonlinearity factor resulting from inadequate and low transportation of protons in the gelatin film under ambient conditions with low relative humidity. This work provides valuable insights into the effects of humidity on the performance of gelatin-based organic synaptic transistors, which could be useful for the development of environment-dependent synaptic devices.

Time-domain analysis of distortion-product otoacoustic emissions using a hydrodynamic cochlea model

Abstract As a by-product of nonlinear amplification in the cochlea, the inner ear emits sound waves in response to two tones with different frequencies. These sound waves are measurable in the ear canal as distortion-product otoacoustic emissions (DPOAEs). DPOAEs putatively consist of two components emerging at different locations in the cochlea. Wave interference between the two components limits the accuracy of DPOAEs as a noninvasive measure of cochlear function. Using short stimulus pulses instead of continuous stimuli, the two DPOAE components can be separated in the time domain due to their different latencies. The present work utilizes a nonlinear hydrodynamic cochlea model to simulate short-pulse DPOAEs in the time domain. When adding irregularities to the mechanical parameters of the model, the simulated DPOAE signals show two distinguishable components and long-lasting beat tones, similar to band-pass filtered experimental data from normal-hearing human subjects. The model results suggest that the beat tones can occur solely due to interference of the coherent-reflection component with the fading nonlinear-distortion component.

Input-output functions of the nonlinear-distortion component of distortion-product otoacoustic emissions in normal and hearing-impaired human ears.

Distortion-product otoacoustic emissions (DPOAEs) arise in the cochlea in response to two tones with frequencies f1 and f2 and mainly consist of two components, a nonlinear-distortion and a coherent-reflection component. Wave interference between these components limits the accuracy of DPOAEs when evaluating the function of the cochlea with conventional continuous stimulus tones. Here, DPOAE components are separated in the time domain from DPOAE signals elicited with short stimulus pulses. The extracted nonlinear-distortion components are used to derive estimated distortion-product thresholds (EDPTs) from semi-logarithmic input-output (I/O) functions for 20 normal-hearing and 21 hearing-impaired subjects. I/O functions were measured with frequency-specific stimulus levels at eight frequencies f2 = 1,…, 8 kHz (f2/f1 = 1.2). For comparison, DPOAEs were also elicited with continuous primary tones. Both acquisition paradigms yielded EDPTs, which significantly correlated with behavioral thresholds (p < 0.001) and enabled derivation of estimated hearing thresholds (EHTs) from EDPTs using a linear regression relationship. DPOAE-component separation in the time domain significantly reduced the standard deviation of EHTs compared to that derived from continuous DPOAEs (p < 0.01). In conclusion, using frequency-specific stimulus levels and DPOAE-component separation increases the reliability of DPOAE I/O functions for assessing cochlear function and estimating behavioral thresholds.

REMOVED: Extraction of distortion-product otoacoustic emission source components and its relevance for objective audiometry

Distortion-product otoacoustic emissions (DPOAEs) arise as byproduct of nonlinear amplification in the cochlea in response to two stimulus tones of different frequencies. They are assumed to comprise mainly two components, a nonlinear-distortion component and a coherent-reflection component, which may cause wave interference in recordings with conventional, continuous stimulus tones, hampering the accuracy of DPOAEs when evaluating cochlear function. Here, DPOAEs are recorded with short stimulus pulses enabling the separation of the two source components in the time domain. Comparison with conventional stimulation endorses the importance of reliable source separation when using DPOAEs in clinical audiometry or research.

A Dynamical Threshold for Cardiac Delayed Afterdepolarization-Mediated Triggered Activity

Ventricular myocytes are excitable cells whose voltage threshold for action potential (AP) excitation is ∼−60 mV at which INa is activated to give rise to a fast upstroke. Therefore, for a short stimulus pulse to elicit an AP, a stronger stimulus is needed if the resting potential lies further away from the INa threshold, such as in hypokalemia. However, for an AP elicited by a long duration stimulus or a diastolic spontaneous calcium release, we observed that the stimulus needed was lower in hypokalemia than in normokalemia in both computer simulations and experiments of rabbit ventricular myocytes. This observation provides insight into why hypokalemia promotes calcium-mediated triggered activity, despite the resting potential lying further away from the INa threshold. To understand the underlying mechanisms, we performed bifurcation analyses and demonstrated that there is a dynamical threshold, resulting from a saddle-node bifurcation mainly determined by IK1 and INCX. This threshold is close to the voltage at which IK1 is maximum, and lower than the INa threshold. After exceeding this dynamical threshold, the membrane voltage will automatically depolarize above the INa threshold due to the large negative slope of the IK1-V curve. This dynamical threshold becomes much lower in hypokalemia, especially with respect to calcium, as predicted by our theory. Because of the saddle-node bifurcation, the system can automatically depolarize even in the absence of INa to voltages higher than the ICa,L threshold, allowing for triggered APs in single myocytes with complete INa block. However, because INa is important for AP propagation in tissue, blocking INa can still suppress premature ventricular excitations in cardiac tissue caused by calcium-mediated triggered activity. This suppression is more effective in normokalemia than in hypokalemia due to the difference in dynamical thresholds.

Experimental active spike responses of analog electrical neuron: beyond “integrate-and-fire” transmission

Using an analog electrical FitzHugh–Nagumo neuron including complex threshold excitation (CTE) properties, we analyze its spiking responses on pulse stimulation. The system is subjected to external pulse stimulus and can generate nonlinear integrate-and-fire and resonant responses which are typical for excitable neuronal cells. Following earlier theoretical predictions, we found that for certain parameters range, there is a possibility to trigger a spiking sequence with a finite number of spikes in response to a single short stimulus pulse. Hence, active transformation of N incoming pulses to M outgoing spikes is possible. We also show that the nonlinear electrical circuit with the CTE feature can be effectively synchronized by a periodic pulse train with 1 to M spike frequency ratio.

Active spike transmission in the neuron model with a winding threshold manifold

We analyze spiking responses of excitable neuron model with a winding threshold manifold on a pulse stimulation. The model is stimulated with external pulse stimuli and can generate nonlinear integrate-and-fire and resonant responses typical for excitable neuronal cells (all-or-none). In addition we show that for certain parameter range there is a possibility to trigger a spiking sequence with a finite number of spikes (a spiking message) in the response on a short stimulus pulse. So active transformation of N incoming pulses to M (with M>N) outgoing spikes is possible. At the level of single neuron computations such property can provide an active “spike source” compensating “spike dissipation” due to the integrate-and-fire N to 1 response. We delineate the dynamical mechanism for the N to M transformation based on the winding threshold manifold in the neighborhood of big saddle loop bifurcation. Based on the theoretical predictions, a nonlinear electronic circuit is designed implementing the active transmission in physical conditions.

Reconstruction of hippocampal granule cell electrophysiology by computer simulation

A model of the hippocampal granule cells was created that closely approximated most of the measured intracellular responses from a neuron under a variety of stimulus conditions. This model suggests that: (1) A simple, four-conductance model can account for most of the intracellular behavior of these neurons. (2) The repolarization mechanism in granule cells may be different from that in squid axons. A weak potassium conductance may be present in hippocampal granule neurons, which simultaneously give rise to a small, passive depolarizing afterpotential. (3) The strength duration properties may assist in identifying the electrotonic and sodium channel properties with short stimulus pulse widths. (4) Repetitive firing responses are highly dependent on the cell's recent history of activation and the regulation of the slow potassium conductance and calcium dynamics. (5) The anodic break response is probably not a property of typical granule cells. Through thorough and precise comparison of experimental and model responses, computer simulations can help assembling channel information into verifiable models that accurately reproduce intracellular data.

A wide concentration range olfactometer for delivery of short reproducible odor pulses

A multistage dynamic flow dilution olfactometer allowing the delivery of precisely controlled stimuli over a wide concentration range is presented. Discrete concentration values, in the range 10 −6 to 5.6 × 10 −2 of the saturated vapor of an odorous compound, are obtained from two original concentration by combining 4 dilution stages. Short stimulus pulses, of a known and reproducible concentration and with short rising and falling times, are sampled from steady concentration levels. A programmable controller manages the olfactometer dilution stage selection, the odor stimulus switch and starts the peripheral devices required by the experiment. The programming of olfactometer function makes it an extremely flexible system for various experimental projects in olfaction.