Transient Response Components Research Articles

A harmonic input method for computing time-varying response PSD of linear time-invariant structures subjected to nonstationary stochastic excitations

A number of stochastic response analysis methods have been successfully developed for computing the time-varying response power spectral density (PSD) of structures subjected to nonstationary stochastic excitations. However, how to compute the time-varying response PSD when transient response components have significant impacts on the stochastic responses remains largely unsolved. To this end, this paper proposes a harmonic input method (HIM) for accurately and efficiently computing time-varying response PSD of linear time-invariant structures subjected to nonstationary stochastic excitations, which can consider the transient response impact. The accuracy of the proposed HIM method is verified via comparison against the Monte Carlo simulations of an SDOF system and an MDOF system, which outperforms the classical methods if the transient response components are of significance. This study offers a high-efficient and high-accurate approach for response PSD estimation, particularly for a low-damping structural system subjected to short-duration nonstationary stochastic excitations.

A stochastic analysis method of transient responses using harmonic wavelets, Part 1: Time-invariant structural systems

The transient response components have significant impacts on the stochastic response analysis of structural systems subjected to nonstationary stochastic excitations. However, available stochastic analysis methods focus on the steady-state response components. This paper proposes a novel stochastic analysis method of the transient responses for multi-degree-of-freedom (MDOF) structural systems subjected to nonstationary stochastic excitations based on periodic generalized harmonic wavelets (GHW). Firstly, the importance of transient response components on structural systems is discussed in detail. The equations of motion of the MDOF structural systems are established by using the periodic GHW with considering the rigid displacement components. The linear algebraic equations with respect to the wavelet coefficients of responses and excitations are established, respectively. Then the connection equations of wavelet coefficients in all scales are established to include the transient components in the responses. A stochastic analysis method of transient responses for structural systems is proposed based on the periodic GHW, and the time-varying power spectral density functions of the structural responses are obtained in a semi-analytical form. The proposed method is validated by two numerical examples, which illustrate the high efficiency and high accuracy of the novel method.

Temporally advanced dynamic change of receptive field of lateral geniculate neurons during brief visual stimulation: Effects of brainstem peribrachial stimulation

Processing of visual information in the brain seems to proceed from initial fast but coarse to subsequent detailed processing. Such coarse-to-fine changes appear also in the response of single neurons in the visual pathway. In the dorsal lateral geniculate nucleus (dLGN), there is a dynamic change in the receptive field (RF) properties of neurons during visual stimulation. During a stimulus flash centered on the RF, the width of the RF-center, presumably related to spatial resolution, changes rapidly from large to small in an initial transient response component. In a subsequent sustained component, the RF-center width is rather stable apart from an initial slight widening. Several brainstem nuclei modulate the geniculocortical transmission in a state-dependent manner. Thus, modulatory input from cholinergic neurons in the peribrachial brainstem region (PBR) enhances the geniculocortical transmission during arousal. We studied whether such input also influences the dynamic RF-changes during visual stimulation. We compared dynamic changes of RF-center width of dLGN neurons during brief stimulus presentation in a control condition, with changes during combined presentation of the visual stimulus and electrical PBR-stimulation. The major finding was that PBR-stimulation gave an advancement of the dynamic change of the RF-center width such that the different response components occurred earlier. Consistent with previous studies, we also found that PBR-stimulation increased the gain of firing rate during the sustained response component. However, this increase of gain was particularly strong in the transition from the transient to the sustained component at the time when the center width was minimal. The results suggest that increased modulatory PBR-input not only increase the gain of the geniculocortical transmission, but also contributes to faster dynamics of transmission. We discuss implications for possible effects on visual spatial resolution.

Contextual modulation of orientation discrimination is independent of stimulus processing time

Contextual inhibition of neural activity in the primary visual cortex begins immediately and is most pronounced in the early transient response component. Using backward masking to control available processing time, we investigated whether the interaction between perceptual contextual modulation and processing time reflects the neural dynamics of contextual inhibition. We found that the threshold elevation due to contextual inhibition in an orientation-discrimination task is essentially independent of the available processing time and that it is closely related to contextual inhibition of the early transient response component of orientation-selective neurons in the primary visual cortex.

PHOTORESPONSE MECHANISMS OF THIN SUPERCONDUCTING FILMS AND SUPERCONDUCTING DETECTORS

The photoresponse of ordinary and high-T c superconductors depends critically on the hierarchy of relaxation times, such as the electron–phonon and phonon–electron scattering times, the time of phonon escape from a superconducting film and also the phonon return time. For thin films of cuprates, close to the superconducting transition the following components of transient response are identified. The picosecond photoresponse is attributed to the dynamics of nonequilibrium quasiparticles and Cooper pairs. The nanosecond response is described by the thermal boundary resistance (the Kapitza resistance) between a superconducting film and a substrate. The microsecond response is associated with the phonon diffusion in the substrate. Using experimental results, we deduce the characteristic time of electron–phonon relaxation and parameters of the film-substrate interface. The kinetic inductance photoresponse of superconductors with s- and d-wave pairing far below the superconducting transition is also calculated. We study parameters (responsivity, operating speed and noise equivalent power) of a nonequilibrium detector, in which only electron states are changed under the radiation, while the film phonons stay in thermodynamic equilibrium with the substrate. Our analysis demonstrates that the nonequilibrium superconducting detectors have essential advantages compared to superconducting bolometers and other detectors.

Response Properties of Lateral Line and Auditory Units in the Medulla Oblongata of the Rainbow Trout (Oncorhynchus Mykiss)

ABSTRACT The objective of this study was to investigate the response properties of second-order lateral line and auditory neurones in the medulla oblongata of the rainbow trout. The frequency response of 20 medullary units was measured by recording single-unit spike activity in response to a mechanical stimulus provided by an oscillating membrane or by a small vibrating sphere. These 20 units were categorized, according to their frequency response properties, into two classes. Ten units responded to relatively low frequencies (<50Hz) and showed a maximum in the frequency response between 70 and 120Hz. The other units responded to higher frequencies, showing a maximum in the frequency response above 150Hz. Significant differences between these two classes were also observed with respect to recording site, latency and sensitivity. It is concluded that these two classes of medullary units are lateral line units and auditory units, respectively. In the medulla, the input from the two mechanosensory systems appears to be mainly processed separately. The majority of mechanically sensitive units (95%) showed a sustained, phase-locked response; 25% displayed a transient response component, mostly in addition to a sustained response component. All units were spontaneously active, with a mean firing rate of 27spikes s−1. Two units responded to a visual stimulus. No topographical representation of lateral line receptive fields was found in the caudal part of the medulla. The response characteristics of primary afferents reported in the literature differ from those of the medullary units of this study, so we conclude that the latter are higher-order units. Medullary lateral line units stimulated by the vibrating sphere appeared to be less sensitive than units stimulated by the vibrating membrane. The sensitivity of the units and the size of their receptive fields indicate that lateral line input converges in the medial nucleus.

Neurotransmitter receptors mediating excitatory input to cells in the cat lateral geniculate nucleus. I. Lagged cells

1. Synaptic mechanisms that might explain the functional properties of the recently discovered class of lagged cells in the dorsal lateral geniculate nucleus (LGN) were analyzed with electrophysiological and pharmacologic techniques. To study the type of excitatory amino acid (EAA) receptor that mediates visual responses of lagged cells, we recorded the response of single cells to a stationary flashing light spot before, during, and after microiontophoretic application of antagonists and agonists to EAA receptors. 2. The visual response of the lagged cells could be almost completely blocked by an antagonist to the N-methyl-D-aspartate (NMDA) receptor. The degree of suppression was dose dependent, and the average maximum degree of suppression for all the cells was 94%. NMDA enhanced the response, and this enhancement was antagonized by NMDA antagonists. A quisqualate/kainate receptor antagonist had no significant effect on the lagged cells. 3. These findings indicate that the visual response in lagged cells is dependent upon activation of NMDA receptors, which may directly result from activation of retinal inputs. 4. No pharmacologic difference was seen between lagged X- and Y-cells, or between lagged ON- and OFF-center cells. 5. gamma-Aminobutyric acid-A (GABA-A) receptor antagonists were used to study whether the characteristic lag of the visual response and the suppression of the initial transient response component of the lagged cells are dependent on geniculate inhibition. Beside enhancement of the visual response, the GABA antagonists strongly reduced the lag of the visual response, and an initial transient response component occurred instead of the initial suppression. The lag remained slightly longer than for nonlagged cells, and the peak firing rate of the transient was below values typical for nonlagged cells, indicating that the lagged cell properties are dependent on other factors beside GABA-A receptor-mediated inhibition. 6. The enhanced visual response during iontophoresis of GABA antagonists could be completely blocked by simultaneous iontophoresis of an NMDA-receptor antagonist. This gives further support to the hypothesis that the retinal input to these cells is mediated by NMDA receptors. 7. The NMDA-receptor/ionophore complex mediates excitatory postsynaptic potentials (EPSPs) characterized by slow rise and decay times and long duration. The ionophore is characterized by a voltage-dependent blockade that makes these receptors particularly sensitive to inhibitory input. The temporal interplay between the slow NMDA receptor-mediated EPSPs and the fast GABA receptor-mediated inhibitory postsynaptic potentials (IPSPs) may explain the characteristic response properties of the lagged cells.

Neurotransmitter receptors mediating excitatory input to cells in the cat lateral geniculate nucleus. II. Nonlagged cells

1. We studied the type of receptor for excitatory amino acids (EAA) that mediates visual responses of nonlagged cells in the dorsal lateral geniculate nucleus (LGN) by recording the visual response of single cells to a stationary flashing spot before, during, and after iontophoretical application of antagonists and agonists to EAA receptors. 2. The visual response of the nonlagged cells was strongly suppressed, in a dose-dependent manner, by the specific quisqualate/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). The average degree of suppression was 74%. Quisqualate enhanced the visual response. 3. Specific antagonists to the N-methyl-D-aspartate (NMDA) receptor had a weak suppressing effect on most nonlagged cells. The average degree of suppression was 22%. Measurement of such weak effects was complicated by the considerable spontaneous fluctuations of responsivity in the LGN cells. In the majority of cells where the visual response was suppressed by NMDA antagonists, the tonic response component was more strongly suppressed than the initial transient response component. The visual response was enhanced by NMDA, and this enhancement was antagonized by NMDA antagonists. 4. These findings suggest that the excitatory input from the retina to nonlagged LGN cells is mainly mediated by non-NMDA receptors. The non-NMDA receptors mediate fast EPSPs, and this can explain the fast onset and offset of the visual response of the nonlagged cells. 5. The generally small contribution from NMDA receptors to the visual response of the nonlagged cells might reflect a minor involvement of these receptors in the retinal input, or it could be related to the excitatory input to LGN from the visual cortex. 6. To study whether the expression of NMDA receptors was related to modulatory brain stem input to LGN, we examined the effects of the NMDA antagonists when the visual response was enhanced with gamma-aminobutyric acid (GABA) antagonists or acetylcholine (ACh). Neither of these pharmacologic manipulations consistently increased the relative contribution of NMDA receptors to the visual response. 7. No pharmacologic difference was found between nonlagged X- and Y-cells, or between ON- and OFF-center cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Response to rates of luminance change of sustained and transient cells in the cat lateral geniculate nucleus and optic tract.

We recorded the response of sustained (X) and transient (Y) cells in the cat lateral geniculate nucleus (LGN) and optic tract to a stationary spot while the spot luminance was increased and decreased with a constant rate (linear luminance functions), or modulated sinusoidally. The spot filled the receptive field center, and was surrounded by an annulus of fixed luminance. The LGN X cells seemed to perform a differentiation-like operation in the time domain at slow temporal modulations, giving information about rate of luminance change. To the linear luminance functions the cells responded with a constant firing rate. The on-center cells were activated during increasing luminance, the off-center cells during decreasing luminance. This firing rate increased monotonically with rate of luminance change. To low-frequency sinusoidal modulations the cells had a marked negative phase shift. The response of the LGN Y cells had a transient component shortly after the luminance started to increase (on-center cells) or decrease (off-center cells), followed by a secondary, gradually changing component. The peak of the transient component occurred on average when the response of the X cells increased most rapidly. To low-frequency sinusoidal modulation the average negative phase shift of this peak was twice the average of the X cells. The Y system could accordingly provide information about rate of change in the response of the X system. In the optic tract the X fiber response resembled the LGN X cell response in most respects. The Y fibers had only a weak transient response component, so this component was accentuated in the thalamic relay. Also the sensitivity for rate of luminance change was increased in LGN.

Conservation of adapting components in auditory-nerve responses.

The responses of single auditory-nerve fibers of Mongolian gerbil were studied using tonal stimuli. The peristimulatory adaptation of firing rate in response to tone bursts presented in quiet and during a background stimulus is described quantitatively. The total transient response which can be produced to the onset of a tone burst, whether presented in quiet or as an intensity increment, is limited and appears to demonstrate a form of conservation. Specifically, the total numbers of spikes produced by the rapidly adapting component, and the slower short-term adaptation component, are proportional at all intensities, and are limited for each fiber. Furthermore, when an incremental stimulus is presented on a background, the total transient response to the background and to the increment is limited and depends upon the final intensity, not the background intensity. When the presumed underlying synaptic drive is derived by removing the effects of refractoriness from the spike train, the same conservation of the transient response components, and proportionality between rapid and short-term components, are observed.

Experimental determination of the linear biphasic constitutive coefficients of human fetal proximal femoral chondroepiphysis

The mechanical properties of the cartilaginous regions of the proximal femoral epiphysis are an important factor in load transmission through the hip joint of young children. Cylindrical test specimens excised from the chondroepiphysis of human stillborn femoral heads were subjected to uniaxial loading in peripherally-unconfined compression, using a ramp/plateau input strain history. The corresponding load vs time curves were analyzed in terms of a recent analytical solution for a linear biphasic material (the well-known KLM model), allowing calculation of that model's three fundamental constitutive coefficients (permeability, equilibrium modulus and solid-phase Poisson ratio) for this material. The numerical algorithm developed to evaluate the biphasic solution yielded very precise replication of previously published KLM parametric plots. When fitted to experimental load histories, however, the model provided only a rather loose approximation of specimen behavior, due apparently to a substantial underestimation of the transient response component associated with interstitial fluid transport. Averaged over the series, the best-fit values for permeability (2.51 × 10 −15 m 4 N s −1) and equilibrium modulus (0.699 M Pa) were in the range of values accepted for human adult articular cartilage. A consequence of the coarseness of the analytical curve fits was that a solid-phase Poisson ratio of 0.0 was inferred for all specimens. The permeability vs equilibrium modulus exhibited a nearly linear ( r = 0.74) inverse relationship similar to that reported for adult articular cartilage.