FM Responses Research Articles

Background noise exerts diverse effects on the cortical encoding of foreground sounds.

In natural listening conditions, many sounds must be detected and identified in the context of competing sound sources, which function as background noise. Traditionally, noise is thought to degrade the cortical representation of sounds by suppressing responses and increasing response variability. However, recent studies of neural network models and brain slices have shown that background synaptic noise can improve the detection of signals. Because acoustic noise affects the synaptic background activity of cortical networks, it may improve the cortical responses to signals. We used spike train decoding techniques to determine the functional effects of a continuous white noise background on the responses of clusters of neurons in auditory cortex to foreground signals, specifically frequency-modulated sweeps (FMs) of different velocities, directions, and amplitudes. Whereas the addition of noise progressively suppressed the FM responses of some cortical sites in the core fields with decreasing signal-to-noise ratios (SNRs), the stimulus representation remained robust or was even significantly enhanced at specific SNRs in many others. Even though the background noise level was typically not explicitly encoded in cortical responses, significant information about noise context could be decoded from cortical responses on the basis of how the neural representation of the foreground sweeps was affected. These findings demonstrate significant diversity in signal in noise processing even within the core auditory fields that could support noise-robust hearing across a wide range of listening conditions.NEW & NOTEWORTHY The ability to detect and discriminate sounds in background noise is critical for our ability to communicate. The neural basis of robust perceptual performance in noise is not well understood. We identified neuronal populations in core auditory cortex of squirrel monkeys that differ in how they process foreground signals in background noise and that may contribute to robust signal representation and discrimination in acoustic environments with prominent background noise.

Effect of Varying Phase Between Frequency and Amplitude Modulation on Bone Conduction Auditory Steady State Responses

Auditory steady state response (ASSR) testing provides a means to objectively estimate hearing levels in newborns and adults for whom behavioral tests prove difficult. When testing these patient groups, it is preferable that clear responses to both air and bone conduction stimuli are obtained in a short amount of time. Much of the literature addressing ASSRs, such as investigations of stimulus and recording parameters, have focused on air conduction ASSRs. The aim of this investigation was to study the amplitudes, latencies, and test times of bone conduction ASSRs elicited using amplitude- (AM), frequency- (FM), and mixed-modulated (MM) stimuli and provide suggestions for optimum recording parameters. Bone and air conduction multiple ASSRs were recorded from two groups of 20 normal-hearing adults using the Multiple Auditory Steady State Response research system. AM, FM, and MM sinusoidal tones were used (0.5-, 1-, 2-, and 4-kHz carrier frequencies), which were modulated between 78 and 92 Hz. AM depth was 100% and FM depth was 20%. ASSR amplitudes and latencies (calculated using the "preceding cycles" technique) were analyzed for MM phase settings across the cycle from 0° at 45° intervals and compared with AM responses. Optimum phase settings for bone and air conduction ASSRs were calculated using a sinusoidal model based on the amplitude data. Similar effects of stimulus type and carrier frequency were observed for bone and air conduction ASSRs. AM responses were larger in amplitude compared with FM responses. MM (at all phase settings tested) and AM response latencies increased with decreasing carrier frequency. MM phase setting had a significant (p < 0.01) sinusoidal effect on ASSR amplitudes, compared with AM responses, at 1, 2, and 4 kHz but not 0.5 kHz for air conduction and 1 and 2 kHz but not 0.5 and 4 kHz for bone conduction. Using a sinusoidal function to model this effect, MM phase settings (±95% confidence intervals) of 318° (295 to 350°) and 295° (290 to 310°) are predicted to evoke the largest responses for bone conduction ASSRs at 1 and 2 kHz, respectively. Phase settings of 293° (285 to 310°), 300° (280 to 310°), and 280° (255 to 330°) are predicted for air conduction ASSRs at 1, 2, and 4 kHz, respectively. MM phase setting had little effect on estimated latency. Test times were significantly (p < 0.01) affected by phase setting with both increases and decreases being observed. Test times for ASSRs at 1, 2, and 4 kHz could be significantly reduced if the estimated optimum phase settings are used. Different stimuli can significantly affect the amplitudes of bone conduction ASSRs. These effects are similar to those observed for air conduction ASSRs. MM stimuli with specific phase settings evoke larger bone conduction ASSRs compared with AM and FM stimuli alone, and calculations show that the time taken to obtain these responses is reduced. Implementation of the suggested optimum settings will promote efficient collection of bone conduction, and indeed air conduction, ASSR data.

Adapting Australian health facilities to cope with climate‐related extreme weather events

PurposeThe purpose of this paper is to discuss the current state of FM preparedness required to deal with the risks to healthcare delivery posed by climate change‐related extreme weather events.Design/methodology/approachSelected stakeholders were invited to participate in targeted focus groups that, using the ROMS methodology, explored the status of current knowledge and preparedness of the NSW health system to deal with the expected demands imposed by increasing incidences of extreme weather events. Findings are summarised and discussed in terms of the key stakeholder objectives identified. Further areas of required research are then discussed.FindingsThe key objectives of the stakeholders were readily agreed, however a lack of information regarding the quantifiable impacts forecast to be associated with climate change constrained the development of other than generic strategies for dealing with these impacts. Further areas of research included assessment of changing demand for health services, likely physical impacts on facilities and their adequacy in coping with these, implementation strategies for augmenting coping capacity and associated costs, plus the need for integrating disaster planning and management strategies to ensure the continuity of operation of health facilities during extreme weather events.Originality/valueThe paper outlines the status of current knowledge regarding the likely impact of climate change‐related extreme weather events on healthcare infrastructure. It explores key issues and determines where future work should be undertaken to ensure that rigorous FM responses are available to cope with a clear and identified threat to the health of the Australian, and similar communities.

The Rorschach test and Gilles de la Tourette’s syndrome: A pilot case–control study

Background: To date only three studies, all lacking a control group, have investigated the personality style underlying tic disorders or Tourette’s syndrome (TS), using the projective Rorschach test. Despite the recent controversy about its proper use in clinical practice and research, the Rorschach test may be useful to explore personality organization in TS. The research hypothesis under investigation in this exploratory study was that young TS patients exhibit a peculiar personality organization, with significantly increased indicators of poor emotional control, aggression, anxiety and depression, on the Rorschach test, compared to healthy controls. Methods: We analyed the Rorschach protocols of 17 newly diagnosed paediatric TS patients and 17 age- and sex-matched healthy controls. Results: Compared with the controls young TS patients recorded significantly lower mean percentages of F and F+ responses and a significantly higher number of FM and FC responses, thus showing reduced control of emotions and difficulty in integrating aggressivity. Furthermore, associations emerged between comorbidity with ADHD and increased emotional instability and between severity of tics and of OCD and a pattern of emotional hyper-control and application of rigid defensive mechanisms. Conclusions: These results seem to confirm the existence of a peculiar personality style in young TS patients and may have important implications, both pathogenetic and therapeutic.

Magnetic behaviour governed by Co spin transitions in LaCo1−xTixO3 (0 ⩽ x ⩽ 0.5) perovskite oxides

Perovskite-type cobaltites LaCo1−xTixO3 (0 ⩽ x ⩽ 0.5) synthesized by the liquid mix technique were characterized by x-ray diffraction, neutron diffraction, XPS, magnetic susceptibility and magnetization versus magnetic field measurements. Structural symmetry changes from rhombohedral (S.G. ) for 0.05 ⩽ x ⩽ 0.20 to orthorhombic (S.G. Pbnm) for 0.25 ⩽ x ⩽ 0.40. The bond distances dependence on the composition, x, has been analysed from x-ray and neutron diffraction data. Both the structural and the spectroscopic (from XPS) results suggest that Ti4+ substitution implies the stabilization of low-spin CoIII ions and the evolution of magnetic moments in the paramagnetic zone is also coherent with this assumption. For small values of x both FM responses and the absence of saturation in the M versus H curves at 2 K are interpreted in terms of a magnetic frustration. For x > 0.25 the cobalt cations seem to stabilize as CoIII (rather than Co3+) coexisting with Co2+. In these more substituted perovskites, AFM Co2+–O–Co2+ interactions become predominant, leading to a well established AFM ordering for x = 0.4 and 0.5.

Heterogeneous Neuronal Responses to Frequency-Modulated Tones in the Primary Auditory Cortex of Awake Cats

Previous studies in anesthetized animals reported that the primary auditory cortex (A1) showed homogenous phasic responses to FM tones, namely a transient response to a particular instantaneous frequency when FM sweeps traversed a neuron's tone-evoked receptive field (TRF). Here, in awake cats, we report that A1 cells exhibit heterogeneous FM responses, consisting of three patterns. The first is continuous firing when a slow FM sweep traverses the receptive field of a cell with a sustained tonal response. The duration and amplitude of FM response decrease with increasing sweep speed. The second pattern is transient firing corresponding to the cell's phasic tonal response. This response could be evoked only by a fast FM sweep through the cell's TRF, suggesting a preference for fast FM. The third pattern was associated with the off response to pure tones and was composed of several discrete response peaks during slow FM stimulus. These peaks were not predictable from the cell's tonal response but reliably reflected the time when FM swept across specific frequencies. Our A1 samples often exhibited a complex response pattern, combining two or three of the basic patterns above, resulting in a heterogeneous response population. The diversity of FM responses suggests that A1 use multiple mechanisms to fully represent the whole range of FM parameters, including frequency extent, sweep speed, and direction.

Simplified Model of the Dynamic Thermal Tuning Behavior of VCSELs

In this letter, a simplified thermal model is used to derive a closed-form expression for the frequency dependency of the current-to-wavelength tuning coefficient (FM response) of vertical-cavity surface-emitting lasers (VCSELs). It explains the experimentally observed square root law and shows very good agreement with measured FM responses of two different VCSEL types. It yields significantly improved accuracy compared to the usually assumed first-order low-pass model.

Measurements of frequency responses of three-section distributed Bragg reflector lasers and application to modulation spectroscopy

The feasibility for wavelength modulation spectroscopy using a three-section distributed Bragg reflector (DBR) laser modulated at the phase section has been investigated. The FM and AM responses have been measured versus frequency from dc to 3 GHz at various positions on the tuning curve. The modulation efficiency as a function of the tuning currents combined with modulation frequency has been studied. In particular the behaviour at the positions of laser mode jumps has been emphasised as well as consistency of the recorded spectra across the entire tuning range. These tests were performed with a tuneable Fabry–Perot filter. The operation conditions were adapted for high and sustained FM response for modulation spectroscopy over a wide wavelength range. A conventional Bragg grating is used as an example of practical application.

Analog direct-modulation behavior of semiconductor laser transmitters using optical FM demodulation

In this paper, we report a theoretical investigation of the analog modulation performance of a semiconductor laser transmitter which employs the direct optical FM demodulation. This analysis is based on the rate equations in which Langevin noise functions are included. The optical FM response has firstly been derived using a chirp-power relation deduced from the rate equations. The chirp-to-power ratio (CPR) has secondly been computed and illustrated showing the effect of the novel damping parameter phenomenologically introduced in the rate equation for the optical phase. The curves show that the conventional CPR can be notably reduced using FM demodulation. Beside this, the comparison of the FM responses with prior studies show that our model may be more appropriate to describe optical FM behavior of many types of single-mode lasers. The relative-output noise (RON) at the transmitter has also been determined and subsequently used to calculate the carrier-to-noise ratio (CNR) available. It is shown that the transmitter inherent noise would significantly degrade as a result of the conversion of the optical frequency noise into intensity noise. Consequently, intrinsic noise is expected practically to contribute the most to CNR at high powers. This feature can be more easily appreciated through curves illustrating CNR and is in contrast to previous studies in which the receiver shot noise was assumed to be the main noise term.

Analytical expressions for the FM and AM responses of an integrated laser-modulator

The need for low-chirp and compact transmitters for high-bit-rate optical links has led to the development of integrated laser electroabsorption modulators (ILM). We have investigated feedback effects inducing frequency chirp by developing a model treating the ILM as a whole and obtained analytical expressions of the FM and AM responses. The variation of the frequency chirp with the residual facet reflectivity of the modulator section is calculated. The model predicts the unusual peak in the measured frequency responses and has been used to define design rules.

Performance implications of the thermal-induced frequency drift in fast wavelength switched systems with heterodyne detection

The frequency modulation response and tuning dynamics of a two-section DFB laser are determined by carrier and thermal modulation of the active layer refractive index. In response to a step change in the injection current for switching between channels in a wavelength division multiplexed system, the optical frequency changes rapidly due to the carrier effect, and then slowly drifts toward a steady state value due to the thermal effect. For wavelength switched applications with heterodyne detection, the drifting of the optical frequency broadens the spectrum of the IF signal and may impose a limit on the time that the signal remains within the IF passband (residency time). The IF spectral broadening and residency time are investigated theoretically and experimentally. Based upon a minimum mean square error fit between experimental and theoretical FM responses, the dependence of the spectral broadening and residency time on the bias condition of the laser and the optical frequency switching interval is characterized.

Dynamic properties of push-pull DFB semiconductor lasers

Using the spatially dependent multimode rate equations, we present a systematic study of small-signal dynamics of push-pull DFB lasers. The various spatial effects such as the longitudinal spatial hole burning, nonlinear gain compression, side-mode contribution, and push-pull modulation are all analyzed in a self-consistent manner. With the closed form expressions for the AM and FM responses, we show explicitly that the resonance frequency and the first cut-off frequency of push-pull DFB lasers are determined by the frequency spacing and the threshold gain difference between the lasing mode and its closest antisymmetric side mode, respectively. Numerical results reveal that a high modulation speed with a very low frequency chirp can be achieved with the push-pull DFB lasers.

Analysis of FM and AM responses of a tunable three-electrode DBR laser diode

Starting with the wave propagation equation, neglecting spatial and spectral hole-burning effects, and using a solving method based on the Green's function, the rate equations are derived for a multielectrode laser. By applying them to a three-electrode DBR laser, tunability and linewidth performance are evaluated, and the modulation characteristics such as the amplitude response and the frequency response are analyzed. It is shown that amplitude modulation can be completely suppressed, and at the same time, a pure frequency modulation obtained if currents are related by an adequate equation. Since this laser diode has many advantages such as a wide wavelength tunability and a narrow spectral linewidth, it will be suitable as a lightwave oscillator or transmitter in future communication systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Recursive simulation models of the semiconductor laser modulation characteristics for accurate performance evaluation of coherent optical CPFSK systems

Modeling of the semiconductor laser nonuniform FM response and residual IM modulation is essential for the computer-aided design of coherent optical communication systems. This paper presents an accurate simulation model, in which the FM and IM responses are represented by recursive digital filters derived directly from measurements. A comparison with previous models of the bibliography reveals the advantages of the current approach. The proposed modeling procedure is applied in the case of a single-electrode DFB laser. This DFB model, in combination with a semi-analytical technique for the evaluation of the error probability, is used to study the influence of the modulation characteristics on the performance of a coherent heterodyne CPFSK system with differential receiver operating at 1 Gb/s. Theoretical and experimental results are in excellent agreement.

Intensity modulation technique using a directly frequency-modulated semiconductor laser and an interferometer

Harmonic distortion in semiconductor laser direct frequency modulation is described with the aid of a nonlinear differential equation for the photon density including gain damping. Curves are presented showing the fundamental, second-order harmonic and third-order harmonic FM responses. It is found that systems using a frequency-modulated laser and an interferometer would exhibit lower distortion levels than those using direct intensity modulation of the laser diode.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effect of spatial hole burning on FM response in DFB laser diodes

AbstractIn this article, the factors of aP and δP are introduced to describe the effects of all optical field longitudinal nonuniformity on the threshold condition. The rate equation model, including spatial hole burning, is established. The two factors are calculated from the rate and coupled mode equations and analytical expressions of FM responses are derived. FM responses of DFB laser diodes are also analyzed numerically.

Requirements for high-frequency LD FM response in an optical CPFSK heterodyne delay demodulation system

The required high-frequency laser diode (LD) FM responses for a continuous phase frequency shift keying (CPFSK) heterodyne delay modulation system were investigated. The degradations due to insufficient high-frequency FM response are evaluated, considering the FSK waveform distortion, which gives undesired phase error at the demodulation circuit. From the phase error, bit error rate (BER) degradation is calculated. The calculated results indicate that the delay-time adjustment for the demodulation circuit is effective in minimizing the power penalty. The theoretical evaluation, including the demodulation circuit optimization, explains the experimental results fairly well. Using the same evaluation procedure, required high-frequency LD FM responses are derived. The results, together with the required low-frequency FM responses, give guidelines for transmitter LD selection for coherent CPFSK systems. >

FM characteristics and compact modules for coherent semiconductor lasers coupled to an external cavity

Measured FM responses of a semiconductor laser optically coupled off-axis to a confocal Fabry-Perot cavity are discussed. It was found that this cavity acted as a frequency discriminator and as a phase comparator for slow and fast frequency fluctuations, respectively. The crossover between them was determined by a half linewidth of the cavity. On the basis of these investigations, two kinds of coherent semiconductor laser modules were constructed. External Fabry-Perot cavities were made by using an optical fiber and a hemispherical microlens. The linewidths of these lasers were less than 25 kHz. >

FM and FSK response of tunable two-electrode DFB lasers and their performance with noncoherent detection

Two-electrode distributed-feedback (DFB) lasers show promise for combining high speed and frequency tunability for frequency-division-multiplexed-frequency-shift-keyed (FDM-FSK) networks. The FM and FSK responses of such lasers have been measured up to modulation frequencies of approximately 1 GHz. Using these lasers in a noncoherent detection system in which a fiber Fabry-Perot tunable optical filter converts an FSK signal into an amplitude-shift-keyed (ASK) format, a 10/sup -9/ BER was observed up to 1 Gb/s. Nonuniform FM response and consequent tone broadening of the optical FSK spectra can lead to system power penalties due to optical-filtering effects. Thus, for a given FM response, the behavior of these lasers in FSK optical systems can be projected.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

12 GHz FM bandwidth for a 1530 nm DFB laser

The FM response of a 1530 nm DFB laser has been measured using a novel birefringent fibre interferometer. The FM response is flat out to 12 GHz when operated at 9mW/facet, corresponding to a linewidth of 15 MHz, and there is a 15–25 ps delay between the FM and AM responses.