Low Characteristic Frequency Research Articles

Tonotopic Optimization for Temporal Processing in the Cochlear Nucleus.

In the auditory system, sounds are processed in parallel frequency-tuned circuits, beginning in the cochlea. Auditory nerve fibers reflect this tonotopy and encode temporal properties of acoustic stimuli by "locking" discharges to a particular stimulus phase. However, physiological constraints on phase-locking depend on stimulus frequency. Interestingly, low characteristic frequency (LCF) neurons in the cochlear nucleus improve phase-locking precision relative to their auditory nerve inputs. This is proposed to arise through synaptic integration, but the postsynaptic membrane's selectivity for varying levels of synaptic convergence is poorly understood. The chick cochlear nucleus, nucleus magnocellularis (NM), exhibits tonotopic distribution of both input and membrane properties. LCF neurons receive many small inputs and have low input thresholds, whereas high characteristic frequency (HCF) neurons receive few, large synapses and require larger currents to spike. NM therefore presents an opportunity to study how small membrane variations interact with a systematic topographic gradient of synaptic inputs. We investigated membrane input selectivity and observed that HCF neurons preferentially select faster input than their LCF counterparts, and that this preference is tolerant of changes to membrane voltage. We then used computational models to probe which properties are crucial to phase-locking. The model predicted that the optimal arrangement of synaptic and membrane properties for phase-locking is specific to stimulus frequency and that the tonotopic distribution of input number and membrane excitability in NM closely tracks a stimulus-defined optimum. These findings were then confirmed physiologically with dynamic-clamp simulations of inputs to NM neurons. One way that neurons represent temporal information is by phase-locking, which is discharging in response to a particular phase of the stimulus waveform. In the auditory system, central neurons are optimized to retain or improve phase-locking precision compared with input from the auditory nerve. However, the difficulty of this computation varies systematically with stimulus frequency. We examined properties that contribute to temporal processing both physiologically and in a computational model. Neurons processing low-frequency input benefit from integration of many weak inputs, whereas those processing higher frequencies progressively lose precision by integration of multiple inputs. Here, we reveal general features of input-output optimization that apply to all neurons that process time varying input.

Optimization study of a piezoelectric bistable generator with doubled voltage frequency using harmonic balance method

Bistable generator for vibration energy harvesting is one of the most promising solutions to reach practically enhanced performance. Due to the buckled–spring–mass–specific behavior, the deformation of the piezoelectric transducer is nonlinearly dependent on the displacement, especially in the inter-well motion case. An analytical model is developed using harmonic balance analysis for the buckled–spring–mass generator architecture. Contrary to the usual method, a special doubled frequency voltage solution in the inter-well motion case is assumed in harmonic balance analysis to obtain an accurate predictive model, which is validated by experimental results. The influence of five critical parameters on the performance is thoroughly discussed. Design rules are then deduced: low damping ratio, properly high coupling level, matched load, optimal buckling level, and low characteristic frequency are required to get optimal performance in the inter-well motion case. Besides, we show some interesting results about the parameter optimization study.

The immediate effects of acoustic trauma on excitation and inhibition in the inferior colliculus: A Wiener-kernel analysis

Noise-induced tinnitus and hyperacusis are thought to correspond to a disrupted balance between excitation and inhibition in the central auditory system. Excitation and inhibition are often studied using pure tones; however, these responses do not reveal inhibition within the excitatory pass band. Therefore, we used a Wiener-kernel analysis, complemented with singular value decomposition (SVD), to investigate the immediate effects of acoustic trauma on excitation and inhibition in the inferior colliculus (IC).Neural responses were recorded from the IC of three anesthetized albino guinea pigs before and immediately after a one-hour bilateral exposure to an 11-kHz tone of 124 dB SPL. Neural activity was recorded during the presentation of a 1-h continuous 70 dB SPL Gaussian-noise stimulus. Spike trains were subjected to Wiener-kernel analysis in which the second-order kernel was decomposed into excitatory and inhibitory components using SVD.Hearing thresholds between 3 and 22 kHz were elevated (13–47 dB) immediately after acoustic trauma. The presence and frequency tuning of excitation and inhibition in units with a low characteristic frequency (CF; < 3 kHz) was not affected, inhibition disappeared whereas excitation was not affected in mid-CF units (3 < CF < 11 kHz), and both excitation and inhibition disappeared in high-CF units (CF > 11 kHz). This specific differentiation could not be identified by tone-evoked receptive-field analysis, in which inhibitory responses disappeared in all units, along with excitatory responses in high-CF units.This study is the first to apply Wiener-kernel analysis, complemented with SVD, to study the effects of acoustic trauma on spike trains derived from the IC. With this analysis, a reduction of inhibition and preservation of good response thresholds was shown in mid-CF units immediately after acoustic trauma. These neurons may mediate noise-induced tinnitus and/or hyperacusis. Moreover, an immediate profound high-frequency hearing loss was reflected by reduced evoked firing rates and loss of both excitation and inhibition in high-CF units.

PENDEXE: A novel energy harvesting concept for low frequency human waistline

We report a novel concept of harvesting energy from low frequency and low force human gait movement from a device attached at the waistline. Due to the rather low characteristic frequencies of the human gait it is difficult to harvest energy piezoelectrically from such movement. The concept reported here uses a pendulum to exploit kinetic energy available from walking and jogging and later converting it into electrical energy by using compliant piezoelectric unimorphs. The pendulum with a fundamental frequency of approximately 2Hz is directed to excite the piezoelectric unimorphs at their natural frequency of 50Hz by a mechanical frequency up-conversion technique. The maximum power output is nearly 300μW. The concept, fabrication of novel rotor-pendulum based harvesting unit and experimentation are discussed in detail.

Suppression of putative tinnitus-related activity by extra-cochlear electrical stimulation.

Studies on animals have shown that noise-induced hearing loss is followed by an increase of spontaneous firing at several stages of the central auditory system. This central hyperactivity has been suggested to underpin the perception of tinnitus. It was shown that decreasing cochlear activity can abolish the noise-induced central hyperactivity. This latter result further suggests that an approach consisting of reducing cochlear activity may provide a therapeutic avenue for tinnitus. In this context, extra-cochlear electric stimulation (ECES) may be a good candidate to modulate cochlear activity and suppress tinnitus. Indeed, it has been shown that a positive current applied at the round window reduces cochlear nerve activity and can suppress tinnitus reliably in tinnitus subjects. The present study investigates whether ECES with a positive current can abolish the noise-induced central hyperactivity, i.e., the putative tinnitus-related activity. Spontaneous and stimulus-evoked neural activity before, during and after ECES was assessed from single-unit recordings in the inferior colliculus of anesthetized guinea pigs. We found that ECES with positive current significantly decreases the spontaneous firing rate of neurons with high characteristic frequencies, whereas negative current produces the opposite effect. The effects of the ECES are absent or even reversed for neurons with low characteristic frequencies. Importantly, ECES with positive current had only a marginal effect on thresholds and tone-induced activity of collicular neurons, suggesting that the main action of positive current is to modulate the spontaneous firing. Overall, cochlear electrical stimulation may be a viable approach for suppressing some forms of (peripheral-dependent) tinnitus.

Short-term synaptic depression is topographically distributed in the cochlear nucleus of the chicken.

In the auditory system, sounds are processed in parallel frequency-tuned circuits, beginning in the cochlea. Activity of auditory nerve fibers reflects this frequency-specific topographic pattern, known as tonotopy, and imparts frequency tuning onto their postsynaptic target neurons in the cochlear nucleus. In birds, cochlear nucleus magnocellularis (NM) neurons encode the temporal properties of acoustic stimuli by "locking" discharges to a particular phase of the input signal. Physiological specializations exist in gradients corresponding to the tonotopic axis in NM that reflect the characteristic frequency (CF) of their auditory nerve fiber inputs. One feature of NM neurons that has not been investigated across the tonotopic axis is short-term synaptic plasticity. NM offers a rather homogeneous population of neurons with a distinct topographical distribution of synaptic properties that is ideal for the investigation of specialized synaptic plasticity. Here we demonstrate for the first time that short-term synaptic depression (STD) is expressed topographically, where unitary high CF synapses are more robust with repeated stimulation. Correspondingly, high CF synapses drive spiking more reliably than their low CF counterparts. We show that postsynaptic AMPA receptor desensitization does not contribute to the observed difference in STD. Further, rate of recovery from depression, a presynaptic property, does not differ tonotopically. Rather, we show that another presynaptic feature, readily releasable pool (RRP) size, is tonotopically distributed and inversely correlated with vesicle release probability. Mathematical model results demonstrate that these properties of vesicle dynamics are sufficient to explain the observed tonotopic distribution of STD.

The emergence of abnormal hypersynchronization in the anatomical structural network of human brain

Brain activity depends on transient interactions between segregated neuronal populations. While synchronization between distributed neuronal clusters reflects the dynamics of cooperative patterns, the emergence of abnormal cortical hypersynchronization is typically associated with spike-wave discharges, which are characterized by a sudden appearance of synchronous around 3Hz large amplitude spike-wave discharges of the electroencephalogram. While most existing studies focus on the cellular and synaptic mechanisms, the aim of this article is to study the role of structural connectivity in the origin of the large-scale synchronization of the brain. Simulating oscillatory dynamics on a human brain network, we find the space-time structure of the coupling defined by the anatomical connectivity and the time delays can be the primary component contributing to the emergence of global synchronization. Our results suggest that abnormal white fiber connections may facilitate the generation of spike-wave discharges. Furthermore, while neural populations can exhibit oscillations in a wide range of frequency bands, we show that large-scale synchronization of the brain only occurs at low frequencies. This may provide a potential explanation for the low characteristic frequencies of spike-wave discharges. Finally, we find the global synchronization has a clear anterior origin involving discrete areas of the frontal lobe. These observations are in agreement with existing brain recordings and in favor of the hypothesis that initiation of spike-wave discharges originates from specific brain areas. Further graph theory analysis indicates that the original areas are highly ranked across measures of centrality. These results underline the crucial role of structural connectivity in the generation of spike-wave discharges.

Inhibitory synaptic release properties are topographically distributed in auditory circuitry

Inhibitory synaptic release properties are topographically distributed in auditory circuitry

Classification of vocalizations by recordings from the auditory midbrain

The temporal and spatial properties of responses to complex stimuli in the central nucleus of the inferior colliculus (ICC), the main converging station in the auditory midbrain, can provide evidence for coding principles in the auditory system and are relevant for the design of neuroprosthesis. We study responses from guinea pigs to a set of eleven species-specific vocalizations which show a wide range of spectral contents, envelope types, frequency and amplitude modulations. The envelopes of the acoustically presented stimuli are characterized as complex or periodic impulses and have various degrees of periodicity. The frequency content ranges from harmonic strucutres to broad spectral distributions. The data analyzed were multi-unit recordings taken simultaneously from 32 positions in the ICC of guinea pigs using a double shank electrode. Peristimulus time histograms (PSTHs) of the high dimensional recordings were classified by linear discriminant analysis in order to evaluate the spatial and temporal distribution of stimulus-related information without the assumption of a specific coding scheme. Neighboring neural populations respond in a similar manner and have highly correlated PSTHs. Combining responses from different positions improves the classification performance for distant postions which do not show correlation, but not for adjacent positions. Separability of responses along the tonotopic gradient of the ICC to vocalizations shows great variation according to their spectral properties. Low-frequency stimuli are found to be better separable in low characteristic frequency (CF) lamina than complex envelope or broad spectral stimuli. Some stimuli of the latter type (squeal, whistle) are nearly perfectly discriminated in multi-units with high CF whereas discrimination for low-frequency stimuli is less good (60 - 70 %) in this region, see Figure ​Figure1.1. For multi-units positioned in the mid-frequency range along the tonotopic axis all stimuli can be separated at a level similar to the result obtained in the low/high CF regions for the respective preferred stimuli. Figure 1 Confusion matrix for the classification of neural responses to periodic low-frequency stimuli (low chutter, drr) and complex-envelope broad-spectrum stimuli (squeal, low whistle) for a multi-unit in an iso-frequency lamina with a characteristic frequency ... We interpret the results as a consequence of the combination of several frequencies in the vocalizations that are better covered by the widely tuned receptive fields of the neurons in the mid-frequency range than by the receptive fields of the units with CFs in the extreme frequency ranges.

Mechanisms underlying azimuth selectivity in the auditory cortex of the pallid bat

This study focused on mechanisms underlying azimuth selectivity in the primary auditory cortex (A1) of pallid bats. The pallid bat listens to prey-generated noise (5–35 kHz) to localize and hunt terrestrial prey. The region of A1 tuned between 5 and 35 kHz consists of two clusters of neurons distinguished by interaural intensity difference (IID) selectivity: binaurally inhibited (EI) and peaked. The first aim of this study was to use sequential dichotic/free-field stimulation to test the hypothesis that IID is the primary cue underlying azimuth selectivity in neurons tuned in the prey-generated noise frequency band. IID selectivity and ear directionality at the neuron's characteristic frequency (CF) were used to predict azimuth selectivity functions. The predicted azimuth selectivity was compared with the actual azimuth selectivity from the same neurons. Prediction accuracy was similarly high for EI neurons and peaked neurons with low CF, whereas predictions were increasingly inaccurate with increasing CF among the peaked neurons. The second aim of this study was to compare azimuth selectivity obtained with noise and CF tones to determine the extent to which stimulus bandwidth influences azimuth selectivity in neurons with different binaural properties. The azimuth selectivity functions were similar for the two stimuli in the majority of EI neurons. A greater percentage of peaked neurons showed differences in their azimuth selectivity for noise and tones. This included neurons with multiple peaks when tested with tones and a single peak when tested with noise. Taken together, data from the two aims suggest that azimuth tuning of EI neurons is primarily dictated by IID sensitivity at CF. Peaked neurons, particularly those with high CF, may integrate IID sensitivity across frequency to generate azimuth selectivity for broadband sound. The data are consistent with those found in cat and ferret A1 in that binaurally facilitated neurons depend to a greater extent (compared to EI neurons) on spectral integration of binaural properties to generate azimuth selectivity for broadband stimuli.

Axonal Branching Patterns as Sources of Delay in the Mammalian Auditory Brainstem: A Re-Examination

In models of temporal processing, time delays incurred by axonal propagation of action potentials play a prominent role. A pre-eminent model of temporal processing in audition is the binaural model of Jeffress (1948), which has dominated theories regarding our acute sensitivity to interaural time differences (ITDs). In Jeffress' model, a binaural cell is maximally active when the ITD is compensated by an internal delay, which brings the inputs from left and right ears in coincidence, and which would arise from axonal branching patterns of monaural input fibers. By arranging these patterns in systematic and opposite ways for the ipsilateral and contralateral inputs, a range of length differences, and thereby of internal delays, is created so that the ITD is transformed into a spatial activation pattern along the binaural nucleus. We reanalyze single, labeled, and physiologically characterized axons of spherical bushy cells of the cat anteroventral cochlear nucleus, which project to binaural coincidence detectors in the medial superior olive (MSO). The reconstructions largely confirm the observations of two previous reports, but several features are observed that are inconsistent with Jeffress' model. We found that ipsilateral projections can also form a caudally directed delay line pattern, which would counteract delays incurred by caudally directed contralateral projections. Comparisons of estimated axonal delays with binaural physiological data indicate that the suggestive anatomical patterns cannot account for the frequency-dependent distribution of best delays in the cat. Surprisingly, the tonotopic distribution of the afferent endings indicate that low characteristic frequencies are under-represented rather than over-represented in the MSO.

Constant-Phase-Element Behavior Caused by Resistivity Distributions in Films: II. Applications

A normal power-law distribution of local resistivity with a uniform dielectric constant was found to be consistent with the constant-phase element (CPE). An analytic expression, based on the power-law resistivity distribution, was found that relates CPE parameters to the physical properties of a film. This expression worked well for such diverse systems as aluminum oxides, oxides on stainless steel, and human skin. Good values for film thickness were obtained, even when previously proposed expressions could not be used or yielded incorrect results. The power-law model yields a CPE impedance behavior in an appropriate frequency range, defined by two characteristic frequencies. Ideal capacitive behavior is seen above the upper characteristic frequency and below the lower characteristic frequency. A symmetric CPE response between the characteristic frequencies can be obtained by adding a parallel resistive pathway.

The Effects of the TiO&lt;sub&gt;2&lt;/sub&gt; Films Thickness on the Performance of Quasi-Solid Dye-Sensitized Solar Cells

The dye – sensitized solar cell (DSSC) is an attractive and promising device for solar cell applications that have been intensively investigated worldwide. DSSC consists of a namo TiO2 film of the photo electrode, dye molecules absorbed on the surface of TiO2 film, an electrolyte layer and a Pt counter electrode. Among these, the nanoporous TiO2 film plays an important role because it can adsorb a large amount of dye molecules which provide electrons. Therefore, the TiO2 film affects the cell performance. In this paper, the characteristics of DSSCs with different TiO2 film thicknesses were studied by using electrochemical impedance spectroscopy(EIS). The impedance component attributed to TiO2|electrolyte interface indicated that the small semicircle and low characteristic frequency was essential for high performance DSSC. An optimum overall conversion efficiency( ％) of 5.54% was obtained in the DSSC assembled with the TiO2 film thickness of 8.86μm.

Envelope Coding in Auditory Nerve Fibers Following Noise-Induced Hearing Loss

Recent perceptual studies suggest that listeners with sensorineural hearing loss (SNHL) have a reduced ability to use temporal fine-structure cues, whereas the effects of SNHL on temporal envelope cues are generally thought to be minimal. Several perceptual studies suggest that envelope coding may actually be enhanced following SNHL and that this effect may actually degrade listening in modulated maskers (e.g., competing talkers). The present study examined physiological effects of SNHL on envelope coding in auditory nerve (AN) fibers in relation to fine-structure coding. Responses were compared between anesthetized chinchillas with normal hearing and those with a mild-moderate noise-induced hearing loss. Temporal envelope coding of narrowband-modulated stimuli (sinusoidally amplitude-modulated tones and single-formant stimuli) was quantified with several neural metrics. The relative strength of envelope and fine-structure coding was compared using shuffled correlogram analyses. On average, the strength of envelope coding was enhanced in noise-exposed AN fibers. A high degree of enhanced envelope coding was observed in AN fibers with high thresholds and very steep rate-level functions, which were likely associated with severe outer and inner hair cell damage. Degradation in fine-structure coding was observed in that the transition between AN fibers coding primarily fine structure or envelope occurred at lower characteristic frequencies following SNHL. This relative fine-structure degradation occurred despite no degradation in the fundamental ability of AN fibers to encode fine structure and did not depend on reduced frequency selectivity. Overall, these data suggest the need to consider the relative effects of SNHL on envelope and fine-structure coding in evaluating perceptual deficits in temporal processing of complex stimuli.

Pulsed Noise Experience Disrupts Complex Sound Representations

Cortical sound representations are adapted to the acoustic environment. Early exposure to exponential frequency-modulated (FM) sweeps results in more neurons selective to the experienced sounds. Here we examined the influence of pulsed noise experience on the development of sound representations in the primary auditory cortex (AI) of the rat. In naïve animals, FM sweep direction selectivity depends on the characteristic frequency (CF) of the neuron--low CF neurons tend to select for upward sweeps and high CF neurons for downward sweeps. Such a CF dependence was not observed in animals that had received weeklong exposure to pulsed noise in periods from postnatal day 8 (P8) to P15 or from P24 to P39. In addition, AI tonotopicity, tuning bandwidth, intensity threshold, tone-responsiveness, and sweep response magnitude were differentially affected by the noise experience depending on the exposure time windows. These results are consistent with previous findings of feature-dependent multiple sensitive periods. The different effects induced here by pulsed noise and previously by FM sweeps further indicate that plasticity in cortical complex sound representations is specific to the sensory input.

Selective Electrical Stimulation of the Auditory Nerve Activates a Pathway Specialized for High Temporal Acuity

Deaf people who use cochlear implants show surprisingly poor sensitivity to the temporal fine structure of sounds. One possible reason is that conventional cochlear implants cannot activate selectively the auditory-nerve fibers having low characteristic frequencies (CFs), which, in normal hearing, phase lock to stimulus fine structure. Recently, we tested in animals an alternative mode of auditory prosthesis using penetrating auditory-nerve electrodes that permit frequency-specific excitation in all frequency regions. We present here measures of temporal transmission through the auditory brainstem, from pulse trains presented with various auditory-nerve electrodes to phase-locked activity of neurons in the central nucleus of the inferior colliculus (ICC). On average, intraneural stimulation resulted in significant ICC phase locking at higher pulse rates (i.e., higher "limiting rates") than did cochlear-implant stimulation. That could be attributed, however, to the larger percentage of low-CF neurons activated selectively by intraneural stimulation. Most ICC neurons with limiting rates >500 pulses per second had CFs <1.5 kHz, whereas neurons with lower limiting rates tended to have higher CFs. High limiting rates also correlated strongly with short first-spike latencies. It follows that short latencies correlated significantly with low CFs, opposite to the correlation observed with acoustical stimulation. These electrical-stimulation results reveal a high-temporal-acuity brainstem pathway characterized by low CFs, short latencies, and high-fidelity transmission of periodic stimulation. Frequency-specific stimulation of that pathway by intraneural stimulation might improve temporal acuity in human users of a future auditory prosthesis, which in turn might improve musical pitch perception and speech reception in noise.

Phase-Locked Responses to Tones of Chinchilla Auditory Nerve Fibers: Implications for Apical Cochlear Mechanics

Responses to tones with frequency < or = 5 kHz were recorded from auditory nerve fibers (ANFs) of anesthetized chinchillas. With increasing stimulus level, discharge rate-frequency functions shift toward higher and lower frequencies, respectively, for ANFs with characteristic frequencies (CFs) lower and higher than approximately 0.9 kHz. With increasing frequency separation from CF, rate-level functions are less steep and/or saturate at lower rates than at CF, indicating a CF-specific nonlinearity. The strength of phase locking has lower high-frequency cutoffs for CFs >4 kHz than for CFs < 3 kHz. Phase-frequency functions of ANFs with CFs lower and higher than approximately 0.9 kHz have inflections, respectively, at frequencies higher and lower than CF. For CFs >2 kHz, the inflections coincide with the tip-tail transitions of threshold tuning curves. ANF responses to CF tones exhibit cumulative phase lags of 1.5 periods for CFs 0.7-3 kHz and lesser amounts for lower CFs. With increases of stimulus level, responses increasingly lag (lead) lower-level responses at frequencies lower (higher) than CF, so that group delays are maximal at, or slightly above, CF. The CF-specific magnitude and phase nonlinearities of ANFs with CFs < 2.5 kHz span their entire response bandwidths. Several properties of ANFs undergo sharp transitions in the cochlear region with CFs 2-5 kHz. Overall, the responses of chinchilla ANFs resemble those in other mammalian species but contrast with available measurements of apical cochlear vibrations in chinchilla, implying that either the latter are flawed or that a nonlinear "second filter" is interposed between vibrations and ANF excitation.

Effects of neonatal partial deafness and chronic intracochlear electrical stimulation on auditory and electrical response characteristics in primary auditory cortex

The use of cochlear implants in patients with severe hearing losses but residual low-frequency hearing raises questions concerning the effects of chronic intracochlear electrical stimulation (ICES) on cortical responses to auditory and electrical stimuli. We investigated these questions by studying responses to tonal and electrical stimuli in primary auditory cortex (AI) of two groups of neonatally deafened cats with residual high-threshold, low-frequency hearing. One group were implanted with a multi-channel intracochlear electrode at 8 weeks of age, and received chronic ICES for up to 9 months before cortical recording. Cats in the other group were implanted immediately prior to cortical recording as adults. In all cats in both groups, multi-neuron responses throughout the rostro-caudal extent of AI had low characteristic frequencies (CFs), in the frequency range of the residual hearing, and high-thresholds. Threshold and minimum latency at CF did not differ between the groups, but in the chronic ICES animals there was a higher proportion of electrically but not acoustically excited recording sites. Electrical response thresholds were higher and latencies shorter in the chronically stimulated animals. Thus, chronic implantation and ICES affected the extent of AI that could be activated by acoustic stimuli and resulted in changes in electrical response characteristics.

Cochlear nonlinearity in normal-hearing subjects as inferred psychophysically and from distortion-product otoacoustic emissions

The aim was to investigate the correlation between estimates of compression ratio, compression threshold, and cochlear gain for normal-hearing subjects inferred from temporal masking curves (TMC) and distortion-product otoacoustic emission (DPOAEs) input-output (I/O) curves. Special care was given to reduce the influence of DPOAE fine structure on the I/O curves. Data were collected for 10 subjects and for cochlear places with characteristic frequencies (CFs) of 0.5, 1, 2, and 4 kHz. Except at 4 kHz, little correlation was found between ratio estimates obtained with the two methods. One reason is that the DPOAE I/O curves show plateaus or dips which result in unexpectedly high compression estimates. Moderately high correlation was found between the compression-threshold estimates obtained with the two methods, although DPOAE-based values were systematically lower than those based on the TMCs. Both methods confirmed that compression ratio and threshold are approximately constant across the CF range considered. Cochlear gain could be estimated from TMCs only and it was found to increase slightly with increasing CF. It is discussed that DPOAE I/O curves may be inappropriate to infer precise individual nonlinear cochlear characteristics in normal-hearing listeners, particularly at low CFs [Work supported by IMSERSO 131/06, PROFIT CIT-390000-2005-4, MEC BFU-2006-07536, and The Oticon Foundation.]

Timing and Spectral Properties of the Accreting Millisecond Pulsar SWIFT J1756.9−2508

SWIFT J1756.9-2508 is one of the few accreting millisecond pulsars (AMPs) discovered to date. We report here the results of our analysis of its aperiodic X-ray variability, as measured with the Rossi X-Ray Timing Explorer during the 2007 outburst of the source. We detect strong (~35%) flat-topped broadband noise throughout the outburst with low characteristic frequencies (~0.1 Hz). This makes SWIFT J1756.9-2508 similar to the rest of AMPs and to other low-luminosity accreting neutron stars when they are in their hard states, and enables us to classify this AMP as an atoll source in the extreme island state. We also find a hard tail in its energy spectrum extending up to 100 keV, fully consistent with such source and state classification.