Suyash Narendra Joshi

A Model of Electrically Stimulated Auditory Nerve Fiber Responses with Peripheral and Central Sites of Spike Generation

A computational model of cat auditory nerve fiber (ANF) responses to electrical stimulation is presented. The model assumes that (1) there exist at least two sites of spike generation along the ANF and (2) both an anodic (positive) and a cathodic (negative) charge in isolation can evoke a spike. A single ANF is modeled as a network of two exponential integrate-and-fire point-neuron models, referred to as peripheral and central axons of the ANF. The peripheral axon is excited by the cathodic charge, inhibited by the anodic charge, and exhibits longer spike latencies than the central axon; the central axon is excited by the anodic charge, inhibited by the cathodic charge, and exhibits shorter spike latencies than the peripheral axon. The model also includes subthreshold and suprathreshold adaptive feedback loops which continuously modify the membrane potential and can account for effects of facilitation, accommodation, refractoriness, and spike-rate adaptation in ANF. Although the model is parameterized using data for either single or paired pulse stimulation with monophasic rectangular pulses, it correctly predicts effects of various stimulus pulse shapes, stimulation pulse rates, and level on the neural response statistics. The model may serve as a framework to explore the effects of different stimulus parameters on psychophysical performance measured in cochlear implant listeners.

Reliability of procedures used for scaling loudness

In this study, 16 normally hearing listeners judged the loudness of 1000-Hz sinusoids using magnitude estimation (ME), magnitude production (MP), and categorical loudness scaling (CLS). Listeners in each of four groups completed the loudness scaling tasks in a different sequence on the first visit (ME, MP, CLS; MP, ME, CLS; CLS, ME, MP; CLS, MP, ME), and the order was reversed on the second visit. This design made it possible to compare the reliability of estimates of the slope of the loudness function across procedures in the same listeners. The ME data were well fitted by an inflected exponential (INEX) function, but a modified power law was used to obtain slope estimates for both ME and MP. ME and CLS were more reliable than MP. CLS results were consistent across groups, but ME and MP results differed across groups in a way that suggested influence of experience with CLS. Although CLS results were the most reproducible, they do not provide direct information about the slope of the loudness function bec...

Temporal integration of loudness measured using categorical loudness scaling and matching procedures

Temporal integration of loudness of 1 kHz tones with 5 and 200 ms durations was assessed in four subjects using two loudness measurement procedures: categorical loudness scaling (CLS) and loudness matching. CLS provides a reliable and efficient procedure for collecting data on the temporal integration of loudness and previously reported nonmonotonic behavior observed at mid-sound pressure level levels is replicated with this procedure. Stimuli that are assigned to the same category are effectively matched in loudness, allowing the measurement of temporal integration with CLS without curve-fitting, interpolation, or assumptions concerning the form of the loudness growth function.

A framework for computational modelling of interaural time difference discrimination of normal and hearing-impaired listeners

Different computational models have been developed to study the interaural time difference (ITD) perception. However, only few have used a physiologically inspired architecture to study ITD discrimination. Furthermore, they do not include aspects of hearing impairment. In this work, a framework was developed to predict ITD thresholds in listeners with normal and impaired hearing. It combines the physiologically inspired model of the auditory periphery proposed by Zilany, Bruce, Nelson, and Carney [(2009). J. Acoust. Soc. Am. 126(5), 2390-2412] as a front end with a coincidence detection stage and a neurometric decision device as a back end. It was validated by comparing its predictions against behavioral data for narrowband stimuli from literature. The framework is able to model ITD discrimination of normal-hearing and hearing-impaired listeners at a group level. Additionally, it was used to explore the effect of different proportions of outer- and inner-hair cell impairment on ITD discrimination.

Effect of hearing loss on spectral weighting of broadband signals

Several studies have shown that normal hearing (NH) listeners place greater weight on the edge frequencies of broadband sounds when they are asked to pick the louder of two intervals in a sample discrimination task. However, little is known about the effects of hearing loss on spectral weighting for broadband sounds. In a classic study of sample discrimination, Doherty and Lutfi (1996) found that listeners with hearing loss (HL) placed greater weight on frequencies in the region of the HL and proposed that components that were less audible were weighted higher. The current study investigated the effect of audibility and hearing loss on spectral weighting for loudness judgments using two spectrally overlapping 18-tone complexes (from 208- to 8708-Hz). Four NH and nine HL listeners judged the loudness of tone complexes in two conditions where the mean level of each tone in the complex was set to either 10-dB SL or 75-dB SPL. The results show that listeners placed greater weight on the high-frequency edges o...

Spectral weighting of interaural time- and level differences for broadband signals

An important ability of the auditory system is to localize sound sources in complex acoustical environments. Two important cues for localization are interaural time- and level differences (ITD, ILD). The sensitivity to these cues differs across frequency and has previously been estimated through frequency-specific detection thresholds. Detection thresholds of ITDs/ILDs are, however, affected by stimulus energy in remote spectral regions, referred to as binaural interference. In this study, the spectral weights of ITD- and ILD cues in the lateralization of a broadband signal was investigated using regression analysis. The stimuli consisted of eleven 1-ERB-wide noise bands (442 Hz-5544 Hz) containing ITD or ILD cues. In experiment 1, ITDs or ILDs were applied to the noise bands and roved independently on every trial. In experiment 2, the noise bands centred at 442 Hz and 5544 Hz were removed to investigate the effect of stimulus bandwidth. In experiment 3, the same two noise bands were present, but containe...

Modelling the effect of pulse-rate on coding of interaural time differences in listeners with cochlear implants

CIs stimulate the auditory nerve fibers (ANF) with a train of amplitude modulated current pulses. Depending on polarity, the pulses can generate spikes at different sites along the ANF. The latency difference between spikes generated at the central and the peripheral axons was found to be up to 200 μs in cats and up to 450 μs in humans. These timing differences could be the reason underlying the poor performance of CI listeners in ITD perception. A model of ANF responses to electrical stimulation (Joshi et al., 2016), which includes two sites of spike generation along the ANF was used to simulate the ANF responses to constant-amplitude and modulated pulse trains for different pulse-rates. The fidelity of the temporal coding was quantified by calculating the phase-locking value. The results show that an increase in pulse-rate leads to higher uncertainty in the site of spike generation, reduction in phase-locking, and increase in variance of its distribution. This may account for impaired ITD thresholds obs...

Effects of harmonic roving on pitch discrimination

Performance in pitch discrimination tasks is limited by variability intrinsic to listeners which may arise from peripheral auditory coding limitations or more central noise sources. The present study aimed at quantifying such “internal noise” by estimating the amount of harmonic roving required to impair pitch discrimination performance. Fundamental-frequency difference limens (F0DLs) were obtained in normal-hearing listeners with and without musical training for complex tones filtered between 1.5 and 3.5 kHz with F0s of 300 Hz (resolved harmonics) and 75 Hz (unresolved harmonics). The harmonicity of the tone complexes was varied by systematically roving the frequency of individual harmonics, which was taken from a Gaussian distribution centered on the nominal frequency in every stimulus presentation. The amount of roving was determined by the standard deviation of this distribution, which varied between 0% and 16% of the tested F0. F0DLs for resolved harmonics remained unaffected for up to 6% roving, and increased thereafter. For unresolved harmonics, performance remained stable up to larger roving values. The results demonstrate a systematic relationship between F0DLs and stimulus variability that could be used to quantify the internal noise and provide strong constraints for physiologically inspired models of pitch perception.

Nonadditivity of forward and simultaneous masking.

The current study measured the additional masking obtained for combinations of forward and simultaneous maskers as a function of forward masker bandwidth, signal delay, and simultaneous masker level. The effects of the two individual maskers were equated in all conditions. Additional masking increased with increasing masker level, increasing signal delay, and decreasing masker bandwidth. The portion of the simultaneous masker that made the greater contribution to additional masking was the part that overlapped with the signal, not with the forward masker. The changes in additional masking observed as a function of forward masker bandwidth and the interaction between the effects of forward and simultaneous maskers call into question the use of additional masking as a measure of basilar membrane compression and present problems for the use of simultaneous noise to simulate hearing loss.

Sequential dependencies in magnitude scaling of loudness

Ten normally hearing listeners used a programmable sone-potentiometer knob to adjust the level of a 1000-Hz sinusoid to match the loudness of numbers presented to them in a magnitude production task. Three different power-law exponents (0.15, 0.30, and 0.60) and a log-law with equal steps in dB were used to program the sone-potentiometer. The knob settings systematically influenced the form of the loudness function. Time series analysis was used to assess the sequential dependencies in the data, which increased with increasing exponent and were greatest for the log-law. It would be possible, therefore, to choose knob properties that minimized these dependencies. When the sequential dependencies were removed from the data, the slope of the loudness functions did not change, but the variability decreased. Sequential dependencies were only present when the level of the tone on the previous trial was higher than on the current trial. According to the attention band hypothesis [Green and Luce, 1974, Perception & Psychophysics] these dependencies arise from a process similar to selective attention, but observations of rapid adaptation of neurons in the inferior colliculus based on stimulus level statistics [Dean et al., 2005, Nature Neuroscience] would also account for the data. [Supported by NIH]