Rippled Noise Research Articles

Acoustic change complex for assessing speech discrimination in normal-hearing and hearing-impaired infants

ObjectiveThis study examined (1) the utility of a clinical system to record acoustic change complex (ACC, an event-related potential recorded by electroencephalography) for assessing speech discrimination in infants, and (2) the relationship between ACC and functional performance in real life. MethodsParticipants included 115 infants (43 normal-hearing, 72 hearing-impaired), aged 3–12 months. ACCs were recorded using [szs], [uiu], and a spectral rippled noise high-pass filtered at 2 kHz as stimuli. Assessments were conducted at age 3–6 months and at 7–12 months. Functional performance was evaluated using a parent-report questionnaire, and correlations with ACC were examined. ResultsThe rates of onset and ACC responses of normal-hearing infants were not significantly different from those of aided infants with mild or moderate hearing loss but were significantly higher than those with severe loss. On average, response rates measured at 3–6 months were not significantly different from those at 7–12 months. Higher rates of ACC responses were significantly associated with better functional performance. ConclusionsACCs demonstrated auditory capacity for discrimination in infants by 3–6 months. This capacity was positively related to real-life functional performance. SignificanceACCs can be used to evaluate the effectiveness of amplification and monitor development in aided hearing-impaired infants.

The Maturation of the Acoustic Change Complex in Response to Iterated Ripple Noise in 'Normal'-Hearing Infants, Toddlers, and Adults.

Infants and toddlers are still being evaluated for their hearing sensitivity but not their auditory-processing skills. Iterated rippled noise (IRN) stimuli require the auditory system to utilize the temporal periodicity and autocorrelate the iterations to perceive pitch. This study investigated the acoustic change complex (ACC) elicited by IRN in "normal"-hearing infants, toddlers, and adults to determine the maturation of cortical processing of IRN stimuli. Cortical responses to filtered white noise (onset) concatenated with IRN stimuli (d = 10 milliseconds, gain = 0.7 dB: 4-32 iterations) were recorded in quiet, alert participants. Participants included 25 infants (2.5-15 months), 27 toddlers (22-59 months), and 8 adults (19-25 years) with "normal" hearing sensitivity. Cortical auditory-evoked responses were recorded for each participant, including the onset response to the noise and an ACC to the transition from noise to IRN. Group differences were assessed using repeated-measures analyses of variance. Most infants had a replicable onset (P) response, while only about half had a measurable ACC (PACC) response to the high-saliency IRN condition. Most toddlers had onset responses and showed a P-NACC response to the IRN16 and IRN32 conditions. Most of the toddler group had responses present to the onset and showed a P-NACC response to all IRN conditions. Toddlers and adults showed similar P-NACC amplitudes; however, adults showed an increase in N1ACC amplitude with increase in IRN iterations (i.e., increased salience). While cortical responses to the percept of sound as determined by the onset response (P) to a stimulus are present in most infants, ACC responses to IRN stimuli are not mature in infancy. Most toddlers as young as 22 months, however, exhibited ACC responses to the IRN stimuli even when the pitch saliency was low (e.g., IRN4). The findings of the current study have implications for future research when investigating maturational effects on ACC and the optimal choice of stimuli.

Diotic and Dichotic Mechanisms of Discrimination Threshold in Musicians and Non-Musicians.

The perception of harmonic complexes provides important information for musical and vocal communication. Numerous studies have shown that musical training and expertise are associated with better processing of harmonic complexes, however, it is unclear whether the perceptual improvement associated with musical training is universal to different pitch models. The current study addresses this issue by measuring discrimination thresholds of musicians (n = 20) and non-musicians (n = 18) to diotic (same sound to both ears) and dichotic (different sounds to each ear) sounds of four stimulus types: (1) pure sinusoidal tones, PT; (2) four-harmonic complex tones, CT; (3) iterated rippled noise, IRN; and (4) interaurally correlated broadband noise, called the “Huggins” or “dichotic” pitch, DP. Frequency difference limens (DLF) for each stimulus type were obtained via a three-alternative-forced-choice adaptive task requiring selection of the interval with the highest pitch, yielding the smallest perceptible fundamental frequency (F0) distance (in Hz) between two sounds. Music skill was measured by an online test of musical pitch, melody and timing maintained by the International Laboratory for Brain Music and Sound Research. Musicianship, length of music experience and self-evaluation of musical skill were assessed by questionnaire. Results showed musicians had smaller DLFs in all four conditions with the largest group difference in the dichotic condition. DLF thresholds were related to both subjective and objective musical ability. In addition, subjective self-report of musical ability was shown to be a significant variable in group classification. Taken together, the results suggest that music-related plasticity benefits multiple mechanisms of pitch encoding and that self-evaluation of musicality can be reliably associated with objective measures of perception.

Longitudinal auditory pathophysiology following mild blast-induced trauma.

Blast-induced hearing difficulties affect thousands of veterans and civilians. The long-term impact of even a mild blast exposure on the central auditory system is hypothesized to contribute to lasting behavioral complaints associated with mild blast traumatic brain injury (bTBI). Although recovery from mild blast has been studied separately over brief or long time windows, few, if any, studies have investigated recovery longitudinally over short-term and longer-term (months) time windows. Specifically, many peripheral measures of auditory function either recover or exhibit subclinical deficits, masking deficits in processing complex, real-world stimuli that may recover differently. Thus, examining the acute time course and pattern of neurophysiological impairment using appropriate stimuli is critical to better understanding and intervening in bTBI-induced auditory system impairments. Here, we compared auditory brainstem response, middle-latency auditory-evoked potentials, and envelope following responses. Stimuli were clicks, tone pips, amplitude-modulated tones in quiet and in noise, and speech-like stimuli (iterated rippled noise pitch contours) in adult male rats subjected to mild blast and sham exposure over the course of 2 mo. We found that blast animals demonstrated drastic threshold increases and auditory transmission deficits immediately after blast exposure, followed by substantial recovery during the window of 7-14 days postblast, although with some deficits remaining even after 2 mo. Challenging conditions and speech-like stimuli can better elucidate mild bTBI-induced auditory deficit during this period. Our results suggest multiphasic recovery and therefore potentially different time windows for treatment, and deficits can be best observed using a small battery of sound stimuli.NEW & NOTEWORTHY Few studies on blast-induced hearing deficits go beyond simple sounds and sparsely track postexposure. Therefore, the recovery arc for potential therapies and real-world listening is poorly understood. Evidence suggested multiple recovery phases over 2 mo postexposure. Hearing thresholds largely recovered within 14 days and partially explained recovery. However, midlatency responses, responses to amplitude modulation in noise, and speech-like pitch sweeps exhibited extended changes, implying persistent central auditory deficits and the importance of subclinical threshold shifts.

The Yost approach to pitch perception: What we can learn from noise-like stimuli

One of Bill's major contributions to psychoacoustics is his innovative work using rippled noise (RN) to understand pitch. Bill has argued that RN-pitch percepts can be accounted for by models based on autocorrelation. Working with Bill, we showed that behavioral responses obtained from chinchillas to RNs generally paralleled those obtained from humans, suggesting the underlying processing is similar between the two species. Recent work from my lab using noise-vocoded harmonic tone complexes (NV-HTCs) was inspired by Bill's approach with RNs. NV-HTCs can also evoke pitch percepts like RNs, but present a challenge to Bill's autocorrelation approach, because they can have strong harmonic structures with weak or no periodicities. However, when NV-HTCs are passed through a gammatone filterbank model, weak stimulus periodicities become augmented in summary correlograms. An analytical model based on summary correlograms for NV-HTC responses from (1) humans in a magnitude estimation task and (2) chinchillas in a stimulus generalization task suggests that the underlying processing does indeed differ between the two species. Specifically, chinchillas appear to process the envelope whereas humans appear to process the fine structure. This talk will explore why this apparent processing difference exists, despite the similarity in chinchilla and human auditory filter bandwidths.

Detection of modulated rippled noise

Bill Yost's study of modulation detection interference (MDI) was in part motivated by his earlier work with temporally modulated rippled noise (RN) which indicated difficulty monitoring a single auditory channel in the presence of cross-channel modulation. The present work further evaluated parameters in the processing of dynamic spectral profiles. In the first experiment, gain in RN generation was sinusoidally modulated. If the gain sign remained unchanged during modulation, modulation resulted in a roughly constant increment in ripple-detection thresholds across change in modulation rate from 2 to 128 Hz. However, cross-sign gain modulation led to a lowpass result with thresholds increasing with modulation rate. A similar lowpass result was obtained in the second ripple-detection experiment in which delay in RN generation was modulated. The third experiment measured thresholds for detecting delay modulation. Thresholds, in terms of Weber ratios, rose with increasing modulation rate from 2 to 8 Hz. Across experiments, the lowpass results contrast with MDI in which interference decreases with increasing modulation rate. With dynamic spectral profiles, there is significant envelope modulation of individual auditory channels due to stimulus frequency excursions. Results suggest a limitation imposed by cross-channel envelope incoherence in utilizing this envelope modulation.

Comorbidity of Auditory Processing, Attention, and Memory in Children With Word Reading Difficulties.

ObjectivesTo document the auditory processing, visual attention, digit memory, phonological processing, and receptive language abilities of individual children with identified word reading difficulties.DesignTwenty-five children with word reading difficulties and 28 control children with good word reading skills participated. All children were aged between 8 and 11 years, with normal hearing sensitivity and typical non-verbal intelligence. Both groups of children completed a test battery designed to assess their auditory processing, visual attention, digit memory, phonological processing, and receptive language.ResultsWhen compared to children who were good readers, children with word reading difficulties obtained significantly lower average scores on tests of auditory processing, including the frequency pattern test, gaps in noise, frequency discrimination, Dichotic Digit difference Test, and Listening in Spatialized Noise. The two groups did not differ on the discrimination measures of sinusoidal amplitude modulation or iterated rippled noise. The results from children with word reading difficulties showed that 5 children (20%) had comorbid deficits in auditory processing, visual attention, and backward digit memory; whereas 12 children (48%) had comorbid auditory processing and visual attention deficits only, and 2 children (8%) had comorbid deficits in auditory processing and digit memory; the remaining children had only auditory processing, visual attention, or digit memory deficits.ConclusionThe current study highlights the general co-existence of auditory processing, memory, and visual attention deficits in children with word reading difficulties. It is also noteworthy, however, that only one fifth of the current cohort had deficits across all measured tasks. Hence, our results also show the significant individual variability inherent in children with word reading difficulties.

Cortical Pitch Response Components Correlate with the Pitch Salience of Resolved and Unresolved components of Mandarin Tones.

Cortical pitch responses (CPRs) are generated at the initiation of pitch-bearing sounds. CPR components have been shown to reflect the pitch salience of iterated rippled noise with different temporal periodicity. However, it is unclear whether features of the CPR correlate with the pitch salience of resolved and unresolved harmonics of speech when the temporal periodicity is identical, and whether CPRs could be a neural index for auditory cortical pitch processing. In this study, CPRs were recorded to two speech sounds: a set including only resolved harmonics and a set including only unresolved harmonics. Speech-shaped noise preceding and following the speech was used to temporally discriminate the neural activity coding the onset of acoustic energy from the onset of time-varying pitch. Analysis of CPR peak latency and peak amplitude (Na) showed that the peak latency to speech sounds with only resolved harmonics was significantly shorter than for sounds with unresolved harmonics (p = 0.01), and that peak amplitude to sounds with only resolved harmonics was significantly higher than for sounds with unresolved harmonics (p <; 0.001). Further, the CPR peak phase locking value in response to sounds with only resolved harmonics was significantly higher than to sounds with only unresolved harmonics (p <; 0.001). Our findings suggest that the CPR changes with pitch salience and that CPR is a potentially useful indicator of auditory cortical pitch processing.

Efficiency in glimpsing vowel sequences in fluctuating makers: Effects of temporal fine structure and temporal regularity.

Listeners' efficiency in glimpsing the target speech in amplitude-modulated maskers may depend on whether the target is perceptually segregated from the masker and on the temporal predictability of the target. Using synthesized vowel sequences as the target, recognition of vowel sequences in simultaneous amplitude-modulated noise maskers was measured as the signal-to-noise ratio (SNR) and the masker modulation rate were systematically varied. In Experiment I (Exp. I), the temporal fine structure of the target was degraded by synthesizing the vowels using iterated rippled noise as the glottal source. In Experiment II (Exp. II), the vowel sequences were constructed so that they were not isochronous, but instead contained randomized intervals between adjacent vowels. Results were compared to the predictions from a dip-listening model based on short-term SNR. The results show no significant facilitative effect of temporal fine structure cues on vowel recognition (Exp. I). The model predictions significantly overestimated vowel-recognition performance in amplitude-modulated maskers when the temporal regularity of the target was degraded (Exp. II), suggesting the influence of temporal regularity on glimpsing efficiency. Furthermore, the overestimations by the model were greater at lower SNRs and selective to moderate masker modulation rates (between 2 and 6 Hz).

Visualization of the auditory pathway in rats with 18F-FDG PET activation studies based on different auditory stimuli and reference conditions including cochlea ablation.

Activation studies with positron emission tomography (PET) in auditory implant users explained some of the mechanisms underlying the variability of achieved speech comprehension. Since future developments of auditory implants will include studies in rodents, we aimed to inversely translate functional PET imaging to rats. In normal hearing rats, activity in auditory and non-auditory regions was studied using 18F-fluorodeoxyglucose (18F-FDG) PET with 3 different acoustic conditions: sound attenuated laboratory background, continuous white noise and rippled noise. Additionally, bilateral cochlea ablated animals were scanned. 3D image data were transferred into a stereotaxic standard space and evaluated using volume of interest (VOI) analyses and statistical parametric mapping (SPM). In normal hearing rats alongside the auditory pathway consistent activations of the nucleus cochlearis (NC), olivary complex (OC) and inferior colliculus (IC) were seen comparing stimuli with background. In this respect, no increased activation could be detected in the auditory cortex (AC), which even showed deactivation with white noise stimulation. Nevertheless, higher activity in the AC in normal hearing rats was observed for all 3 auditory conditions against the cochlea ablated status. Vice versa, in ablated status activity in the olfactory nucleus (ON) was higher compared to all auditory conditions in normal hearing rats. Our results indicate that activations can be demonstrated in normal hearing animals based on 18F-FDG PET in nuclei along the central auditory pathway with different types of noise stimuli. However, in the AC missing activation with respect to the background advises the need for more rigorous background noise attenuation for non-invasive reference conditions. Finally, our data suggest cross-modal activation of the olfactory system following cochlea ablation–underlining, that 18F-FDG PET appears to be well suited to study plasticity in rat models for cochlear implantation.

Human frequency following responses to iterated rippled noise with positive and negative gain: Differential sensitivity to waveform envelope and temporal fine-structure

The perceived pitch of iterated rippled noise (IRN) with negative gain (IRNn) is an octave lower than that of IRN with positive gain (IRNp). IRNp and IRNn have identical waveform envelopes (ENV), but differing stimulus waveform fine structure (TFS), which likely accounts for this perceived pitch difference. Here, we examine whether differences in the temporal pattern of phase-locked activity reflected in the human brainstem Frequency Following Response (FFR) elicited by IRNp and IRNn can account for the differences in perceived pitch for the two stimuli. FFRs using a single onset polarity were measured in 13 normal-hearing, adult listeners in response to IRNp and IRNn stimuli with 2 ms, and 4 ms delay. Autocorrelation functions (ACFs) and Fast Fourier Transforms (FFTs) were used to evaluate the dominant periodicity and spectral pattern (harmonic spacing) in the phase-locked FFR neural activity. For both delays, the harmonic spacing in the spectra corresponded more strongly with the perceived lowering of pitch from IRNp to IRNn, compared to the ACFs. These results suggest that the FFR elicited by a single polarity stimulus reflects phase-locking to both stimulus ENV and TFS. A post-hoc experiment evaluating the FFR phase-locked activity to ENV (FFRENV), and TFS (FFRTFS) elicited by IRNp and IRNn confirmed that only the phase-locked activity to the TFS, reflected in FFRTFS, showed differences in both spectra and ACF that closely matched the pitch difference between the two stimuli. The results of the post-hoc experiment suggests that pitch-relevant information is preserved in the temporal pattern of phase-locked activity and suggests that the differences in stimulus ENV and TFS driving the pitch percept of IRNp and IRNn are preserved in the brainstem neural response. The scalp recorded FFR may provide for a noninvasive analytic tool to evaluate the relative contributions of envelope and temporal fine-structure in the neural representation of complex sounds in humans.

Perception of noise-vocoded tone complexes: A time domain analysis based on an auditory filterbank model

When a wideband harmonic tone complex (wHTC) is passed through a noise vocoder, the resulting sounds can have spectra with large peak-to-valley ratios, but little or no periodicity strength in the autocorrelation functions. We measured judgments of pitch strength for normal-hearing listeners for noise-vocoded wideband harmonic tone complexes (NV-wHTCs) relative to standard and anchor stimuli. The standard was a 1-channel NV-wHTC and the anchor was either the unprocessed wHTC or an infinitely-iterated rippled noise (IIRN). Although there is variability among individuals, the magnitude judgment functions obtained with the IIRN anchor suggest different listening strategies among individuals. In order to gain some insight into possible listening strategies, test stimuli were analyzed at the output of an auditory filterbank model based on gammatone filters. The weak periodicity strengths of NV-wHTCs observed in the stimulus autocorrelation functions are augmented at the output of the gammatone filterbank model. Six analytical models of pitch strength were evaluated based on summary correlograms obtained from the gammatone tone filterbank. The results of the filterbank analysis suggest that, contrary to the weak or absent periodicity strengths in the stimulus domain, temporal cues contribute to pitch strength perception of noise-vocoded harmonic stimuli such that listeners' judgments of pitch strength reflect a nonlinear, weighted average of the temporal information between the fine structure and the envelope.

Hearing sensitivity to gliding rippled spectrum patterns.

The sensitivity of human hearing to gliding rippled spectrum patterns of sound was investigated. The test signal was 2-oct wide rippled noise with the ripples gliding along the frequency scale. Both ripple density and gliding velocity were frequency-proportional across the signal band; i.e., the density was specified in ripples/oct and the velocity was specified in oct/s and ripple/s. The listener was required to discriminate between a test signal with gliding ripples and a non-rippled reference signal. Limits of gliding velocity were measured as a function of ripple density. The ripple gliding velocity limit decreased with an increasing ripple density: from 388.9 oct/s (388.9 ripple/s) at a ripple density of 1 ripple/oct to 11.3 oct/s (79.1 ripple/s) at a density of 7 ripple/oct. These tendencies could be approximated by log/log regression functions with slopes of 1.71 for the velocity expressed in oct/s and 0.71 for the velocity expressed in ripple/s. A qualitative model based on combined action of the excitation-pattern and the temporal-processing mechanism is suggested to explain the results.

Gliding ripple speed thresholds for rippled spectra

Rippled noise is a productive model of natural signals with complex spectrum patterns. It was used as a test signal to measure spectrum-pattern resolution both in normal-hearing listeners and in hearing-impaired listeners and users of cochlear implants. However, a variety of natural auditory signals feature combined spectro-temporal patterns. These signals may be modeled by rippled noise with ripples. In the present study, ripple gliding speed thresholds were measured in normal-hearing listeners. The highest gliding speed (expressed in oct/s or ripples/s) at which the gliding ripple pattern could be distinguished from a non-rippled noise was determined. The ripple gliding speed threshold expressed in ripple/s was inversely-proportional to the ripple density (ripples per octave, RPO); it decreased from approximately 400–500 ripple/s at RPO of 1/oct to approximately 50 ripple/s at RPO of 10/oct. Expressed in oct/s, the threshold decreased from 400 to 500 oct/s at RPO of 1/oct to 5 oct/s at RPO of 10/oct. Thus, the ripples fused into non-rippled spectrum at gliding speed more than 400 to 500 Hz/oct. At this speed, 400 to 500 ripples per second glide through an octave frequency band. [Work supported by The Russian Science Foundation, Grant 16-15-10046.]Rippled noise is a productive model of natural signals with complex spectrum patterns. It was used as a test signal to measure spectrum-pattern resolution both in normal-hearing listeners and in hearing-impaired listeners and users of cochlear implants. However, a variety of natural auditory signals feature combined spectro-temporal patterns. These signals may be modeled by rippled noise with ripples. In the present study, ripple gliding speed thresholds were measured in normal-hearing listeners. The highest gliding speed (expressed in oct/s or ripples/s) at which the gliding ripple pattern could be distinguished from a non-rippled noise was determined. The ripple gliding speed threshold expressed in ripple/s was inversely-proportional to the ripple density (ripples per octave, RPO); it decreased from approximately 400–500 ripple/s at RPO of 1/oct to approximately 50 ripple/s at RPO of 10/oct. Expressed in oct/s, the threshold decreased from 400 to 500 oct/s at RPO of 1/oct to 5 oct/s at RPO of ...

Effect of musical training on EEG measures of spectro-temporal processing

Recent studies show that musical training enhances auditory processing abilities such as sensitivity to temporal fine structure and narrowband frequency resolution. Little is known about the effect of musical training on broadband spectro-temporal processing using rippled noise. This study evaluated whether musician enhancement in frequency resolution can be generalized to broadband spectro-temporal resolution using electrophysiological measures. We tested the hypothesis that musicians have enhanced broadband spectral resolution abilities compared to non-musicians. Spectral processing was measured using P1 N1 P2 responses to spectro-temporally modulated rippled noise. The test stimulus was spectro-temporally modulation wideband (4 octaves) rippled noise from 350 to 5600 Hz. EEG recordings were made using a 32-channel standard 10/20 configuration BioSemi system with a sampling rate of 4 kHz. For each condition, 1000 repetitions of the test stimuli were presented at 100% modulation with 2 Hz temporal modulation via electrically shielded ER-3A earphones, using a passive listening paradigm. The amplitude and latency of the P1-N1-P2 components were measured to determine whether musicians had better sensitivity to spectro-temporal modulation compared to age-matched non-musicians. Results to date show slight musician enhancement in spectral processing. The clinical implications of these findings for auditory-rehabilitation will be discussed.

The effects of carrier bandwidth and intensity on spectral ripple perception in listeners with hearing loss

Listeners with hearing loss often need to listen to speech signals at high intensities to ensure proper audibility. Spectral processing deteriorates at high intensities, but signal acoustics such as bandwidth and filtering can be used to mitigate this deterioration. This study evaluated the effect of carrier bandwidth on broadband spectral processing at high intensities for individuals with hearing loss. Spectral modulation detection thresholds were measured using a rippled noise carrier with varying bandwidths from 1 to 4 octaves, to assess whether individuals with hearing loss were more susceptible to the deleterious effects of high-intensity compared to their counterparts with normal hearing. Results show that spectral processing degrades at high intensities. Listeners with hearing loss benefited from the increase in signal bandwidth at high intensities much more than their counterparts with normal hearing. Our findings suggest that spectral processing for broadband signals involve within and across ch...

Auditory stream segregation of iterated rippled noises by normal-hearing and hearing-impaired listeners.

Individuals with hearing loss are thought to be less sensitive to the often subtle variations of acoustic information that support auditory stream segregation. Perceptual segregation can be influenced by differences in both the spectral and temporal characteristics of interleaved stimuli. The purpose of this study was to determine what stimulus characteristics support sequential stream segregation by normal-hearing and hearing-impaired listeners. Iterated rippled noises (IRNs) were used to assess the effects of tonality, spectral resolvability, and hearing loss on the perception of auditory streams in two pitch regions, corresponding to 250 and 1000 Hz. Overall, listeners with hearing loss were significantly less likely to segregate alternating IRNs into two auditory streams than were normally hearing listeners. Low pitched IRNs were generally less likely to segregate into two streams than were higher pitched IRNs. High-pass filtering was a strong contributor to reduced segregation for both groups. The tonality, or pitch strength, of the IRNs had a significant effect on streaming, but the effect was similar for both groups of subjects. These data demonstrate that stream segregation is influenced by many factors including pitch differences, pitch region, spectral resolution, and degree of stimulus tonality, in addition to the loss of auditory sensitivity.

Perception of Iterated Rippled Noise Periodicity in Cochlear Implant Users

Pitch perception is more challenging for individuals with cochlear implants (CIs) than normal-hearing subjects because the signal processing by CIs is restricted. Processing and perceiving the periodicity of signals may contribute to pitch perception. Whether individuals with CIs can discern pitch within an iterated rippled noise (IRN) signal is still unclear. In a prospective controlled psychoacoustic study with 34 CI users and 15 normal-hearing control subjects, the difference limen between IRN signals with different numbers of iterations was measured. In 7 CI users and 15 normal-hearing control listeners with single-sided deafness, pitch matching between IRN and harmonic complex tones was measured. The pitch onset response (POR) following signal changes from white noise to IRN was measured electrophysiologically. The CI users could discriminate different numbers of iteration in IRN signals, but worse than normal-hearing listeners. A POR was measured for both normal-hearing subjects and CI users increasing with the pitch salience of the IRN. This indicates that the POR could serve as an objective measure to monitor progress during audioverbal therapy after CI surgery.

Does a pitch center exist in auditory cortex? A mismatch negativity study

Pitch sensation can be evoked by different acoustic cues which should be processed by different strategies for extracting pitch. However, in resent studies a general pitch center was hypothesized exist at some level of the auditory pathway for a general pitch sensation independent of other acoustic features. In this study, the existence of pitch center was examined by testing that whether the mismatch negativity (MMN) response, an index of pre-attentive auditory processing, can be elicited by pitch deviations in a passive oddball paradigm that composed of four different pitch-evoking stimuli (i.e., sinusoidal tone, resolved complex, iterated rippled noise, and pulse train). Although the pitch-evoking cues in these pitch-evoking stimuli were disparate, they evoked a fixed pitch of 200 Hz (or 283 Hz in a reversed oddball paradigm). Occasionally a deviant sound was encountered which evoked a different pitch (283 Hz, or 200 Hz in the reversed oddball paradigm). The results showed no significant MMN response was elicited by the deviant, regardless of the type of pitch-evoking stimuli, suggesting the absence of pitch center in the pre-attentive auditory processing stage.

Reconstructing spectral cues for sound localization from responses to rippled noise stimuli.

Human sound localization in the mid-saggital plane (elevation) relies on an analysis of the idiosyncratic spectral shape cues provided by the head and pinnae. However, because the actual free-field stimulus spectrum is a-priori unknown to the auditory system, the problem of extracting the elevation angle from the sensory spectrum is ill-posed. Here we test different spectral localization models by eliciting head movements toward broad-band noise stimuli with randomly shaped, rippled amplitude spectra emanating from a speaker at a fixed location, while varying the ripple bandwidth between 1.5 and 5.0 cycles/octave. Six listeners participated in the experiments. From the distributions of localization responses toward the individual stimuli, we estimated the listeners’ spectral-shape cues underlying their elevation percepts, by applying maximum-likelihood estimation. The reconstructed spectral cues resulted to be invariant to the considerable variation in ripple bandwidth, and for each listener they had a remarkable resemblance to the idiosyncratic head-related transfer functions (HRTFs). These results are not in line with models that rely on the detection of a single peak or notch in the amplitude spectrum, nor with a local analysis of first- and second-order spectral derivatives. Instead, our data support a model in which the auditory system performs a cross-correlation between the sensory input at the eardrum-auditory nerve, and stored representations of HRTF spectral shapes, to extract the perceived elevation angle.