Task-modulated Sensitivity to Vocal Pitch in the Dorsal Premotor Cortex during Multitalker Speech Recognition.

It has long been known that listening to speech activates inferior frontal (pre-)motor regions in addition to a more dorsal premotor site (dPM). Recent work shows that dPM, located adjacent to laryngeal motor cortex, responds to low-level acoustic speech cues including vocal pitch, and the speech envelope, in addition to higher-level cues such as phoneme categories. An emerging hypothesis is that dPM is part of a general auditory-guided laryngeal control circuit that plays a role in producing speech and other voluntary auditory-vocal behaviors. We recently reported a study in which dPM responded to vocal pitch during a degraded speech recognition task, but only when speech was rated as unintelligible; dPM was more robustly modulated by the categorical difference between intelligible and unintelligible speech. Contrary to the general auditory-vocal hypothesis, this suggests intelligible speech is the primary driver of dPM. However, the same pattern of results was observed in pitch-sensitive auditory cortex. Crucially, vocal pitch was not relevant to the intelligibility judgment task, which may have facilitated processing of phonetic information at the expense of vocal pitch cues. The present fMRI study (n = 25) tests the hypothesis that, for a multitalker task that emphasizes pitch for talker segregation, left dPM and pitch-sensitive auditory regions will respond to vocal pitch regardless of overall speech intelligibility. This would suggest that pitch processing is indeed a primary concern of this circuit, apparent during perception only when the task demands it. Spectrotemporal modulation distortion was used to independently modulate vocal pitch and phonetic content in two-talker (male/female) utterances across two conditions (Competing, Unison), only one of which required pitch-based segregation (Competing). A Bayesian hierarchical drift-diffusion model was used to predict speech recognition performance from patterns of spectrotemporal distortion imposed on each trial. The model's drift rate parameter, a d'-like measure of performance, was strongly associated with vocal pitch for Competing but not Unison. Using a second Bayesian hierarchical model, we identified regions where behaviorally relevant acoustic features were related to fMRI activation in dPM. We regressed the hierarchical drift-diffusion model's posterior predictions of trial-wise drift rate, reflecting the relative presence or absence of behaviorally relevant acoustic features from trial to trial, against trial-wise activation amplitude. A significant positive association with overall drift rate, reflecting vocal pitch and phonetic cues related to overall intelligibility, was observed in left dPM and bilateral auditory cortex in both conditions. A significant positive association with "pitch-restricted" drift rate, reflecting only the relative presence or absence of behaviorally relevant pitch cues, regardless of the presence or absence of phonetic content (intelligibility), was observed in left dPM, but only in the Competing condition. Interestingly, the same effect was observed in bilateral auditory cortex but in both conditions. A post hoc mediation analysis ruled out the possibility that decision load was responsible for the observed pitch effects. These findings suggest that processing of vocal pitch is a primary concern of the auditory-cortex-dPM circuit, although during perception core pitch, processing is carried out by auditory cortex with a potential modulatory influence from dPM.

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.

Syllable-constituent perception by hearing-aid users: Common factors in quiet and noise.

The abilities of 59 adult hearing-aid users to hear phonetic details were assessed by measuring their abilities to identify syllable constituents in quiet and in differing levels of noise (12-talker babble) while wearing their aids. The set of sounds consisted of 109 frequently occurring syllable constituents (45 onsets, 28 nuclei, and 36 codas) spoken in varied phonetic contexts by eight talkers. In nominal quiet, a speech-to-noise ratio (SNR) of 40 dB, scores of individual listeners ranged from about 23% to 85% correct. Averaged over the range of SNRs commonly encountered in noisy situations, scores of individual listeners ranged from about 10% to 71% correct. The scores in quiet and in noise were very strongly correlated, R = 0.96. This high correlation implies that common factors play primary roles in the perception of phonetic details in quiet and in noise. Otherwise said, hearing-aid users' problems perceiving phonetic details in noise appear to be tied to their problems perceiving phonetic details in quiet and vice versa.

Spectrotemporal modulation sensitivity for hearing-impaired listeners: dependence on carrier center frequency and the relationship to speech intelligibility.

Poor speech understanding in noise by hearing-impaired (HI) listeners is only partly explained by elevated audiometric thresholds. Suprathreshold-processing impairments such as reduced temporal or spectral resolution or temporal fine-structure (TFS) processing ability might also contribute. Although speech contains dynamic combinations of temporal and spectral modulation and TFS content, these capabilities are often treated separately. Modulation-depth detection thresholds for spectrotemporal modulation (STM) applied to octave-band noise were measured for normal-hearing and HI listeners as a function of temporal modulation rate (4-32 Hz), spectral ripple density [0.5-4 cycles/octave (c/o)] and carrier center frequency (500-4000 Hz). STM sensitivity was worse than normal for HI listeners only for a low-frequency carrier (1000 Hz) at low temporal modulation rates (4-12 Hz) and a spectral ripple density of 2 c/o, and for a high-frequency carrier (4000 Hz) at a high spectral ripple density (4 c/o). STM sensitivity for the 4-Hz, 4-c/o condition for a 4000-Hz carrier and for the 4-Hz, 2-c/o condition for a 1000-Hz carrier were correlated with speech-recognition performance in noise after partialling out the audiogram-based speech-intelligibility index. Poor speech-reception and STM-detection performance for HI listeners may be related to a combination of reduced frequency selectivity and a TFS-processing deficit limiting the ability to track spectral-peak movements.

Vowel identification by amplitude and phase contrast.

Vowel identification is largely dependent on listeners' access to the frequency of two or three peaks in the amplitude spectrum. Earlier work has demonstrated that, whereas normal-hearing listeners can identify harmonic complexes with vowel-like spectral shapes even with very little amplitude contrast between "formant" components and remaining harmonic components, listeners with hearing loss require greater amplitude differences. This is likely the result of the poor frequency resolution that often accompanies hearing loss. Here, we describe an additional acoustic dimension for emphasizing formant versus non-formant harmonics that may supplement amplitude contrast information. The purpose of this study was to determine whether listeners were able to identify "vowel-like" sounds using temporal (component phase) contrast, which may be less affected by cochlear loss than spectral cues, and whether overall identification improves when congruent temporal and spectral information are provided together. Five normal-hearing and five hearing-impaired listeners identified three vowels over many presentations. Harmonics representing formant peaks were varied in amplitude, phase, or a combination of both. In addition to requiring less amplitude contrast, normal-hearing listeners could accurately identify the sounds with less phase contrast than required by people with hearing loss. However, both normal-hearing and hearing-impaired groups demonstrated the ability to identify vowel-like sounds based solely on component phase shifts, with no amplitude contrast information, and they also showed improved performance when congruent phase and amplitude cues were combined. For nearly all listeners, the combination of spectral and temporal information improved identification in comparison to either dimension alone.

Cortical encoding of signals in noise: effects of stimulus type and recording paradigm.

OBJECTIVES: Perception-in-noise deficits have been demonstrated across many populations and listening conditions. Many factors contribute to successful perception of auditory stimuli in noise, including neural encoding in the central auditory system. Physiological measures such as cortical auditory-evoked potentials (CAEPs) can provide a view of neural encoding at the level of the cortex that may inform our understanding of listeners' abilities to perceive signals in the presence of background noise. To understand signal-in-noise neural encoding better, we set out to determine the effect of signal type, noise type, and evoking paradigm on the P1-N1-P2 complex. DESIGN: Tones and speech stimuli were presented to nine individuals in quiet and in three background noise types: continuous speech spectrum noise, interrupted speech spectrum noise, and four-talker babble at a signal-to-noise ratio of -3 dB. In separate sessions, CAEPs were evoked by a passive homogenous paradigm (single repeating stimulus) and an active oddball paradigm. RESULTS: The results for the N1 component indicated significant effects of signal type, noise type, and evoking paradigm. Although components P1 and P2 also had significant main effects of these variables, only P2 demonstrated significant interactions among these variables. CONCLUSIONS: Signal type, noise type, and evoking paradigm all must be carefully considered when interpreting signal-in-noise evoked potentials. Furthermore, these data confirm the possible usefulness of CAEPs as an aid to understand perception-in-noise deficits.

Discrimination of time-reversed harmonic complexes by normal-hearing and hearing-impaired listeners.

Normal-hearing (NH) listeners and hearing-impaired (HI) listeners detected and discriminated time-reversed harmonic complexes constructed of equal-amplitude harmonic components with fundamental frequencies (F0s) ranging from 50 to 800 Hz. Component starting phases were selected according to the positive and negative Schroeder-phase algorithms to produce within-period frequency sweeps with relatively flat temporal envelopes. Detection thresholds were not affected by component starting phases for either group of listeners. At presentation levels of 80 dB SPL, NH listeners could discriminate the two waveforms nearly perfectly when the F0s were less than 300-400 Hz but fell to chance performance for higher F0s. HI listeners performed significantly poorer, with reduced discrimination at several of the F0s. In contrast, at a lower presentation level meant to nearly equate sensation levels for the two groups, NH listeners' discrimination was poorer than HI listeners at most F0s. Roving presentation levels had little effect on performance by NH listeners but reduced performance by HI listeners. The differential impact of roving level suggests a weaker perception of timbre differences and a greater susceptibility to the detrimental effects of experimental uncertainty in HI listeners.

Perception of dissonance by people with normal hearing and sensorineural hearing loss.

The purpose of this study was to determine whether the perceived sensory dissonance of pairs of pure tones (PT dyads) or pairs of harmonic complex tones (HC dyads) is altered due to sensorineural hearing loss. Four normal-hearing (NH) and four hearing-impaired (HI) listeners judged the sensory dissonance of PT dyads geometrically centered at 500 and 2000 Hz, and of HC dyads with fundamental frequencies geometrically centered at 500 Hz. The frequency separation of the members of the dyads varied from 0 Hz to just over an octave. In addition, frequency selectivity was assessed at 500 and 2000 Hz for each listener. Maximum dissonance was perceived at frequency separations smaller than the auditory filter bandwidth for both groups of listners, but maximum dissonance for HI listeners occurred at a greater proportion of their bandwidths at 500 Hz than at 2000 Hz. Further, their auditory filter bandwidths at 500 Hz were significantly wider than those of the NH listeners. For both the PT and HC dyads, curves displaying dissonance as a function of frequency separation were more compressed for the HI listeners, possibly reflecting less contrast between their perceptions of consonance and dissonance compared with the NH listeners.

Acoustic reflexes to Schroeder-phase harmonic complexes in normal-hearing and hearing-impaired individuals.

Harmonic complexes generated with positive or negative Schroeder-phases may result in differences in cochlear excitation, even though their long-term spectra and amplitudes are equal. As a measure of possible differences in cochlear excitation resulting from these harmonic complexes, thresholds and growth of the acoustic reflex were assessed in normal-hearing and hearing-impaired subjects. Harmonic complexes with fundamental frequencies of 50, 100, and 200 Hz were constructed with positive and negative-Schroeder phases. In normal-hearing subjects, acoustic reflex thresholds for the 50- and 100-Hz fundamental waveforms were typically lower for negative Schroeder-phase complexes than for positive Schroeder phase stimuli. At the highest fundamental frequency of 200 Hz, there were no significant threshold differences due to phase. Hearing-impaired subjects showed a similar pattern for thresholds between the two phase selections, but with smaller differences than those observed in normal-hearing subjects. At levels above reflex threshold, the magnitude of the acoustic reflex was greater for the negative-phase than the positive-phase stimuli for the lowest fundamental frequency, but no significant differences were observed at fundamental frequencies of 100 and 200 Hz. These results are consistent with generally greater cochlear excitation in response to negative than to positive Schroeder-phase stimuli when the fundamental frequency is sufficiently low. Increased excitation may reflect a synchronization of response across a wide band of frequencies in the cochlea when the rate of frequency sweep within periods of these harmonic complexes is appropriately matched to timing characteristics of the traveling wave.