Assessing the relationship between talker normalization and spectral contrast effects in speech perception.

The perception of any given sound is influenced by surrounding sounds. When successive sounds differ in their spectral compositions, these differences may be perceptually magnified, resulting in spectral contrast effects (SCEs). For example, listeners are more likely to perceive /ɪ/ (low F1) following sentences with higher F1 frequencies; listeners are also more likely to perceive /ɛ/ (high F1) following sentences with lower F1 frequencies. Previous research showed that SCEs for vowel categorization were attenuated when sentence contexts were spoken by different talkers [Assgari and Stilp. (2015). J. Acoust. Soc. Am. 138(5), 3023-3032], but the locus of this diminished contextual influence was not specified. Here, three experiments examined implications of variable talker acoustics for SCEs in the categorization of /ɪ/ and /ɛ/. The results showed that SCEs were smaller when the mean fundamental frequency (f0) of context sentences was highly variable across talkers compared to when mean f0 was more consistent, even when talker gender was held constant. In contrast, SCE magnitudes were not influenced by variability in mean F1. These findings suggest that talker variability attenuates SCEs due to diminished consistency of f0 as a contextual influence. Connections between these results and talker normalization are considered.

The identity of a speech sound can be affected by the spectrum of a preceding stimulus in a contrastive manner. Although such aftereffects are often reduced in people with hearing loss and cochlear implants (CIs), one recent study demonstrated larger spectral contrast effects in CI users than in normal-hearing (NH) listeners. The present study aimed to shed light on this puzzling finding. We hypothesized that poorer spectral resolution leads CI users to rely on different acoustic cues not only to identify speech sounds but also to adapt to the context. Thirteen postlingually deafened adult CI users and 33 NH participants (listening to either vocoded or unprocessed speech) participated in this study. Psychometric functions were estimated in a vowel categorization task along the /I/ to /ε/ (as in "bit" and "bet") continuum following a context sentence, the long-term average spectrum of which was manipulated at the level of either fine-grained local spectral cues or coarser global spectral cues. In NH listeners with unprocessed speech, the aftereffect was determined solely by the fine-grained local spectral cues, resulting in a surprising insensitivity to the larger, global spectral cues utilized by CI users. Restricting the spectral resolution available to NH listeners via vocoding resulted in patterns of responses more similar to those found in CI users. However, the size of the contrast aftereffect remained smaller in NH listeners than in CI users. Only the spectral contrasts used by listeners contributed to the spectral contrast effects in vowel identification. These results explain why CI users can experience larger-than-normal context effects under specific conditions. The results also suggest that adaptation to new spectral cues can be very rapid for vowel discrimination, but may follow a longer time course to influence spectral contrast effects.

Cortical mechanisms of talker normalization in fluent sentences

1 The Relevance of Psychophysics for Speech Perception.- The role of psychophysics in understanding speech perception.- Central and peripheral processes in the perception of speech and nonspeech sounds.- Psychophysics versus specialized processes in speech perception: an alternative perspective.- Speech perception and the role of long-term memory.- Levels of representation of phonemes and bandwidth of spectral-temporal integration.- General discussion of session 1.- 2 Separation of Acoustic Events.- The meaning of duplex perception: sounds as transparant objects.- Perceptual separation of speech from concurrent sounds.- Sound separation and auditory perceptual organization.- On the significance of spectral synchrony for signal detection.- Auditory enhancement in speech perception.- General discussion of session 2.- 3 Dynamic Aspects.- Trading relations, acoustic cue integration, and context effects in speech perception.- Perceptual integration of rise time and silence in affricate/fricative and pluck/bow continua.- Reversal of the rise-time cue in the affricate/fricative contrast: an experiment on the silence of sound.- Possible acoustic bases for the perception of voicing contrasts.- Is there a natural sensitivity at 20ms in relative tone-onset-time continua? A reanalysis of Hirsh's (1959) data.- Auditory constraints on speech perception.- Studies of possible psychoacoustic factors underlying speech perception.- Perception of tone, band, and formant sweeps.- Psychophysical representation of stop consonant and temporal masking in speech.- Effects of stimulus dynamics on frequency discrimination.- Extending the search for a psychophysical basis for dynamic phonetic patterns.- General discussion of session 3.- 4 Timbre (Peripheral Constraints and Central Processes in the Perception of Complex Signals)..- Psychophysics of audio signal processing and the role of pitch in speech..- Does the human auditory system include large scale spectral integration?.- Some aspects of the sound of speech sounds.- Involvement of the critical band in identification, perceived distance, and discrimination of vowels.- Profile analysis and speech perception.- General discussion of session 4.- 5 Physiological Correlates of Speech Perception.- Peripheral auditory processing of speech information: implications from a physiological study of intensity discrimination.- Organization of the cochlear nucleus for information processing.- Changes in the phonemic quality and neural representation of a vowel by alteration of the relative phase of harmonics near F1.- Phase vowels.- Nonlinear responses, in the auditory nerve to vowel-related complex stimuli.- Discussion of physiological correlates of speech perception.- General discussion of session 5.- 6 Primary Speech Percepts.- English and French speech processing: some psycholinguistic investigations.- Units of organization and analysis in the perception of speech.- Implications from infant speech studies on the unit of perception.- General discussion of session 6.- 7 Psychophysics and Speech Perception in the Hearing-Impaired.- Relationship between psychophysical abilities and speech perception for subjects with unilateral and bilateral cochlear hearing impairments.- Speech-reception threshold in a fluctuating background sound and its possible relation to temporal auditory resolution.- Differences in listening strategies between normal and hearing-impaired listeners.- Critical bands in the perception of speech signals by normal and sensorineural hearing loss listeners.- Phase and the hearing-impaired.

Background Speech perception in noisy environments is significantly influenced by a listener’s ability to segregate competing auditory streams, a process in which fundamental frequency (F0) plays a crucial role, particularly under informational masking conditions. F0 differences, quantified in semitone shifts, facilitate stream segregation by enhancing perceptual distinctiveness between target and masker signals. Although this phenomenon has been extensively studied in various languages, there is limited research on its effects in Malayalam, a Dravidian language characterized by rich prosodic features. The present study investigated how semitone-based variations in F0 affect the perception of low-frequency PB Malayalam words in the presence of gender-specific speech maskers. Twenty-seven young adults (13 males, 14 females) aged 18 to 25 years, all native Malayalam speakers with normal hearing sensitivity, participated in the study. An adaptive SNR-50 paradigm was employed to assess speech perception across eight experimental conditions involving four semitone shifts (0, 4, 8, and 12 ST) and two-talker babble maskers of either male or female voices. Stimuli were presented binaurally, and participants completed a closed-set image selection task to indicate their responses. Results Significant main effects were observed for masker gender, with better performance in the different-gender (male) masker condition. Although semitone shifts alone did not yield significant differences, a significant interaction between semitone and masker gender was noted. Performance was consistently highest at 0 ST across both masker types. Conclusion These findings suggest that while increased F0 separation can aid speech perception, voice naturalness at 0 ST may offer greater perceptual stability. Results have implications for understanding pitch-based stream segregation in Malayalam and improving clinical speech perception strategies.

The present study provides an elaboration of the "double-weak" theory of speech perception proposed by Nearey (1990, 1991). In this framework, the objects of speech perception (and production) are viewed as neither primarily auditory nor primarily gestural; rather, they are abstract, symbolic elements lawfully constrained to map onto relatively simple (but not entirely transparent) patterns in both domains. Speech cannot be understood unless both articulation and acoustics are considered: Many production strategies appear to be directed at achieving acoustically-oriented goals, yet most context effects in speech perception seem to be motivated by the consequences of gestural overlap. The double-weak framework suggests that speech perception and speech production are less-than-perfect inverses of each other. Despite long-term accommodation of each for the demands of the other, real-time production and perception may operate as autonomous subsystems. A family of perceptual models is discussed that provides varying degrees of approximation to "ideal solutions" (in the sense of minimizing error rate) to classifying production data exhibiting contextual interactions. Members of this family that provide substantial, yet incomplete, compensation for the consequences of gestural overlap appear to be adequate to account for the results of many speech perception experiments. Such partial perceptual compensation allows, in principle, for the kind of imperfect "error correction" discussed by Ohala (1981, 1990) in conjunction with hypo- and hypercorrection phenomena.

The ability to perceive speech is related to the listener's ability to differentiate among frequencies (i.e., spectral resolution). Cochlear implant (CI) users exhibit variable speech-perception and spectral-resolution abilities, which can be attributed in part to the extent of electrode interactions at the periphery (i.e., spatial selectivity). However, electrophysiological measures of peripheral spatial selectivity have not been found to correlate with speech perception. The purpose of this study was to evaluate auditory processing at the periphery and cortex using both simple and spectrally complex stimuli to better understand the stages of neural processing underlying speech perception. The hypotheses were that (1) by more completely characterizing peripheral excitation patterns than in previous studies, significant correlations with measures of spectral selectivity and speech perception would be observed, (2) adding information about processing at a level central to the auditory nerve would account for additional variability in speech perception, and (3) responses elicited with spectrally complex stimuli would be more strongly correlated with speech perception than responses elicited with spectrally simple stimuli. Eleven adult CI users participated. Three experimental processor programs (MAPs) were created to vary the likelihood of electrode interactions within each participant. For each MAP, a subset of 7 of 22 intracochlear electrodes was activated: adjacent (MAP 1), every other (MAP 2), or every third (MAP 3). Peripheral spatial selectivity was assessed using the electrically evoked compound action potential (ECAP) to obtain channel-interaction functions for all activated electrodes (13 functions total). Central processing was assessed by eliciting the auditory change complex with both spatial (electrode pairs) and spectral (rippled noise) stimulus changes. Speech-perception measures included vowel discrimination and the Bamford-Kowal-Bench Speech-in-Noise test. Spatial and spectral selectivity and speech perception were expected to be poorest with MAP 1 (closest electrode spacing) and best with MAP 3 (widest electrode spacing). Relationships among the electrophysiological and speech-perception measures were evaluated using mixed-model and simple linear regression analyses. All electrophysiological measures were significantly correlated with each other and with speech scores for the mixed-model analysis, which takes into account multiple measures per person (i.e., experimental MAPs). The ECAP measures were the best predictor. In the simple linear regression analysis on MAP 3 data, only the cortical measures were significantly correlated with speech scores; spectral auditory change complex amplitude was the strongest predictor. The results suggest that both peripheral and central electrophysiological measures of spatial and spectral selectivity provide valuable information about speech perception. Clinically, it is often desirable to optimize performance for individual CI users. These results suggest that ECAP measures may be most useful for within-subject applications when multiple measures are performed to make decisions about processor options. They also suggest that if the goal is to compare performance across individuals based on a single measure, then processing central to the auditory nerve (specifically, cortical measures of discriminability) should be considered.

Previous studies, using modulation stimuli, on the relative effects of frequency resolution and time resolution on CI users’ speech perception failed to reach a consistent conclusion. In this study, frequency change detection and temporal gap detection were used to investigate the frequency resolution and time resolution of CI users, respectively. Psychophysical and neurophysiological methods were used to simultaneously investigate the effects of frequency and time resolution on speech perception in post-lingual cochlear implant (CI) users. We investigated the effects of psychophysical results [frequency change detection threshold (FCDT), gap detection threshold (GDT)], and acoustic change complex (ACC) responses (evoked threshold, latency, or amplitude of ACC induced by frequency change or temporal gap) on speech perception [recognition rate of monosyllabic words, disyllabic words, sentences in quiet, and sentence recognition threshold (SRT) in noise]. Thirty-one adult post-lingual CI users of Mandarin Chinese were enrolled in the study. The stimuli used to induce ACCs to frequency changes were 800-ms pure tones (fundamental frequency was 1,000 Hz); the frequency change occurred at the midpoint of the tones, with six percentages of frequency changes (0, 2, 5, 10, 20, and 50%). Temporal silences with different durations (0, 5, 10, 20, 50, and 100 ms) were inserted in the middle of the 800-ms white noise to induce ACCs evoked by temporal gaps. The FCDT and GDT were obtained by two 2-alternative forced-choice procedures. The results showed no significant correlation between the CI hearing threshold and speech perception in the study participants. In the multiple regression analysis of the influence of simultaneous psychophysical measures and ACC responses on speech perception, GDT significantly predicted every speech perception index, and the ACC amplitude evoked by the temporal gap significantly predicted the recognition of disyllabic words in quiet and SRT in noise. We conclude that when the ability to detect frequency changes and the temporal gap is considered simultaneously, the ability to detect frequency changes may have no significant effect on speech perception, but the ability to detect temporal gaps could significantly predict speech perception.

The aim of this study was to examine the role of fundamental frequency (F0) information in improving speech perception of individuals with a cochlear implant (CI) who use a contralateral hearing aid (HA). The authors hypothesized that in bilateral-bimodal (CI/HA) users the perception of natural prosody speech would be superior to the perception of speech with monotonic flattened F0 contour, whereas in unilateral CI users the perception of both speech signals would be similar. They also hypothesized that in the CI/HA listening condition the speech perception scores would improve as a function of the magnitude of the difference between the F0 characteristics of the target speech signal and the F0 characteristics of the competitors, whereas in the CI-alone condition such a pattern would not be recognized, or at least not as clearly. Two tests were administered to 29 experienced CI/HA adult users who, regardless of their residual hearing or speech perception abilities, had chosen to continue using an HA in the nonimplanted ear for at least 75% of their waking hours. In the first test, the difference between the perception of speech characterized by natural prosody and speech characterized by monotonic flattened F0 contour was assessed in the presence of babble noise produced by three competing male talkers. In the second test the perception of semantically unpredictable sentences was evaluated in the presence of a competing reversed speech sentence spoken by different single talkers with different F0 characteristics. Each test was carried out under two listening conditions: CI alone and CI/HA. Under both listening conditions, the perception of speech characterized by natural prosody was significantly better than the perception of speech in which monotonic F0 contour was flattened. Differences between the scores for natural prosody and for monotonic flattened F0 speech contour were significantly greater, however, in the CI/HA condition than with CI alone. In the second test, the overall scores for perception of semantically unpredictable sentences in the presence of all competitors were higher in the CI/HA condition in the presence of all competitors. In both listening conditions, scores increased significantly with increasing difference between the F0 characteristics of the target speech signal and the F0 characteristics of the competitor. The higher scores obtained in the CI/HA condition than with CI alone in both of the task-specific tests suggested that the use of a contralateral HA provides improved low-frequency information, resulting in better performance by the CI/HA users.

Effects of testosterone on speech production and perception: Linking hormone levels in males to vocal cues and female voice attractiveness ratings

The phonetic distinction between the stop consonant /b/ and semivowel /w/ is cued primarily by duration of the initial formant transition. Syllable duration, however, is a context that influences human adults' and infants' perception of this distinction. For example, when overall syllable duration is long (296 ms), infants show poor discrimination of syllables with transition durations of 16 and 40 ms, but good discrimination of syllables with transition durations of 40 and 64 ms. However, when overall syllable duration is short (80 ms), the opposite is true [P. D. Eimas and J. L. Miller, Science 209, 1140–1141 (1980)]. The current study addresses whether rhesus and Japanese macaques' perception of the /b-w/ distinction is also influenced by syllable duration: Six CV syllables used in the Eimas and Miller (1980) study were presented to five macaques using a same-different discrimination paradigm. Results showed that macaques' pattern of discrimination was similar to the pattern shown by infants, thus supporting an auditory-based mechanism for the effect of syllable duration in phonetic perception. [Work supported by NSF and NIH.]

Functional imaging studies of speech perception have revealed extensive involvement of the dorsolateral temporal lobes in aspects of speech and voice processing. In the current study, fMRI was used as a functional imaging method to address the perception of speech in different masking conditions. Throughout the scanning experiment, subjects were directed to listen to a female talker with whom they had been familiarized previously. In addition to the acoustic noise generated by the scanner, different kinds of masking sound were also presented diotically, at SPL levels determined by pilot testing. The masking sounds were signal correlated noise, spectrally rotated speech, and a second female talker. The results show a network of activation in the bilateral temporal lobes, prefrontal cortex and parietal lobes that were commonly activated across all masking conditions. Controlling for the effects of energetic masking, informational masking gave rise exclusively to clusters of activity in bilateral STG, with peak level activations in left anterior-mid STG and posterior STS/STG and right primary auditory cortex. Post-test intelligibility measures were used to reveal greater activation in the left temporal lobe, which was associated with higher performance scores.

According to P. K. Kuhl (1991), a perceptual magnet effect occurs when discrimination accuracy is lower among better instances of a phonetic category than among poorer instances. Three experiments examined the perceptual magnet effect for the vowel /i/. In Experiment 1, participants rated some examples of /i/ as better instances of the category than others. In Experiment 2, no perceptual magnet effect was observed with materials based on Kuhl's tokens of /i/ or with items normed for each participant. In Experiment 3, participants labeled the vowels developed from Kuhl's test set. Many of the vowels in the nonprototype /i/ condition were not categorized as /i/s. This finding suggests that the comparisons obtained in Kuhl's original study spanned different phonetic categories.

Two synthetic speech syllables, differing in pitch, were presented over one loudspeaker situated at one of the following azimuths: 0 degrees (in from of the subject); 45 degrees, 90 degrees, and 135 degrees to the left or to the right; and 180 degrees. The subject's task was to report the high-pitch syllable. The pattern of performance can be described in terms of two component effects: a decreasing anterior-posterior gradient, and right-side advantage. Competition between messages from one direction only appears to be as effective in determining auditory spatial effects as competition between spatially distinct sources.

The “Perceptual Magnet Effect” (PME) is characterized by perceptual warping in favor of stronger assimilation near the prototypical sound against other nonprotypical variants. This ERP study adopted a modified oddball paradigm to examine the PME predictions. The participants were 18 normal adults. The stimuli were computer-synthesized /a/ sounds and their acoustic controls by replacing the formant frequencies with sinewaves. Prior to the ERP sessions, the prototype (P) and nonprototype (NP) were chosen based on behavioral identification and goodness rating. Then 4 variants were created with ±40 mels apart in reference to P or NP, respectively, in either the first formant (F1) or second formant (F2). Mismatch negativity (MMN) responses were derived with the P (or NP) as the standard and the 4 variants as the deviant. The same ERP procedure was used for the nonspeech stimuli. Behavioral results showed that individual subjects consistently rated vowel category goodness based on the F2/F1 ratio. MMN amplitudes in the P and NP conditions showed a pattern consistent with the PME model. This pattern was observed for both speech and nonspeech stimuli. These results provide neural evidence in support of the PME that may extend to general auditory processing of spectral patterns.