Influence of fundamental frequency on stop-consonant voicing perception: a case of learned covariation or auditory enhancement?

For stimuli modeling stop consonants varying in the acoustic correlates of voice onset time (VOT), human listeners are more likely to perceive stimuli with lower f0's as voiced consonants--a pattern of perception that follows regularities in English speech production. The present study examines the basis of this observation. One hypothesis is that lower f0's enhance perception of voiced stops by virtue of perceptual interactions that arise from the operating characteristics of the auditory system. A second hypothesis is that this perceptual pattern develops as a result of experience with f0-voicing covariation. In a test of these hypotheses, Japanese quail learned to respond to stimuli drawn from a series varying in VOT through training with one of three patterns of f0-voicing covariation. Voicing and f0 varied in the natural pattern (shorter VOT, lower f0), in an inverse pattern (shorter VOT, higher f0), or in a random pattern (no f0-voicing covariation). Birds trained with stimuli that had no f0-voicing covariation exhibited no effect of f0 on response to novel stimuli varying in VOT. For the other groups, birds' responses followed the experienced pattern of covariation. These results suggest f0 does not exert an obligatory influence on categorization of consonants as [VOICE] and emphasize the learnability of covariation among acoustic characteristics of speech.

Neighboring spectral content influences vowel identification.

Four experiments explored the relative contributions of spectral content and phonetic labeling in effects of context on vowel perception. Two 10-step series of CVC syllables ([bVb] and [dVd]) varying acoustically in F2 midpoint frequency and varying perceptually in vowel height from [delta] to [epsilon] were synthesized. In a forced-choice identification task, listeners more often labeled vowels as [delta] in [dVd] context than in [bVb] context. To examine whether spectral content predicts this effect, nonspeech-speech hybrid series were created by appending 70-ms sine-wave glides following the trajectory of CVC F2's to 60-ms members of a steady-state vowel series varying in F2 frequency. In addition, a second hybrid series was created by appending constant-frequency sine-wave tones equivalent in frequency to CVC F2 onset/offset frequencies. Vowels flanked by frequency-modulated glides or steady-state tones modeling [dVd] were more often labeled as [delta] than were the same vowels surrounded by nonspeech modeling [bVb]. These results suggest that spectral content is important in understanding vowel context effects. A final experiment tested whether spectral content can modulate vowel perception when phonetic labeling remains intact. Voiceless consonants, with lower-amplitude more-diffuse spectra, were found to exert less of an influence on vowel perception than do their voiced counterparts. The data are discussed in terms of a general perceptual account of context effects in speech perception.

Role of experience for language-specific functional mappings of vowel sounds.

Studies involving human infants and monkeys suggest that experience plays a critical role in modifying how subjects respond to vowel sounds between and within phonemic classes. Experiments with human listeners were conducted to establish appropriate stimulus materials. Then, eight European starlings (Sturnus vulgaris) were trained to respond differentially to vowel tokens drawn from stylized distributions for the English vowels /i/ and /I/, or from two distributions of vowel sounds that were orthogonal in the F1-F2 plane. Following training, starlings' responses generalized with facility to novel stimuli drawn from these distributions. Responses could be predicted well on the bases of frequencies of the first two formants and distributional characteristics of experienced vowel sounds with a graded structure about the central "prototypical" vowel of the training distributions. Starling responses corresponded closely to adult human judgments of "goodness" for English vowel sounds. Finally, a simple linear association network model trained with vowels drawn from the avian training set provided a good account for the data. Findings suggest that little more than sensitivity to statistical regularities of language input (probability-density distributions) together with organizational processes that serve to enhance distinctiveness may accommodate much of what is known about the functional equivalence of vowel sounds.

Depolarizing the perceptual magnet effect.

In recent years there has been a great deal of interest in demonstrations of the so-called "Perceptual-Magnet Effect" (PME). In these studies, AX-discrimination tasks purportedly reveal that discriminability of speech sounds from a single category varies with judged phonetic "goodness" of the sounds. However, one possible confound is that category membership is determined by identification of sounds in isolation, whereas, discrimination tasks include pairs of stimuli. In the first experiment of the current study, identification and goodness judgments were obtained for vowels (/i/-/e/) presented in pairs. A substantial shift in phonetic identity was evidenced with changes in the context vowel. In a second experiment, listeners participated in an AX-discrimination task with the vowel pairs from the first experiment. Using the contextual identification functions from the first experiment, predictions of discriminability were calculated using the classic tenets of Categorical Perception. Obtained discriminability functions were well accounted for by predictions from identification. There was no additional unexplained variance that required the proposal of "perceptual magnets." These results suggest that PME may be nothing more than further demonstration that general discriminability is greater for cross-category stimulus pairs than for within-category pairs.

General contrast effects in speech perception: effect of preceding liquid on stop consonant identification.

When members of a series of synthesized stop consonants varying acoustically in F3 characteristics and varying perceptually from /da/ to /ga/ are preceded by /al/, subjects report hearing more /ga/ syllables relative to when each member is preceded by /ar/ (Mann, 1980). It has been suggested that this result demonstrates the existence of a mechanism that compensates for coarticulation via tacit knowledge of articulatory dynamics and constraints, or through perceptual recovery of vocal-tract dynamics. The present study was designed to assess the degree to which these perceptual effects are specific to qualities of human articulatory sources. In three experiments, series of consonant-vowel (CV) stimuli varying in F3-onset frequency (/da/-/ga/) were preceded by speech versions or nonspeech analogues of /al/ and /ar/. The effect of liquid identity on stop consonant labeling remained when the preceding VC was produced by a female speaker and the CV syllable was modeled after a male speaker's productions. Labeling boundaries also shifted when the CV was preceded by a sine wave glide modeled after F3 characteristics of /al/ and /ar/. Identifications shifted even when the preceding sine wave was of constant frequency equal to the offset frequency of F3 from a natural production. These results suggest an explanation in terms of general auditory processes as opposed to recovery of or knowledge of specific articulatory dynamics.

Perceptual compensation for coarticulation by Japanese quail (Coturnix coturnix japonica).

When members of a series of synthesized stop consonants varying in third-formant (F3) characteristics and varying perceptually from /da/ to /ga/ are preceded by /al/, human listeners report hearing more /ga/ syllables than when the members of the series are preceded by /ar/. It has been suggested that this shift in identification is the result of specialized processes that compensate for acoustic consequences of coarticulation. To test the species-specificity of this perceptual phenomenon, data were collected from nonhuman animals in a syllable "labeling" task. Four Japanese quail (Coturnix coturnix japonica) were trained to peck a key differentially to identify clear /da/ and /ga/ exemplars. After training, ambiguous members of a /da/-/ga/ series were presented in the context of /al/ and /ar/ syllables. Pecking performance demonstrated a shift which coincided with data from humans. These results suggest that processes underlying "perceptual compensation for coarticulation" are species-general. In addition, the pattern of response behavior expressed is rather common across perceptual systems.

Effect of voice quality on perceived height of English vowels.

Across a variety of languages, phonation type and vocal-tract shape systematically covary in vowel production. Breathy phonation tends to accompany vowels produced with a raised tongue body and/or advanced tongue root. A potential explanation for this regularity, based on a hypothesized interaction between the acoustic effects of vocal-tract shape and phonation type, is evaluated. It is suggested that increased spectral tilt and first-harmonic amplitude resulting from breathy phonation interact with the lower-frequency first formant resulting from a raised tongue body to produce a perceptually 'higher' vowel. To test this hypothesis, breathy and modal versions of vowel series modelled after male and female productions of English vowel pairs /i/ and /i/, /u/ and /[symbol: see text]/, and /lamda/ and /a/ were synthesized. Results indicate that for most cases, breathy voice quality led to more tokens being identified as the higher vowel (i.e. /i/, /u/, /lamda/). In addition, the effect of voice quality is greater for vowels modelled after female productions. These results are consistent with a hypothesized perceptual explanation for the covariation of phonation type and tongue-root advancement in West African languages. The findings may also be relevant to gender differences in phonation type.

Spectral discontinuities and the vowel length effect.

Perception of voicing for stop consonants in consonant-vowel syllables can be affected by the duration of the following vowel so that longer vowels lead to more "voiced" responses. On the basis of several experiments, Green, Stevens, and Kuhl (1994) concluded that continuity of fundamental frequency (f0), but not continuity of formant structure, determined the effective length of the following vowel. In an extension of those efforts, we found here that both effects were critically dependent on particular f0s and formant values. First, discontinuity in f0 does not necessarily preclude the vowel length effect because the effect maintains when f0 changes from 200 to 100 Hz, and 200-Hz partials extend continuously through test syllables. Second, spectral discontinuity does preclude the vowel length effect when formant changes result in a spectral peak shifting to another harmonic. The results indicate that the effectiveness of stimulus changes for sustaining or diminishing the vowel length effect depends critically on particulars of spectral composition.

Perception of voicing for syllable-initial stops at different intensities: does synchrony capture signal voiceless stop consonants?

In response to stop consonants with longer F1-cutback duration, the dominant synchronization of mid- and high-CF chinchilla auditory-nerve fibers changes from frequencies near F2 to frequencies near F1 at onset of voicing [D. G. Sinex and L. P. McDonald, J. Acoust. Soc. Am. 85, 1995-2004 (1989)]. If this change in neural synchronization is perceptually relevant for human listeners, then it may be predicted that changes in stimulus intensity and changes in the frequency difference between lower (F1) and higher (F2/F3) stimulus components should both affect perception of voicing. In a series of experiments, multiple continua of synthesized CVs varying in F1 cutback of the consonantal portion were played to listeners at levels ranging from 40 to 80 dB SPL. Across experiments, the frequency difference between F1 and F2 was manipulated by changing the onset frequency of F1 or F2. Subjects labeled more initial stops as voiceless as a function of increasing stimulus level and of decreasing frequency difference between F1 and F2. There was also an interaction between stimulus intensity and the frequency difference between F1 and F2 such that the effect of intensity was greater for smaller differences. These effects were reliable across a number of synthetic F1-cutback series, and the effect of intensity extended to a digitally edited series of hybrid CVs in which F1-cutback was varied by cross splicing naturally produced /da/ and /ta/. The effect of overall stimulus intensity was not affected by amplitude of prevocalic aspiration energy or by the presence or absence of release bursts. The results provide evidence for the perceptual significance of synchrony encoding of voicing for stop consonants.