Minimal spectral contrast of formant peaks for vowel recognition as a function of spectral slope

In four experiments we investigated whether listeners can locate the formants of vowels not only from peaks, but also from spectral “shoulders”—features that give rise to zero crossings in the third, but not the first, differential of the excitation pattern—as hypothesized by Assmann and Summerfield (1989). Stimuli were steady-state approximations to the vowels [a, i, з, u, ?] created by summing the first 45 harmonics of a fundamental of 100 Hz. Thirty-nine harmonics had equal amplitudes; the other 6 formed three pairs that were raised in level to define three “formants.” An adaptive psychophysical procedure determined the minimal difference in level between the 6 harmonics and the remaining 39 at which the vowels were identifiably different from one another. These thresholds were measured through simulated communication channels, giving overall slopes to the excitation patterns of the five vowels that ranged from ?1 dB/erb to +2 dB/erb. Excitation patterns of the threshold stimuli were computed, and the locations of formants were estimated from zero crossings in the first and third differentials. With the more steeply sloping communication channels, some formants of some vowels were represented as shoulders rather than peaks, confirming the predictions of Assmann and Summerfield’s models. We discuss the limitations of the excitation pattern model and the related issue of whether the location of formants can be computed from spectral shoulders in auditory analysis.     

Perception of temporal order in vowel sequences with and without formant transitions.

Temporal-order perception of phoneme segments in running speech is much superior to temporal-order perception in repeating vowel sequences. The more rapid rates possible in running speech may be due largely to the presence of formant transitions. In a series of five experiments we observed that many temporal-order misjudgements of repeating vowels can be explained in terms of auditory stream segregation, triggered for the most part by discontinuities in first-formant frequencies of adjacent vowels. Streaming, however, can be suppressed by formant transitions appropriate for the perception of stop consonants and by continuous transitions resembling those in coarticulated vowels. At rapid sequence rates, when the constraints of auditory streaming are removed, correct temporal-order identification is limited by linguistic transformations of vowels into other phoneme segments.     

The role of vowel formant frequencies and duration in the perception of foreign accent

ABSTRACT

This study used synthesis to manipulate vowel formant frequencies and durations to evaluate their role on foreign accent perception. Formant frequencies and durations for the vowels /æ/, /?/, and /a/ were manipulated with changes toward and away from the mean native English and Spanish-accented values from Sidaras, S. K., Alexander, J. E. D., & Nygaard, L. C. (2009. Perceptual learning of systematic variation in Spanish-accented speech. The Journal of the Acoustical Society of America, 125, 3306–3316). Native listeners rated these stimuli on degree of accentedness and comprehensibility. Gradual changes in formant frequencies from native to non-native values impacted /a/ negatively, /?/ positively, and /æ/ minimally. Effects of vowel duration on either type of ratings were small and restricted to vowel-specific interactions. The current findings suggest that vowel formant frequencies are primary cues to foreign accent. Their influence depends upon whether or not frequencies could reflect alternative vowel categories.     

Acoustic and phonological factors in vowel identification

Vowels are better identified in a consonantal syllabic context than as isolated vowels. This finding is contrary to predictions from traditional theories of vowel perception. The poor perception of isolated vowels might be attributed to a lack of dynamic acoustic cues or to familiarity effects related to the phonological rules of English. Vowel identification tests were conducted using six talkers, nine vowels, and seven syllabic contexts. Consonantal context improved vowel identification; final consonants aided identification more than initial consonants. No consistent support was found for the effect of phonological rules but duration information was seen to play a critical role. Results constitute a challenge to traditional theories of vowel perception.     

Contextual effects in vowel perception I: Anchor-induced contrast effects

Results from recent experiments using a selective adaption paradigm with vowels have been interpreted as the result of the fatigue of a set of feature detectors. These results could also be interpreted, however, as resulting from changes in auditory memory (auditory contrast) or changing response criteria (response bias). In the present studies, subjects listened to vowels under two conditions: an equiprobable control, with each of the stimuli occurring equally often, and an anchoring condition, with one vowel occurring more often than any of the others. Contrast effects were found in that vowel category boundaries tended to shift toward the category of the anchor, relative to the equiprobable control. Results from these experiments were highly similar to previous selective adaptation results and suggest that neither feature detector fatigue nor response criterion changes can adequately account for the adaptation/ anchoring results found with vowels.     

Phonetic discrimination ability of a native vowel contrast in preterm infants

Preterm birth is considered a risk factor for cognitive and linguistic development; however, research focusing on the comparison between preterm and full term infants' early abilities in speech perception is still scarce. In this study, the ability to perceive and discriminate a native vowel contrast by a sample of preterm infants at 4 and 8 months of age (corrected for gestation) has been analyzed using the familiarization-preference procedure and complex stimuli (CV.CV, multispeakers). Results reveal differences at 4 months of age between pre-term and control groups; only the latter successfully categorize and react to the vowel change. By 8 months of age, preterm infants are able to solve the task. A positive correlation was observed between duration of attention (visual fixation measures) and a number of neonatal risk factors. Results show the effects of preterm birth on the processing of complex stimuli and suggest the need to further explore the connection between early speech perception capacities and language development in this at-risk population.     

Linguistic and nonlinguistic auditory processing of rapid vowel formant (F2) modulations in university students with and without developmental dyslexia

To help resolve the issue of whether developmental dyslexia (DD) is related to central auditory processing deficits or to language-specific processing deficits, we had nine dyslectic and nine nondyslectic right-handed undergraduate students perform linguistic (Experiment 1: phoneme identification) and nonlinguistic (Experiment 2: formant rate change detection) tasks. In Experiment 1, subjects listened to synthetic vowels whose second formant (F2) was modulated sinusoidally with F1, F3, and F4 held constant. F2 modulation rate (4-18 Hz) was manipulated within and across stimuli. The groups did not differ in phoneme identification. Experiment 2 was run three times and showed that the control subjects' performance improved across runs whereas the dyslexics' deteriorated across runs (p < .0001), suggesting practice and fatigue effects, respectively. Performance on the two experiments correlated significantly and negatively for the dyslexic subjects only. These results suggest that resource depletion or frontal lobe dysfunction may be implicated in developmental dyslexia.     

Effects of vowel variety and sample lenght on identification of a speaker in a line-up

Subjects (average age 21 years, recruited by personal contact and through a school) were presented with a spoken sentence on tape and then heard six speakers of the same sex, including the original speaker, say the same sentence. They were required to indicate which was the original speaker. The task was repeated with seven different sentences and sets of speakers. One group of subjects heard short sentences containing an average of 2.14 different vowel sounds and 6.28 syllables, another group heard short sentences containing an average of 6.14 vowel sounds (7.28 syllables) and a third group heard longer sentences containing an average of 6.28 vowel sounds (11.00 syllables). Accuracy of speaker identification improved significantly when more vowel sounds were heard, but increased sentence length had no significant effect on performance. Performance was significantly better when the listener was the same sex as the speaker than when the listener was of the other sex.     

Phonemic identification in a phoneme monitoring experiment: The variable role of uncertainty about vowel contexts

Subjects monitored for the syllable-initial phonemes /b/ and /s/, as well as for the syllables containing those phonemes, in lists of nonsense syllables. Time to detect /b/ was a function of the amount of uncertainty as to the identity of the vowel following the target consonant; when uncertainty was low, no difference existed between phoneme and syllable monitoring latencies, but when uncertainty was high, syllables were detected faster than phonemes. Time to detect /s/ was independent of uncertainty concerning the accompanying vowel and was always slower than syllable detection. The role of knowledge of contexts in a phoneme-monitoring task as well as the relative availability of phonemic information to the listener in this task are discussed.     

Comparison of the effect of onset asynchrony on auditory grouping in pitch matching and vowel identification

Previous experiments have shown that when a slightly mistuned harmonic of a complex tone starts more than about 80 msec before the remaining components, it makes a reduced contribution to the pitch of the complex. This contribution decreases to zero by about 300-msec onset asynchrony. In vowel perception, however, analogous experiments have shown that a much shorter asynchrony (around 40 msec) is enough to ensure that a component does not influence a vowel’s phonemic category. The three experiments reported here demonstrate that this difference in the utility of onset time as a grouping cue does not arise because of differences in stimulus structure, but rather is due to the perceptual task. They show that the onset asynchrony needed in a pitch-matching experiment to remove the contribution that a mistuned component makes to the pitch of a vowel is the same as that needed to remove the contribution to the pitch of a flat-spectrum complex tone. They further show that a much smaller onset asynchrony is needed to perceptually remove the same harmonic from a vowel for the calculation of vowel quality. The implication of this result for models of auditory grouping is discussed.     

On the relationship between vowel and consonant identification when cued by the same acoustic information

When listening to speech, do we recognize syllables or phonemes? Information concerning the organization of the decisions involved in identifying a syllable may be elicited by allowing separate phonetic decisions regarding the vowel and consonant constituents to be controlled by the same acoustic information and by looking for evidence of interaction between these decisions. The duration and first formant frequency of the steady-state vocalic segment in synthesized consonant-vowel-consonant syllables were varied to result in responses of /bεd/, /bæd/, /bεt/, and /bæt/. The fact that the duration of the steady-state segment controls both decisions implies that that segment must be included in its entirety in the signal intervals on which the two decisions are based. For most subjects, no further significant interaction between the vocalic and consonantal decision is found beyond the fact that they are both affected by changes in the duration parameter. A model of two separate and independent phonetic decisions based on overlapping ranges of the signal adequately accounts for these data, and no explicit syllable level recognition needs to be introduced.     

Stressed vowel perception in word recognition

The purpose of this study was to measure subjects' ability to detect deliberate stressed, front-vowel misarticulations embedded in two-syllable words. Reaction times to words with various vowel-height misarticulations were examined for 25 women to assess the effect of a specific vowel height change on listeners' ability to recognize a word. Statistical analysis indicated no significant differences between reaction times to initial, stressed vowel changes along the height dimension, suggesting that subjects responded similarly to all vowel errors on the detection task. This finding provides further evidence that stressed-vowel information may serve as a perceptual anchor in guiding a listener during word recognition.     

Phonemic vowel length contrasts in cerebellar disorders

Apraxia of speech and Broca's aphasia both affect voice onset time (VOT) whereas phonemic vowel length distinctions seem to be preserved. Assuming a close cooperation of anterior perisylvian language zones and the cerebellum with respect to speech timing, a similar profile of segment durations must be expected in ataxic dysarthria. In order to test this hypothesis, patients with cerebellar atrophy or cerebellar ischemia were asked to produce sentence utterances including either one of the German lexial items "Rate" (/ra:t(h)e/, 'installment'), "Ratte" (/rat(h)e/, 'rat'), "Gram" (/gra:m/, 'grief'), "Gramm" (/gram/, 'gramm'), "Taten" (/t(h)atn/, 'actions'), or "Daten" (/datn/, 'data'). At the acoustic signal, the duration of the target vowels /a/ and /a:/ as well as the VOT of the word-initial alveolar stops /d/ and /t/ were determined. In addition, a master tape comprising the target words from patients and controls in randomized order was played to three listeners for perceptual evaluation. In accordance with a previous study, first, the cerebellar subjects presented with a reduced categorical separation of the VOT of voiced and unvoiced stop consonants. Second, vowel length distinctions were only compromised in case of the minimal pair "Gram"/"Gramm." In contrast to "Rate"/"Ratte", production of the former lexical items requires coordination of several orofacial structures. Disruption of vowel length contrasts would, thus, depend upon the complexity of the underlying articulatory pattern.     

The missing fundamental in vowel height perception

Traunmüller (1981) suggested that the tonotopic distance between the first formant (F ₁) and the fundamental frequency (F ₀) is a major determinant of perceived vowel height. In the present study, subjects identified a vowel-height continuum ranging in formant pattern from /?/ to /ε/, at fiveF ₀ values. IncreasingF ₀ led to an increased probability of /I/ responses (i.e., the phoneme boundary shifted toward the /ε/ end of the continuum). Various conditions of filtering out the lower harmonics of the stimuli caused only marginal shifts of the phoneme boundary. The experiments provide evidence against interpretations of Traunmüller’s (1981) results that claim that vowel height is determined by the distance betweenF ₁ and the lowest harmonic that is present in the basilar membrane excitation pattern.     

Parameters for vowel substitutions in Broca''s aphasia

Most phonological investigations of aphasic speech have focused on consonants. In this study, the vowel substitutions of five English-speaking Broca's aphasics were investigated with respect to several major explanatory hypotheses. An important possible motivating factor for aphasic substitutions, phonological environment, is an unlikely origin for these errors, since most substitutions occur along the paradigmatic axis. Another prominent hypothesis, markedness, also has no predictive power for the data of this sample. However, an analysis of the five Chomsky and Halle (1968, (The Sound Pattern of English), New York: Harper and Row) vowel features indicates that the two features for vowel height are selectively impaired: There is a strong tendency for lower vowels to replace higher targets. Moreover, the substitution sounds tend to be close to their target sounds along the height axis. Four possible hypotheses are advanced to account for this consistent lowering tendency.     

Decay of auditory memory in vowel discrimination

Two experiments on same-different vowel discrimination are reported. In each, the main variable was the duration of a silent delay between the two items being judged. As would be expected from the assumption that such judgements depend at least partly on auditory sensory memory, longer delays led to poorer discrimination than shorter delays. The auditory memory loss seems to be asymptotic at about 3 sec, whether it is measured by correct discrimination of (as in one part of Experiment 2) by the contextual influence of the first vowel on identification of the second.