Perception Of Voice Onset Time Research Articles

Natural Boundaries in Gap Detection are Related to Categorical Perception of Stop Consonants

The hypothesis that a natural auditory psychophysical discontinuity contributes to a perceptual category boundary between voiced and voiceless English stop consonants was examined. The relationships between voice onset time (VOT) phonetic boundary and gap-detection thresholds for conditions in which the sounds delimiting the gap were acoustically identical or different were examined in 18 native young adult English speakers. It was specifically hypothesized that between-channel gap-detection thresholds, in contrast to within-channel gap thresholds, would be better predictors of categorical VOT values for listeners. The stimuli used in the between-channel gap-detection task were designed such that dissimilar leading and trailing markers of the gap approximated a stop burst and a following vowel, both in terms of temporal and spectral relationships, while being devoid of phonetic identity. The stimuli used to examine the VOT measure were synthesized bilabial stop syllables in a continuum ranging from /ba/ to /pa/. Statistically significant larger gap thresholds were found for the between-channel conditions than for the within-channel condition (p < 0.05). The center frequency of the trailing marker affected the between-channel gap thresholds with the thresholds improving as the center frequency increased (p < 0.05). Statistically significant positive correlations and predictive linear relations were found between VOT phonetic boundaries and between-channel gap thresholds (p < 0.05) but not within-channel gap thresholds (p > 0.05). A relationship between the phonetic boundary of voiced-voiceless speech sounds and the auditory temporal resolution task of detecting gaps placed within dissimilar markers, regardless of the center frequency of the trailing marker noise burst, was demonstrated. Detection of gaps between different nonspeech acoustic markers and categorical perception of VOT seems to share the same underlying perceptual timing mechanisms in native English speakers.

Auditory processing disorder in children with reading disabilities: effect of audiovisual training

Reading disability is associated with phonological problems which might originate in auditory processing disorders. The aim of the present study was 2-fold: first, the perceptual skills of average-reading children and children with dyslexia were compared in a categorical perception task assessing the processing of a phonemic contrast based on voice onset time (VOT). The medial olivocochlear (MOC) system, an inhibitory pathway functioning under central control, was also explored. Secondly, we investigated whether audiovisual training focusing on voicing contrast could modify VOT sensitivity and, in parallel, induce MOC system plasticity. The results showed an altered voicing sensitivity in some children with dyslexia, and that the most severely impaired children presented the most severe reading difficulties. These deficits in VOT perception were sometimes accompanied by MOC function abnormalities, in particular a reduction in or even absence of the asymmetry in favour of the right ear found in average-reading children. Audiovisual training significantly improved reading and shifted the categorical perception curve of certain children with dyslexia towards the average-reading children's pattern of voicing sensitivity. Likewise, in certain children MOC functioning showed increased asymmetry in favour of the right ear following audiovisual training. The training-related improvements in reading score were greatest in children presenting the greatest changes in MOC lateralization. Taken together, these results confirm the notion that some auditory system processing mechanisms are impaired in children with dyslexia and that audiovisual training can diminish these deficits.

The Perception of Voice Onset Time: An fMRI Investigation of Phonetic Category Structure

This study explored the neural systems underlying the perception of phonetic category structure by investigating the perception of a voice onset time (VOT) continuum in a phonetic categorization task. Stimuli consisted of five synthetic speech stimuli which ranged in VOT from 0 msec ([da]) to 40 msec ([ta]). Results from 12 subjects showed that the neural system is sensitive to VOT differences of 10 msec and that details of phonetic category structure are retained throughout the phonetic processing stream. Both the left inferior frontal gyrus (IFG) and cingulate showed graded activation as a function of category membership with increasing activation as stimuli approached the phonetic category boundary. These results are consistent with the view that the left IFG is involved in phonetic decision processes, with the extent of activation influenced by increased resources devoted to resolving phonetic category membership and/or selecting between competing phonetic categories. Activation patterns in the cingulate suggest that it is sensitive to stimulus difficulty and resolving response conflict. In contrast, activation in the posterior left middle temporal gyrus and the left angular gyrus showed modulation of activation only to the "best fit" of the phonetic category, suggesting that these areas are involved in mapping sound structure to its phonetic representation. The superior temporal gyrus (STG) bilaterally showed weaker sensitivity to the differences in phonetic category structure, providing further evidence that the STG is involved in the early analysis of the sensory properties of speech.

Perception of the voicing distinction in speech produced during simultaneous communication

This study investigated the perception of voice onset time (VOT) in speech produced during simultaneous communication (SC). Four normally hearing, experienced sign language users were recorded under SC and speech alone (SA) conditions speaking stimulus words with voiced and voiceless initial consonants embedded in a sentence. Twelve hearing-impaired listeners participated, with three of them randomly assigned to audit the speech sample provided by each one of the four speakers under the SC and SA conditions. In addition, 24 normal hearing listeners were randomly assigned to audit the speech samples produced by the four speakers under the SC and SA conditions, three listeners in noise and three listeners in filtered listening conditions for each of the four speakers. Although results indicated longer sentence durations for SC than SA, results showed no difference in the perception of the voicing distinction for speech produced during SC versus speech produced during SA under either the noise or filtered listening condition, or any difference in perception for the hearing-impaired listeners. This conclusion is consistent with previous research indicating that temporal alterations produced by SC do not produce degradation of temporal or spectral cues in speech or disruption of the perception of specific English phoneme segments. Learning outcomes As a result of this activity, the participant will be able to: (1) describe simultaneous communication; (2) explain the role of simultaneous communication in communication with persons who are hearing-impaired; (3) discuss methods of measuring perception of voice onset time with hearing-impaired listeners and with hearing listeners under filtered and noise conditions; and (4) specify the ability of listeners to perceive the voicing distinction in speech produced during simultaneous communication.

The role of stimulus uncertainty in speech perception

Among the important experimental factors that affect psychophysical measurements of speech perception is stimulus uncertainty. Charles Watson has defined stimulus uncertainty as variation in stimulus parameters from trial to trial and demonstrated its highly degrading effects on a variety of complex auditory signals. Watson, Kelley, and Wroton showed large (×10) elevation of frequency-discrimination thresholds for ‘‘word-length tonal patterns’’ under high uncertainty conditions [J. Acoust. Soc. Am. 60, 1176–1186 (1976)]. Investigations of speech, such as the perception of VOT (voice onset time) in stops [Kewley-Port, Watson, and Foyle, J. Acoust. Soc. Am. 83, 1113–1145 (1988)] and discrimination of vowel formants [Kewley-Port, J. Acoust. Soc. Am. 110 (2001)], have also demonstrated the systematic and profound effects of higher levels of stimulus uncertainty. This presentation will discuss extensions of the concept of stimulus uncertainty that demonstrate the degrading effects of the variability in more natural speech (versus synthetic speech) and longer phonetic context (including sentences) on vowel formant discrimination. Results from normal-hearing and hearing-impaired listeners demonstrating similar detrimental effects of high stimulus uncertainty will also be presented. [Research supported by NIH-NIDCD.]

Physiological evidence for a temporal processing mechanism underlying voice-onset time (VOT) encoding

Despite decades of psychoacoustical research, the detailed neural mechanisms underlying VOT encoding remain obscure. Evidence collected from direct recordings in auditory cortex of human subjects undergoing surgical evaluation for medically intractable epilepsy, and from primary auditory cortex in monkeys, supports a temporal processing mechanism as a principal means by which VOT is encoded by the brain. This mechanism, as proposed by Pisoni [J. Acoust. Soc. Am. 77, 1352–1361 (1977)], argues that the perceptual discrimination of voiced from unvoiced stop consonants is based, in part, on whether consonant release and voicing onset are perceived as occurring simultaneously or sequentially. Neural activity in auditory cortex offers physiologically plausible parallels to this perceptual scheme that can help account for the distribution of typical VOT values used by the majority of the world’s languages, categorical perception of VOT, and perceptual boundary shifts that occur with changes in stop-consonant place of articulation and when nonspeech analogs of VOT are used. These responses in primary auditory cortex are poised to provide powerful inputs to later processing areas, where they can be integrated with other acoustical, visual, and language-related inputs known to modulate VOT perception. [Work supported by DC00657 and DC00120.]

The role of F1 in the perception of voice onset time and voice offset time.

An important speech cue is that of voice onset time (VOT), a cue for the perception of voicing and aspiration in word-initial stops. Preaspiration, an [h]-like sound between a vowel and the following stop, can be cued by voice offset time, a cue which in most respects mirrors VOT. In Icelandic VOffT is much more sensitive to the duration of the preceding vowel than is VOT to the duration of the following vowel. This has been explained by noting that preaspiration can only follow a phonemically short vowel. Lengthening of the vowel, either by changing its duration or by moving the spectrum towards that appropriate for a long vowel, will thus demand a longer VOffT to cue preaspiration. An experiment is reported showing that this greater effect that vowel quantity has on the perception of VOffT than on the perception of VOT cannot be explained by the effect of F1 frequency at vowel offset.

Role of F1 in the perception of voice offset time as a cue for preaspiration

Preaspiration, an [h]-like sound inserted between a vowel and a following stop consonant, can be cued by voice offset time (VOffT), a speech cue which is a mirror image of voice onset time (VOT). Previous research has shown VOffT to be an effective cue for preaspiration and also that the perception of VOffT differs from the perception of VOT in that the former cue is much more sensitive to the duration of a preceding vowel than is VOT to the duration of a following vowel [J. Pind, Q. J. Exp. Psychol. 49A, 745–764 (1996)]. The hypothesis has been put forward that this is due to the fact that preaspiration in Icelandic follows a phonemically short vowel, whereas aspiration can precede either a long or a short vowel. Vowel quantity will therefore affect phoneme boundaries for VOffT, not for VOT. This hypothesis was confirmed in experiments where vowel spectra were used to manipulate quantity rather than vowel duration. The present paper reports experiments investigating whether this effect could possibly be mediated through the effect of F1 on perceived voicing. [Supported by Icelandic Science Foundation and University of Iceland.]

Effects of local speaking rate context on the perception of voice-onset time in initial stop consonants.

This study explored the prediction that local speaking rate context affects the perception of voice-onset time (VOT) in initial stop consonants not only for ambiguous stimuli at the category boundary, but also for good exemplars from the category center [Volaitis and Miller, J. Acoust, Soc. Am. 92, 723-735 (1992)]. Naturally produced exemplars of the voiceless phonetic category were presented in the context of syllables produced at fast or slow speaking rates in a series of perceptual tasks, including phonetic discrimination, identification, and goodness rating. The results from all three tasks revealed no effects of speaking rate context on the perception of VOT in initial stop consonants. Rather, it appears that listeners perceive longer VOTs as better exemplars of the voiceless phonetic category, irrespective of the rate context. Further, an additional condition, in which the instructions and familiarization tasks presented prior to the administration of the perceptual tests were similar to those used by Volaitis and Miller [J. Acoust. Soc. Am. 92, 723-735 (1992)], revealed that the previous results may have been influenced by the experimental design.

Interactions between acoustic dimensions contributing to the perception of voicing

It is well known that several dimensions contribute to the classification of stop consonants as voiced or voiceless. Three Garner speeded classification experiments evaluated the nature of perceptual interactions between such dimensions. Specifically, the experiments examined the potential integrality between pairs of three critical dimensions—voice onset time (VOT), aspiration amplitude, and fundamental frequency (F0) at the onset of voicing. Only within-category (/ka/) syllable stimuli were used to eliminate phonetic classification as a basis for subjects’ responses. Evidence was found for the asymmetric separability of VOT and F0; while variability along F0 significantly influenced VOT classification speed, variability along VOT did not similarly influence F0 classification. A similar pattern of asymmetric separability for F0 and aspiration amplitude was demonstrated, where aspiration variability did not interfere with F0 classification. In contrast, VOT and aspiration amplitude were completely integral, with variability along either dimension influencing classification speed along the other dimension. Furthermore, performance accuracy reflected a time-intensity trade-off between perception of VOT and aspiration. The results indicate that F0 and VOT/aspiration amplitude may contribute to two distinct sets of perceptual analyzers. Implications for phoneme perception will be discussed. [Work supported by NIH.]

Talker continuity and the use of rate information during phonetic perception.

Research has shown that speaking rate provides an important context for the perception of certain acoustic properties of speech. For example, syllable duration, which varies as a function of speaking rate, has been shown to influence the perception of voice onset time (VOT) for syllable-initial stop consonants. The purpose of the present experiments was to examine the influence of syllable duration when the initial portion of the syllable was produced by one talker and the remainder of the syllable was produced by a different talker. A short-duration and a long-duration /bi/-/pi/ continuum were synthesized with pitch and formant values appropriate to a female talker. When presented to listeners for identification, these stimuli demonstrated the typical effect of syllable duration on the voicing boundary: a shorter VOT boundary for the short stimuli than for the long stimuli. An /i/ vowel, synthesized with pitch and formant values appropriate to a male talker, was added to the end of each of the short tokens, producing a new hybrid continuum. Although the overall syllable duration of the hybrid stimuli equaled the original long stimuli, they produced a VOT boundary similar to that for the short stimuli. In a second experiment, two new /i/ vowels were synthesized. One had a pitch appropriate to a female talker with formant values appropriate to a male talker; the other had a pitch appropriate to a male talker and formants appropriate to a female talker. These vowels were used to create two new hybrid continua. In a third experiment, new hybrid continua were created by using more extreme male formant values. The results of both experiments demonstrated that the hybrid tokens with a change in pitch acted like the short stimuli, whereas the tokens with a change in formants acted like the long stimuli. A fourth experiment demonstrated that listeners could hear a change in talker with both sets of hybrid tokens. These results indicate that continuity of pitch but not formant structure appears to be the critical factor in the calculation of speaking rate within a syllable.

Effects of preceding context on the voice-onset-time-category boundary.

The category boundary on a voice-onset-time (VOT) continuum ranging from bin to pin shifts when the stimuli are preceded by different carrier phrases. By using a variety of precursor phrases, by varying the temporal interval between precursor and test word, and by selectively eliminating either voicing information or spectral structure from the precursors, the present experiments show that the context effect is caused by the presence or absence of voicing in the precursor's final segment. The effect decreases with temporal separation but persists over several seconds. In addition, the "baseline" VOT boundary for isolated test words interspersed in a test sequence shifts depending on what precursor stimuli occur in the same sequence. The perception of VOT thus seems to be sensitive to both close and distant manifestations of laryngeal activity, always in an assimilative fashion, which suggests an integrative perceptual mechanism with a long time constant.

Synchronized discharge rate representation of voice-onset time in the chinchilla auditory nerve.

Responses of chinchilla auditory nerve fibers to synthesized stop consonant syllables differing in voice-onset time (VOT) were obtained. The syllables, heard as /ga/-/ka/ or /da/-/ta/, were similar to those previously used by others in psychophysical experiments with human and chinchilla subjects. Synchronized discharge rates of neurons tuned to frequencies near the first formant increased at the onset of voicing for VOTs longer than 20 ms. Stimulus components near the formant or the neuron's characteristic frequency accounted for the increase. In these neurons, synchronized response changes were closely related to the same neuron's average discharge rates [D. G. Sinex and L. P. McDonald, J. Acoust. Soc. Am. 83, 1817-1827 (1988)]. Neurons tuned to frequency regions near the second and third formants usually responded to components near the second formant prior to the onset of voicing. These neurons' synchronized discharges could be captured by the first formant at the onset of voicing or with a latency of 50-60 ms, whichever was later. Since these neurons' average rate responses were unaffected by the onset of voicing, the latency of the synchronized response did provide as much additional neural cue to VOT. Overall, however, discharge synchrony did not provide as much information about VOT as was provided by the best average rate responses. The results are compared to other measurements of the peripheral encoding of speech sounds and to aspects of VOT perception.

Auditory constraints on the perception of voice-onset time: the influence of lower tone frequency on judgments of tone-onset simultaneity.

The experiments reported employed nonspeech analogs of speech stimuli to examine the perceptual interaction between first-formant onset frequency and voice-onset time, acoustic cues to the voicing distinction in English initial stop consonants. The nonspeech stimuli comprised two pure tones varying in relative onset time, and listeners were asked to judge the simultaneity of tone onsets. These judgments were affected by the frequency of the lower tone in a manner that parallels the influence of first-formant onset frequency on voicing judgments. This effect was shown to occur regardless of prior learning and to be systematic over a wide range of lower tone frequencies including frequencies beyond the range of possible first-formant frequencies of speech, suggesting that the effect in speech is not attributable to (tacit) knowledge of production constraints, as some current theories suggest.

A neural model of the mechanisms underlying infant perception of voice-onset time

A neural model of the mechanisms underlying infant perception of voice-onset time

Effects of preceding context on perception of voice onset time

When discriminating pairs of stimuli from an acoustic voice-onset-time (VOT) continuum, English-speaking listeners typically show a performance peak in the region of the phonetic category boundary. An ongoing series of experiments demonstrates that the location and height of this peak are affected by preceding phonetic context. While any preceding context appears to lower discrimination performance, perhaps by interfering with auditory memory, a preceding [s] also shifts the peak towards the short-VOT end of the continuum and, correspondingly, increases “voiceless” responses in a labeling task. This latter effect (trivial within a syllable because of English spelling conventions) occurs even when a word boundary intervenes and does not seem to be mediated by the duration of the silent closure interval following the [s]. A hypothesis currently being pursued is that phonological voicing decisions for word-initial stop consonants are sensitive to the voicing status of preceding phonetic segments. [Work supported by NICHD.]

Effects of speaking rate on the perception of syllable-initial stop consonants

It has been shown that the effect of speaking rate on the perception of voice onset time (VOT) is strongest in the immediate vicinity of the target stop consonant: Listeners interpret a given VOT value in accordance with the duration of the CV syllable containing the stop consonant [A. Q. Summerfield, J. Exp. Psychol. Human Percept. Perform. 7, 1074–1095 (1981); J. L. Miller et al., J. Acoust. Soc. Am. Suppl. 1 76, S89 (1984)]. These studies used stimuli that are unlikely to occur in natural speech. The first study used simplified synthetic VOT continua; the second study used computer-edited natural-speech VOT continua, but in which syllable duration was kept constant, so that vowel duration decreased as VOT increased. The present study explored the effects of speaking rate on the perception of syllable-initial stop consonants in more natural circumstances. Natural-speech [bi-pi] continua were constructed, in which vowel rather than syllable duration was kept constant. Preliminary results indicate that the phoneme boundary is not, or only minimally, affected by changes in vowel duration.

Voice onset time perception in Japanese aphasic patients

This study examines identification of the synthetic speech stimuli [ga] and [ka] of varying voice onset times (VOTs) in 17 Japanese aphasic subjects. Approximately two-thirds of the aphasic subjects showed deterioration in performance of VOT identification of the stimuli. A quantitative analysis of the results indicates that the problems of the majority of these subjects may be attributable not only to difficulty in labeling the stimuli but also to difficulty in processing the acoustic and/or phonetic information of the stimuli.

The effect of sentence timing on the perception of word-initial stop consonants

The influence of temporal context on the perception of voice onset time (VOT) as information for the identity of a word-initial stop consonant was examined in three experiments. Different versions of a ten-word precursor phrase were constructed by recording the phrase at a fast and a slow rate and then combining words from the two original phrases to produce composite phrases with various patterns of rate changes. A final (target) syllable from a seven member /gi/-/ki/ VOT continuum constructed from natural speech was added to the end of the phrase after a variable pause duration. Subjects listened to each phrase version and judged the identity of the target syllable. In general, the VOT boundary was found to shift to shorter values with precursors that contained more fast words and shorter pause durations. Furthermore, while the rate of the words immediately preceding the target appeared to have the greatest effect, there were also significant rate effects due to the rate of stressed words early in the precursor and the pattern of rate changes throughout the precursor. [Work supported by NIMH.]

Perception of voice onset time: A cross language study of American English and Danish

This paper examines the relationship between the production and perception of the voicing distinction for American English and Danish stop consonants in initial position. In a comparison of the production of /p/ and /k/ for the two languages the Danish stops were found to have longer aspiration. It was therefore hypothesized that perceptually, Danish listeners would have a later cross-over point than American listeners. This was tested in a labelling experiment, using computer-edited, naturally produced stimuli. The aspiration was shortened from right to left to produce a series of stimuli ranging in VOT from +10 to +70 msec of the syllables pi, pu, ki, and ku. The listening tests showed a statistically significant difference in the expected direction for the labial stimuli but not for the velars. This may be due to the stimulus range which proved to be less appropriate for Danish listeners than for American listeners.