Synthetic Consonant-vowel Syllables Research Articles

Effects of Spectral Shaping on Speech Auditory Brainstem Responses to Stop Consonant-Vowel Syllables.

Spectral shaping is employed by hearing aids to make consonantal information, such as formant transitions, audible for listeners with hearing loss. How manipulations of the stimuli, such as spectral shaping, may alter encoding in the auditory brainstem has not been thoroughly studied. The aim of this study was to determine how spectral shaping of synthetic consonant-vowel (CV) syllables, varying in their second formant (F2) onset frequency, may affect encoding of the syllables in the auditory brainstem. We employed a repeated measure design. Sixteen young adults (mean = 20.94 years, 6 males) and 11 older adults (mean = 58.60 years, 4 males) participated in this study. Speech-evoked auditory brainstem responses (speech-ABRs) were obtained from each participant using three CV exemplars selected from synthetic stimuli generated for a /ba-da-ga/ continuum. Brainstem responses were also recorded to corresponding three CV exemplars that were spectrally shaped to decrease low-frequency information and provide gain for middle and high frequencies according to a Desired Sensation Level function. In total, six grand average waveforms (3 phonemes [/ba/, /da/, /ga/] X 2 shaping conditions [unshaped, shaped]) were produced for each participant. Peak latencies and amplitudes, referenced to prestimulus baseline, were identified for 15 speech-ABR peaks. Peaks were marked manually using the program cursor on each individual waveform. Repeated-measures analysis of variances were used to determine the effects of shaping on the latencies and amplitudes of the speech-ABR peaks. Shaping effects produced changes within participants in ABR latencies and amplitudes involving onset and major peaks of the speech-ABR waveform for certain phonemes. Specifically, data from onset peaks showed that shaping decreased latency for /ga/ in older listeners, and decreased amplitude onset for /ba/ in younger listeners. Shaping also increased the amplitudes of major peaks for /ga/ stimuli in both groups. Encoding of speech in the ABR waveform may be more complex and multidimensional than a simple demarcation of source and filter information, and may also be influenced by cue intensity and age. These results suggest a more complex subcortical encoding of vocal tract filter information in the ABR waveform, which may also be influenced by cue intensity and age.

Effects of Aging on the Subcortical Encoding of Stop Consonants.

Purpose The main purpose of this study was to evaluate aging effects on the predominantly subcortical (brainstem) encoding of the second-formant frequency transition, an essential acoustic cue for perceiving place of articulation. Method Synthetic consonant-vowel syllables varying in second-formant onset frequency (i.e., /ba/, /da/, and /ga/ stimuli) were used to elicit speech-evoked auditory brainstem responses (speech-ABRs) in 16 young adults (M age = 21 years) and 11 older adults (M age = 59 years). Repeated-measures mixed-model analyses of variance were performed on the latencies and amplitudes of the speech-ABR peaks. Fixed factors were phoneme (repeated measures on three levels: /b/ vs. /d/ vs. /g/) and age (two levels: young vs. older). Results Speech-ABR differences were observed between the two groups (young vs. older adults). Specifically, older listeners showed generalized amplitude reductions for onset and major peaks. Significant Phoneme × Group interactions were not observed. Conclusions Results showed aging effects in speech-ABR amplitudes that may reflect diminished subcortical encoding of consonants in older listeners. These aging effects were not phoneme dependent as observed using the statistical methods of this study.

Newborn Event-Related Potentials Predict Poorer Pre-Reading Skills in Children at Risk for Dyslexia

Earlier results from the Jyväskylä Longitudinal Study of Dyslexia showed that newborn event-related potentials (ERPs) of children with and without familial risk for dyslexia were associated with receptive language and verbal memory skills between 2.5 and 5 years of age. We further examined whether these ERPs (responses to synthetic consonant-vowel syllables /ba/, /da/, /ga/; presented equiprobably with 3,910—7,285 ms interstimulus intervals) predict later pre-reading skills measured before the onset of school (6.5 years of age). In line with our earlier results, the at-risk children (N = 11) with atypical speech processing in the right hemisphere (a slower shift in polarity from positivity to negativity in responses to /ga/ at 540—630 ms) scored significantly lower in phonological skills, rapid naming, and letter knowledge than the control children (N = 10) without enhanced right hemispheric speech processing. These results further extend our earlier findings of newborn ERPs in predicting poorer language skills. These consistent differences in ERPs to speech sounds may have applications in the future for the early identification of children at risk for developmental language problems. This would facilitate well-directed intervention even before reading problems are typically diagnosed.

Interaction of top-down and bottom-up processing on categorical perception.

To investigate the processing of acoustic and semantic information on phonemic categorical perception, two nine-step continua consisting of natural and synthetic consonant-vowel (CV) syllables were presented in four different contexts. For both natural and synthetic speech, /ti/-/pi/ continua varying in spectral tilt were constructed. The four contexts included the isolated CV syllable, a neutral context sentence with the CV at the end, and two additional sentences with varying semantic load. One semantically favored a /p/ response and one a /t/ response. Eleven adults with normal hearing (2 males, 9 females, 21 45 years old, mean age=27) labeled the consonant in the CV syllable. Results showed a pattern of statistically-significant differences between the isolated and neutral context versus the semantically-loaded sentences for both natural and synthetic stimuli, suggesting an effect of semantic context. The differences appeared in the vicinity of the boundaries of the psychometric functions, suggesting an interaction between top-down and bottom-up processing. Strictly bottom-up processing would have shown overlaid response functions, and strictly top-down processing would have shown differences at the endpoints. The pattern of results suggests interactive, parallel rather than hierarchical processing.

Brain Event-Related Potentials (ERPs) Measured at Birth Predict Later Language Development in Children with and Without Familial Risk for Dyslexia

We report associations between brain event-related potentials (ERPs) measured from newborns with and without familial risk for dyslexia and these same children's later language and verbal memory skills at 2.5, 3.5, and 5 years of age. ERPs to synthetic consonant-vowel syllables (/ba/, /da/, /ga/; presented equiprobably with 3,910-7,285 msec interstimulus intervals) were recorded from 26 newborns at risk for familial dyslexia and 23 control infants participating in the Jyväskylä Longitudinal Study of Dyslexia. The correlation and regression analyses showed that the at-risk type of response pattern at birth (a slower shift in polarity from positivity to negativity in responses to /ga/ at 540-630 msec) in the right hemisphere was related to significantly poorer receptive language skills across both groups at the age of 2.5 years. The similar ERP pattern in the left hemisphere was associated with poorer verbal memory skills at the age of 5 years. These results demonstrate that ERPs of newborns may be valid predictors of later language and neurocognitive outcomes.

Event-Related Potentials and Consonant Differentiation in Newborns with Familial Risk for Dyslexia

We measured event-related potentials (ERPs) to synthetic consonant-vowel syllables (/ba/, /da/, /ga/) from 26 newborns with familial risk for dyslexia and 23 control infants participating in the Jyväskylä Longitudinal Study of Dyslexia. The syllables were presented with equal probability and with interstimulus intervals ranging from 3,010 to 7,285 ms. Analyses of averaged ERPs from the latencies identified on the basis of principal component analysis (PCA) revealed significant group differences in stop-consonant processing in several latency ranges. At the latencies of 50-170 ms and 540-630 ms, the responses to /ga/ were larger and more positive than those to /ba/ and /da/ in the right hemisphere in the at-risk group. Between 740 and 940 ms, the responses to /ba/ and /da/ differed between anterior and posterior electrode sites in the control group. These results indicate that the cortical electric activation evoked by speech elements differs between children with and without risk for dyslexia even immediately after birth. Group-related effects at early and late latency ranges of ERPs suggest differences both in the early sound processing based on activation of afferent elements and in later phases of syllable encoding.

Effect of a Single-Channel Wide Dynamic Range Compression Circuit on Perception of Stop Consonant Place of Articulation

Previous studies have shown that altering the amplitude of a consonant in a specific frequency region relative to an adjacent vowel's amplitude in the same frequency region will affect listeners' perception of the consonant place of articulation. Hearing aids with single-channel, fast-acting wide dynamic range compression (WDRC) alter the overall consonant-vowel (CV) intensity ratio by increasing consonant energy. Perhaps one reason WDRC has had limited success in improving speech recognition performance is that the natural amplitude balances between consonant and vowel are altered in crucial frequency regions, thus disturbing the aforementioned amplitude cue for determining place of articulation. The current study investigated the effect of a WDRC circuit on listeners' perception of place of articulation when the relative amplitude of consonant and vowel was manipulated. The stimuli were a continuum of synthetic CV syllables stripped of all place cues except relative consonant amplitudes. Acoustic analysis of the CVs before and after hearing aid processing showed a predictable increase in high-frequency energy, particularly for the burst of the consonant. Alveolar bursts had more high-frequency energy than labial bursts. Twenty-five listeners with normal hearing and 5 listeners with sensorineural hearing loss labeled the consonant sound of the CV syllables in unaided form and after the syllables were recorded through a hearing aid with single-channel WDRC. There were significantly more listeners who were unable to produce a category boundary when labeling the aided stimuli. Of those listeners who did yield a category boundary for both aided and unaided stimuli, there were significantly more alveolar responses for the aided condition. These results can be explained by the acoustic analyses of the aided stimuli.

Voicing judgements by chinchillas trained with a reward paradigm

Experiments were performed to replicate and extend previous findings of similar categorization of voiced/voiceless consonant–vowel (CV) syllables by humans and chinchillas. A reward paradigm was applied to the question of how stimulus range affects the voice-onset-time (VOT) corresponding to the voiced/voiceless category boundary. Each of four adult chinchillas and four human subjects identified synthetic CV syllables as voiced (/ba/, /da/, /ga/) or voiceless (/pa/, /ta/, /ka/) using voiceless standards of either 80 or 120 ms. In both humans and animals, extending the VOT range from 80 to 120 ms shifted category boundaries to longer VOTs, but to a different extent across listeners. Control experiments suggested that listeners were attending to different phonetic cues in a manner that depended on the listener, rather than on species. The results are interpreted in terms of similar contextual effects and use of multiple phonetic cues to voicing in humans and animals.

Evoked intracranial current source distribution tomography: perspectives and restrictions

We report associations between brain event-related potentials (ERPs) measured from newborns with and without familial risk for dyslexia and these same children's later language and verbal memory skills at 2.5, 3.5, and 5 years of age. ERPs to synthetic consonant-vowel syllables (/ba/, /da/, /ga/; presented equiprobably with 3,910-7,285 msec interstimulus intervals) were recorded from 26 newborns at risk for familial dyslexia and 23 control infants participating in the Jyväskylä Longitudinal Study of Dyslexia. The correlation and regression analyses showed that the at-risk type of response pattern at birth (a slower shift in polarity from positivity to negativity in responses to /ga/ at 540-630 msec) in the right hemisphere was related to significantly poorer receptive language skills across both groups at the age of 2.5 years. The similar ERP pattern in the left hemisphere was associated with poorer verbal memory skills at the age of 5 years. These results demonstrate that ERPs of newborns may be valid predictors of later language and neurocognitive outcomes.

A developmental study of vowel perception from brief synthetic consonant–vowel syllables

The purpose of this study was to assess the perceptual role of brief synthetic consonant–vowel syllables as cues for vowels in children and adults. Stop–consonant syllables were synthesized in the context of three vowels, [i, u open aye], with durations of 10, 30, or 46 ms. These syllables were produced with bursts, and contained formant motions and/or formant onset frequencies appropriate for each stop–consonant place of articulation. Eight children at each of five age levels, 5, 6, 7, 9, and 11, and a control group of eight adults were trained to identify the vowel as either /i/, /u/, or /open aye/. The findings showed that children and adults extracted vowel information at a generally high level from stimuli as brief as one glottal pulse (together with initial burst). However, significant effects were found for age, vowel, duration, and transition type. The age effects manifested that even quantal vowels were not identified in an adult-like manner by young children. The duration and transition-type effects revealed that vowel perception was sometimes better for longer syllables with moving formant transitions. Overall, these perceptual findings indicate that syllable identification is more salient in adults than children. [Work supported by NIH, DC00464.]

Focused attention during selective adaptation along a place of articulation continuum.

The current investigation manipulated subjects' attention to adaptor tokens in five selective adaptation experiments. All stimuli were synthetic consonant-vowel syllables, with the consonant varying from [b] to [d] by formant frequency transitions. Two distractor conditions (auditory and visual) were compared to a more typical endpoint-[d alpha] adaptor condition. Distraction from endpoint-[d alpha] adaptors to phonetically distinct [si] and [integral of i] was used to observe whether smaller adaptation effects would result when attention was not focused on adaptor stimuli. In contrast, a focused attention condition required subjects to whisper [b alpha] adaptors right after they were heard. Performance in the focused attention condition was compared to a more typical endpoint-[b alpha] adaptation condition. Results indicated that focused attention did not affect the size of the adaptation effect. Asymmetrical adaptation results for [d alpha] vs [b alpha] adaptors, and a larger amount of adaptation with the presence of contralateral "distractor" syllables, resembled findings in psychoacoustic studies of discrimination and loudness adaptation. These results suggest that two levels of auditory processing (not special to speech perception) were responsible for the observed adaptation effects.

Stimulus Ratio Effects on Speech Discrimination by Children and Adults

This investigation examined the response strategies and discrimination accuracy of adults and children aged 5-10 as the ratio of same to different trials was varied across three conditions of a "change/no-change" discrimination task. The conditions varied as follows: (a) a ratio of one-third same to two-thirds different trials (33% same), (b) an equal ratio of same to different trials (50% same), and (c) a ratio of two-thirds same to one-third different trials (67% same). Stimuli were synthetic consonant-vowel syllables that changed along a place of articulation dimension by formant frequency transition. Results showed that all subjects changed their response strategies depending on the ratio of same-to-different trials. The most lax response pattern was observed for the 50% same condition, and the most conservative pattern was observed for the 67% same condition. Adult response patterns were most conservative across condition. Difference in discrimination accuracy as measured by P(C) were found, with the largest difference in the 5- to 6-year-old group and the smallest change in the adult group. These findings suggest that children's response strategies, like those of adults, can be manipulated by changing the ratio of same-to-different trials. Furthermore, interpretation of sensitivity measures must be referenced to task variables such as the ratio of same-to-different trials.

Effects of hearing loss on utilization of short-duration spectral cues in stop consonant recognition.

The purpose of this experiment was to evaluate the utilization of short-term spectral cues for recognition of initial plosive consonants (/b,d,g/) by normal-hearing and by hearing-impaired listeners differing in audiometric configuration. Recognition scores were obtained for these consonants paired with three vowels (/a,i,u/) while systematically reducing the duration (300 to 10 ms) of the synthetic consonant-vowel syllables. Results from 10 normal-hearing and 15 hearing-impaired listeners suggest that audiometric configuration interacts in a complex manner with the identification of short-duration stimuli. For consonants paired with the vowels /a/ and /u/, performance deteriorated as the slope of the audiometric configuration increased. The one exception to this result was a subject who had significantly elevated pure-tone thresholds relative to the other hearing-impaired subjects. Despite the changes in the shape of the onset spectral cues imposed by hearing loss, with increasing duration, consonant recognition in the /a/ and /u/ context for most hearing-impaired subjects eventually approached that of the normal-hearing listeners. In contrast, scores for consonants paired with /i/ were poor for a majority of hearing-impaired listeners for stimuli of all durations.

Phoneme boundaries in normal and hearing-impaired ears.

Two experiments are reported which explore variables that may complicate the interpretation of phoneme boundary data from hearing-impaired listeners. Fourteen synthetic consonant-vowel syllables comprising a/ba-da-ga/ continuum were used as stimuli. The first experiment examined the influence of presentation level and ear of presentation in normal-hearing subjects. Only small differences in the phoneme boundaries and labeling functions were observed between ears and across presentation levels. Thus monaural presentation and relatively high signal level do not appear to be complicating factors in research with hearing-impaired listeners, at least for these stimuli. The second experiment described a test procedure for obtaining phoneme boundaries in some hearing-impaired listeners that controlled for between-subject sources of variation unrelated to hearing impairment and delineated the effects of spectral shaping imposed by the hearing impairment on the labeling functions. Labeling data were obtained from unilaterally hearing-impaired listeners under three test conditions: in the normal ear without any signal distortion; in the normal ear listening through a spectrum shaper that was set to match the subject's suprathreshold audiometric configuration; and in the impaired ear. The reduction in the audibility of the distinctive acoustic/phonetic cues seemed to explain all or part of the effects of the hearing impairment on the labeling functions of some subjects. For many other subjects, however, other forms of distortion in addition to reduced audibility seemed to affect their labeling behavior.

Psychoacoustic properties of imagined speech stimuli

Synthetic consonant-vowel syllables were generated by factorially crossing four values each on two acoustic continua (VOT and F2-F3 transition onset frequency) to produce 16 stimuli identifiable as /ba/, /pa/, /ta/, and /da/. In addition, four silent stimuli were generated. The complete set of 20 stimuli was presented to subjects in a paired comparison test for scalar judgments of similarity. Silent stimuli were presented paired both with sounded stimuli and with other silent stimuli. On the response sheet sounded stimuli were indicated by an underscore and silent stimuli by “ba,” “pc,” “da,” or “ta,” depending upon which syllable the subject was supposed to “hear.” Subjects were instructed to make comparisons involving silent stimuli by imagining themselves pronouncing the silent syllable. Responses from 13 subjects were analyzed using the INDSCAL individual differences scaling approach [J. Carroll, in Multidimensional Scaling: Theory and Applications in the Behavioral Sciences, edited by Shepard, Romney, and Nerlove (1972)]. The recovered two-dimensional perceptual space clearly showed the four silent stimuli located appropriately with respect to the sounded stimuli, indicating that subjects were capable of making linguistic judgments of sounded and silent stimuli alike. These results provide direct support for prototype models of speech perception.

The effect of age and hearing loss on the identification of synthetic /b, d, g/-stimuli.

A series of speech identification tests using 15 synthetic consonant-vowel syllables with variations in second and third formant transitions were conducted in different age groups with normal hearing and a group of older subjects with a sensorineural hearing loss. Varying degree of categorical perception of three distinct phoneme categories /b, d, g/ was observed in all groups. Responses were depending on start frequency and gliding direction of second and third formant transitions. In the group of 60-80-year-old persons with a hearing loss response percentages were significantly lower and error frequency high. Furthermore, category shift boundaries changed as compared with the group of 20-40 years old. Results in a group of children age 8-15 years, were rather similar to those in the 20-40-year-old group. Both age and age-induced sensorineural hearing loss seemed to influence elderly people's ability to identity the synthetic speech stimuli.

Parellel processing of auditory and phonetic information: A reassessment using speed-accuracy tradeoff functions

In previous experiments I have used “redundancy gains” in choice reaction time (RT) as evidence that the processing of auditory and phonetic information does not occur in a strict serial fashion, but that some form of shared, simultaneous, or parallel processing must be involved [C. C. Wood, Percept. Psychophys. 15, 501–508 (1974)]. The present experiments reassessed this interpretation using speed-accuracy tradeoff functions as dependent variables instead of mean RT [C. C. Wood and J. R. Jennings, Percept. Psychophys. 19, 92–101 (1976)]. Speed-accuracy tradeoff functions (a) permit response criterion effects to be assessed in RT tasks much as signal detection theory permits such effects to be assessed in detection tasks, and (b) provide estimates of the rate of growth of information over time following stimulus onset. The results of these experiments confirm and extend previous evidence of redundancy gains for fundamental frequency and place of articulation in synthetic consonant-vowel (CV) syllables. However, similar tradeoff functions and redundancy gains of similar magnitude were obtained when the CV syllables were reduced to isolated first and second formant transitions. Thus, we cannot rule out the possibility that redundancy gains for the full CV syllables are based upon only an auditory analysis of the formant transitions instead of phonetic feature identification. Some ways to approach this question experimentally will be discussed.

Contrast effects on stop consonant identification.

Changes in the identification of speech sounds following selective adaptation are usually attributed to a reduction in sensitivity of auditory feature detectors. An alternative explanation of these effects is based on the notion of response contrast. In several experiments, subjects identified the initial segment of synthetic consonant-vowel syllables as either the voiced stop [b] or the voiceless stop [ph]. Each test syllable had a value of voice onset time (VOT) that placed it near the English voiced-voiceless boundary. When the test syllables were preceded by a single clear [b] (VOT = -100 msec), subjects tended to identify them as [ph], whereas when they were preceded by an unambiguous [ph] (VOT = 100 msec), the syllables were predominantly labeled [b]. This contrast effect occurred even when the contextual stimuli were velar and the test stimuli were bilabial, which suggests a featural rather than a phonemic basis for the effect. To discount the possibility that these might be instances of single-trial sensory adaptation, we conducted a similar experiment in which the contextual stimuli followed the test items. Reliable contrast effects were still obtained. In view of these results, it appears likely that response contrast accounts for at least some component of the adaptation effects reported in the literature.

Perceptual invariance and onset spectra for stop consonants in different vowel environments.

A series of listening tests with brief synthetic consonant-vowel syllables was carried out to determine whether the initial part of a syllable can provide cues to place of articulation for voiced stop consonants independent of the remainder of the syllable. The data show that stimuli as short as 10-20 ms sampled from the onset of a consonant-vowel syllable, can be reliably identified for consonantal place of articulation, whether the second and higher formants contain moving or straight transitions and whether or not an initial burst is present. In most instances, these brief stimuli also contain sufficient information for vowel indentification. Stimulus continua in which formant transitions ranged from values appropriate to [b], [d], [g] in various vowel environments, and in which stimulus durations were 20 and 46 ms, yielded categorical labeling functions with a few exceptions. These results are consistent with a theory of speech perception in which consonant place of articulation is cued by invariant properties derived from the spectrum sampled in a 10-20 ms time window adjacent to consonantal onset or offset.

Feature analyzers for the phonetic dimensionstop vs. continuant

Perception of sounds along the phonetic dimensionstop vs. continuant was studied by means of a selective adaptation procedure. Subjects first identified a series of synthetic consonant-vowel syllables whose formant transitions varied in duration, slope, and amplitude characteristics. They were perceived as either [ba] or [wa]. After the initial identification test, an adapting stimulus was presented repeatedly, and then the subjects again identified the original test series. Adapting with a stop (either [ba] or [da]) led to a decrease in the number of test stimuli identified as [ba], whereas adapting with the continuant sound [wa] led to an increase in the number of [ba] identification responses. Removing the vowel portion of an adapting stimulus greatly reduced the identification shift only when the resulting stimulus was no longer perceived as speech-like. A reduction in the number of [ba] identifications occurred even when a nonspeech “stop” (the sound of a plucked string) was used as the adapting stimulus, suggesting that phonetic processing is not a necessary condition for an adaptation effect.