Hemispheric lateralization for speech perception in stutterers.

Sixteen adult, right-handed, moderate-to-severe atutterers (12 males, 4 females) and 20 nonstuttering controls (10 males, 10 females) were given a dichotic nonsense-syllable test to determine hemispheric lateralization for speech. Both male and female stutterers evidenced right-ear advantages in syllable identification similar in magnitude to those found for normals. These data confirm other reports of no difference in cerebral speech lateralization for stutterers and nonstutterers and, therefore, lend no support to theories that relate stuttering to abnormalities in cerebral lateralization. Acknowledgments. This study was conducted while the authors were at the University of Connecticut, Storrs. The assistance of Dr. R. L. Webster, the staff of the Institute for Behavioral Research, and the Departments of Psychology and Linguistics at the University of Connecticut is gratefully acknowledged.

Hemispheric specialization for speech perception in four-year-old children from low and middle socio-economic classes.

Four-year-old male and female children from low and middle socio-economic class (SEC) were presented a dichotic syllable task. Both low and middle SEC males evidenced significant right-ear advantages. Neither low nor middle SEC females evidenced a significant right-ear advantage. The similar ear advantage in the low and middle SEC populations replicates a previous study with six-year-olds and suggests that the variations in rearing conditions which occur in low and middle SEC classes does not affect hemispheric lateralization for speech perception.

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 consonant-vowel amplitude ratio in the recognition of voiceless stop consonants by listeners with hearing impairment.

Several authors have evaluated consonant-to-vowel ratio (CVR) enhancement as a means to improve speech recognition in listeners with hearing impairment, with the intention of incorporating this approach into emerging amplification technology. Unfortunately, most previous studies have enhanced CVRs by increasing consonant energy, thus possibly confounding CVR effects with consonant audibility. In this study, we held consonant audibility constant by reducing vowel transition and steady-state energy rather than increasing consonant energy. Performance-by-intensity (PI) functions were obtained for recognition of voiceless stop consonants (/p/, /t/, /k/) presented in isolation (burst and aspiration digitally separated from the vowel) and for consonant-vowel syllables, with readdition of the vowel /a/. There were three CVR conditions: normal CVR, vowel reduction by 6 dB, and vowel reduction by 12 dB. Testing was conducted in broadband noise fixed at 70 dB SPL and at 85 dB SPL. Six adults with sensorineural hearing impairment and 2 adults with normal hearing served as listeners. Results indicated that CVR enhancement did not improve identification performance when consonant audibility was held constant, except at the higher noise level for one listener with hearing impairment. The re-addition of the vowel energy to the isolated consonant did, however, produce large and significant improvements in phoneme identification.

Multidimensional scaling of nasal voice quality.

Listeners judged the dissimilarity of pairs of synthesized nasal voices that varied on 3 dimensions. Separate nonmetric multidimensional scaling (MDS) solutions were calculated for each listener and the group. Similar 3-dimensional solutions were derived for the group and each of the listeners, with the group MDS solution accounting for 83% of the total variance in listeners' judgments. Dimension 1 ("Nasality") accounted for 54% of the variance, Dimension 2 ("Loudness") for 18% of the variance, and Dimension 3 ("Pitch") for 11% of the variance. The 3 dimensions were significantly and positively correlated with objective measures of nasalization, intensity, and fundamental frequency. The results of this experiment are discussed in relation to other MDS studies of voice perception, and there is a discussion of methodological issues for future research.

Recognition of sentences in noise by normal-hearing listeners using simulations of speak-type cochlear implant signal processors.

To assess whether more channels are needed to understand speech in noise than in quiet, we processed speech in a manner similar to that of spectral peak-like cochlear implant processors and presented it at a +2-dB signal-to-noise ratio to normal-hearing listeners for identification. The number of analysis filters varied from 8 to 16, and the number of maximum channel amplitudes selected in each cycle varied from 2 to 16. The results show that more channels are needed to understand speech in noise than in quiet, and that high levels of speech understanding can be achieved with 12 channels. Selecting more than 12 channel amplitudes out of 16 channels did not yield significant improvements in recognition performance.

Neurophysiologic correlates of cross-language phonetic perception.

This study examined neurophysiologic correlates of the perception of native and nonnative phonetic categories. Behavioral and electrophysiologic responses were obtained from Hindi and English listeners in response to a stimulus continuum of naturally produced, bilabial CV stimuli that differed in VOT from -90 to 0 ms. These speech sounds constitute phonemically relevant categories in Hindi but not in English. As expected, the native Hindi listeners identified the stimuli as belonging to two distinct phonetic categories (/ba/ and /pa/) and were easily able to discriminate a stimulus pair across these categories. On the other hand, English listeners discriminated the same stimulus pair at a chance level. In the electrophysiologic experiment N1 and MMN cortical evoked potentials (considered neurophysiologic indices of stimulus processing) were measured. The changes in N1 latency which reflected the duration of pre-voicing across the stimulus continuum were not significantly different for Hindi and English listeners. On the other hand, in response to the /ba/-/pa/ stimulus contrast, a robust MMN was seen only in Hindi listeners and not in English listeners. These results suggest that neurophysiologic levels of stimulus processing reflected by the MMN and N1 are differentially altered by linguistic experience.

Cortical auditory evoked potential correlates of categorical perception of voice-onset time.

The goal of this study was to examine the neural encoding of voice-onset time distinctions that indicate the phonetic categories /da/ and /ta/ for human listeners. Cortical Auditory Evoked Potentials (CAEP) were measured in conjunction with behavioral perception of a /da/-/ta/ continuum. Sixteen subjects participated in identification and discrimination experiments. A sharp category boundary was revealed between /da/ and /ta/ around the same location for all listeners. Subjects' discrimination of a VOT change of equal magnitude was significantly more accurate across the /da/-/ta/ categories than within the /ta/ category. Neurophysiologic correlates of VOT encoding were investigated using the N1 CAEP which reflects sensory encoding of stimulus features and the MMN CAEP which reflects sensory discrimination. The MMN elicited by the across-category pair was larger and more robust than the MMN which occurred in response to the within-category pair. Distinct changes in N1 morphology were related to VOT encoding. For stimuli that were behaviorally identified as /da/, a single negativity (N1) was apparent; however, for stimuli identified as /ta/, two distinct negativities (N1 and N1') were apparent. Thus the enhanced MMN responses and the morphological discontinuity in N1 morphology observed in the region of the /da/-/ta/ phonetic boundary appear to provide neurophysiologic correlates of categorical perception for VOT.

Mechanisms of vowel recognition for Ineraid patients fit with continuous interleaved sampling processors.

Vowel recognition was assessed for eight, cochlear implant patients who use the Ineraid's six-electrode array. Recognition was tested in three conditions: with the Ineraid after years of experience; with a CIS processor at fitting of the processor; and with the CIS processor after 1 month's experience. At the time of fitting of the CIS processor, vowel recognition was not superior to that with the Ineraid. Recognition improved significantly over the period of a month. At 1 month, performance was significantly better with the CIS processor than with the Ineraid. This outcome is interpreted to mean that remapping of the vowel space is necessary following fitting with the CIS processor and some of the remapping occurs over a time period of days or weeks, rather than hours. Vowel errors at one month could be accounted for by two mechanisms. One is that patients attended to low-frequency channels at expense of high-frequency channels, or could not use information in high-frequency channels. The second is that, for diphthongs, patients could not detect frequency change over the course of the utterance.

Distribution of acoustic cues for stop consonant place of articulation in VCV syllables.

Most theoretical accounts of the identification of stop consonant place of articulation have focused on how bursts and formant transitions conspire to signal place in CV syllables. In the present series of experiments we have examined the identification of place in VCV syllables and have found that not only do the burst and opening transitions affect the judgement of place, but so also do the closing transitions and the duration of the closure interval. This outcome is consistent with the outcomes of many other experiments in showing that there are multiple acoustic events which bear on the identification of a given phone and that those acoustic events are distributed over time. Theoretical accounts of place identification based on data of this kind may prove viable than theories based on data from the identification of place in the absolute, syllable-initial position.

The identification of consonants and vowels by cochlear implant patients using a 6-channel continuous interleaved sampling processor and by normal-hearing subjects using simulations of processors with two to nine channels.

OBJECTIVE: To compare the vowel and consonant identification ability of cochlear implant patients using a 6-channel continuous interleaved sampling (CIS) processor and of normal-hearing subjects using simulations of processors with two to nine channels. DESIGN: Subjects, 10 normal-hearing listeners and seven cochlear implant patients, were presented synthetic vowels in /bVt/ context, natural vowels produced by men, women, and girls in /hVd/ context, and consonants in /aCa/ context for identification. Stimuli for the normal-hearing subjects were pre-processed through simulations of implant processors with two to nine channels and were output as the sum of sinusoids at the center frequencies of the analysis filters. RESULTS: Five implant patients' scores fell within the range of normal performance with a 6-channel processor when the patients were tested with synthetic vowels. Four patients' scores fell within the range of normal with a 6-channel processor when the patients were tested with multitalker vowels. Five patients' scores fell within the range of normal for a 6-channel processor for the consonant feature "place of articulation." CONCLUSION: Signal processing technology for cochlear implants has matured sufficiently to allow some patients who use CIS processors and a small number of monopolar electrodes to achieve scores on tests of speech identification that are within the range of scores established by normal-hearing subjects listening to speech processed through a small number of channels.

Exploration of the perceptual magnet effect using the mismatch negativity auditory evoked potential.

The goals of this study were (i) to assess the replicability of the "perceptual magnet effect" [Iverson and Kuhl, J. Acoust. Soc. Am. 97(1), 553-561 (1995)] and (ii) to investigate neurophysiologic processes underlying the perceptual magnet effect by using the mismatch negativity (MMN) auditory evoked potential. A stimulus continuum from /i/ to /e/ was synthesized by varying F1 and F2 in equal mel steps. Ten adult subjects identified and rated the goodness of the stimuli. Results revealed that the prototype was the stimulus with the lowest F1 and highest F2 values and the nonprototype stimulus was close to the category boundary. Subjects discriminated stimulus pairs differing in equal mel steps. The results indicated that discrimination accuracy was not significantly different in the prototype and the nonprototype condition. That is, no perceptual magnet effect was observed. The MMN evoked potential (a preattentive, neurophysiologic index of auditory discrimination) revealed that despite equal mel differences between the stimulus pairs the MMN was largest for the prototype pair (i.e., the pair that had the lowest F1 and highest F2 values). Therefore the MMN appears to be sensitive to within category acoustic differences. Taken together, the behavioral and electrophysiologic results indicate that discrimination of stimulus pairs near a prototype is based on the auditory structure of the stimulus pairs.

Improving consonant intelligibility for Ineraid patients fit with continuous interleaved sampling (CIS) processors by enhancing contrast among channel outputs.

OBJECTIVE: In Experiment 1 the objective was to determine whether patients who have been implanted with the Ineraid electrode array perform better on tests of consonant identification when signals are processed through a continuous interleaved sampling (CIS) processor than when signals are processed through an analogue (Ineraid) processor. In Experiment 2 the objective was to determine, for patients using the CIS strategy, whether identification accuracy for stop consonant place of articulation could be improved by enhancing differences in the patterns of the signal processor channel outputs. DESIGN: In Experiment 1, 16 consonants were presented in VCV format for identification. In Experiment 1 the CIS patients evidence difficulty in identifying /p t k/. Therefore, in Experiment 2 the voiceless stop consonants were presented in two stimulus conditions. In one, the stimuli were unfiltered. In the other, the stimuli were individually filtered so as to enhance the differences in channel outputs for /p/, /t/, and /k/. RESULTS: In Experiment 1 the patients performed better with CIS processors than with analogue processors. In Experiment 2 the "enhanced" stimuli were identified with better accuracy than were the unfiltered stimuli. CONCLUSIONS: We confirm that Ineraid patients achieve higher scores on tests of consonant identification when using a CIS processor than when using an analogue processor. Errors in identification of stop consonant place of articulation, when using a CIS processor, are due to the similarity in the patterns of the processor's channel outputs. By showing that consonant intelligibility can be improved by filtering, we show that we have not reached the limit of speech understanding that can be supported by the population of neural elements remaining in our patients' auditory systems.

Relative spectral change and formant transitions as cues to labial and alveolar place of articulation.

The perceptual salience of relative spectral change [Lahiri et al., J. Acoust. Soc. Am. 76, 391-404 (1984)] and formant transitions as cues to labial and alveolar/dental place of articulation was assessed in a conflicting cue paradigm. The prototype stimuli were produced by two English speakers. The stimuli with conflicting cues to place of articulation were created by altering the spectra of the signals so that the change in spectral energy from signal onset to voicing onset specified one place of articulation while the formant transitions specified the other place of articulation. Listeners' identification of these stimuli was determined principally by the information from formant transitions. This outcome provides no support for the view that the relative spectral change is a significant perceptual cue to stop consonant place of articulation.

Changes in speech intelligibility as a function of time and signal processing strategy for an Ineraid patient fitted with continuous interleaved sampling (CIS) processors.

OBJECTIVE: To assess changes in speech intelligibility as a function of signal processing strategy and as a function of time for one of the first two Ineraid patients in the United States fitted with a continuous interleaved sampling (CIS) signal processor. DESIGN: In Experiment 1, the patient was fitted with a CIS processor and measures of speech intelligibility were taken over a period of 4 mo. These data were compared with data collected with the Ineraid. In Experiment 2, three new signal processing strategies were tested. Measures of speech intelligibility were taken at fitting and after a week's use of the processor. In Experiment 3, the number of channels in the processor was reduced to 5, 4, and 3. Each processor was tested at fitting and after a week's use of the processor. RESULTS: In Experiment 1, immediately on fitting, the CIS processor produced better speech intelligibility for consonants, vowels, and the CID sentences than did the Ineraid. Performance improved over periods ranging from 1 to 4 mo depending on the test material. In Experiment 2, two processors produced significantly better speech intelligibility than did other processors. Most generally, performance dropped slightly when a new processor was fitted and then improved over the course of week. All of the processors produced better speech intelligibility than did the Ineraid. In Experiment 3, five channels allowed similar levels of performance as did six channels. The effect of four and three channels varied as a function of test material. Four CIS channels allowed better performance than did the four analogue channels of the Ineraid. CONCLUSIONS: We conclude 1) that CIS processors can provide much better speech intelligibility than can the analogue processor of the Ineraid; 2) that many CIS strategies, not just one, will produce better speech intelligibility than will the Ineraid; 3) that for this patient, five channels can allow as high a level of word intelligibility as can six channels; 4) that when the number of CIS and analogue channels are equated (at four), the CIS strategy provides better speech intelligibility than does the Ineraid; and 5) that speech intelligibility with CIS processors improves over periods as short as a week and as long as several months after fitting of the processor.