Effects of intensity on the categorical perception of stop consonants and isolated second formant transitions

Identification and discrimination of two-formant [bae-dae-gae] and [pae-tae-kae] synthetic speech stimuli and discrimination of corresponding isolated second formant transitions (chirps) were performed by six subjects. Stimuli were presented at several intensity levels such that the intensity of the F2 transition was equated between speech and nonspeech stimuli, or the overall intensity of the stimulus was equated. At higher intensity (92 dB), b-d-g and p-t-k identification and between-category discrimination performance declined and bilabial-alveolar phonetic boundaries shifted in location on the continuum towards the F2 steady-state frequency. Between-category discrimination improved from performance at 92 dB when 92-dB speech stimuli were simultaneously masked by 60-dB speech noise; alveolar-velar boundaries shifted to a higher frequency location in the 92-dB-plus-noise condition. Chirps were discriminated categorically when presented at 58 dB, but discrimination peaks declined at higher intensities. Perceptual performance for chirps and p-t-k stimuli was very similar, and slightly inferior to performance for b-d-g stimuli, where simultaneous masking by F1 resulted in a lower effective intensity of F2. The results were related to a suggested model involving pitch comparison and transitional quality perceptual strategies.     

The perception of stop consonant voicing by Spanish-English bilinguals

The performance of Spanish-English bilinguals in two perception tasks, using a synthetic speech continuum varying in voice onset time, was compared with the performance of Spanish and English monolinguals. Voice onset time in speech production was also compared between these groups. Results in perception of bilinguals differed from that of both monolingual groups. Results of bilingual production in their two languages conformed with results obtained from each monolingual group. The perceptual results are interpreted in terms of differences in the use of available acoustic cues by bilingual and monolingual listeners of English and Spanish.     

Simple and contingent adaptation effects for place of articulation in stop consonants

Experiments on selective adaptation have shown that the locus of the phonetic category boundary between two segments shifts after repetitive listening to an adapting stimulus. Theoretical interpretations of these results have proposed that adaptation occurs either entirely at an auditory level of processing or at both auditory and more abstract phonetic levels. The present experiment employed two alternating stimuli as adaptors in an attempt to distinguish between these two possible explanations. Two alternating stimuli were used as adaptors in order to test for the presence of contingent effects and to compare these results to simple adaptation using only a single adaptor. Two synthetic CV series with different vowels that varied the place of articulation of the consonant were employed. When two alternating adaptors were used, contingent adaptation effects were observed for the two stimulus series. The direction of the shifts in each series was governed by the vowel context of the adapting syllables. Using the single adaptor data, a comparison was made between the additive effects of the single adaptors and their combined effects when presented in alternating pairs. With voiced adaptors, only within-series adaptation effects were found, and these data were consistent with a on,level model of selective adaptation. However, for the voiceless adaptors, both within- and cross-series adaptation effects were found, suggesting the possible presence of two levels of adaptation to place of articulation. Further, the contingent adaptation effects with the voiceless adaptors seemed to be the result of the additive effects of the two alternating adaptors. This result indicates that previously reported contingent adaptation results may also reflect the net vowel specific adaptation effects after cancellation of other, nonvowel dependent effects and that caution is needed in interpreting such results.     

Some effects of later-occurring information on the perception of stop consonant and semivowel

In three experiments, we determined how perception of the syllable-initial distinction between the stop consonant [b] and the semivowel [w], when cued by duration of formant transitions, is affected by parts of the sound pattern that occur later in time. For the first experiment, we constructed four series of syllables, similar in that each had initial formant transitions ranging from one short enough for [ba] to one long enough for [wa], hut different in overall syllable duration. The consequence in perception was that, as syllable duration increased, the [b-w] boundary moved toward transitions of longer duration. Then, in the second experiment, we increased the duration of the sound by adding a second syllable, [da], (thus creating [bada-wada]), and observed that lengthening the second syllable also shifted the perceived [b-w] boundary in the first syllable toward transitions of longer duration; however, this effect was small by comparison with that produced when the first syllable was lengthened equivalently. In the third experiment, we found that altering the structure of the syllable had an effect that is not to be accounted for by the concomitant change in syllable duration: lengthening the syllable by adding syllable-final transitions appropriate for the stop consonant [d] (thus creating [bad-wad]) caused the perceived [b-w] boundary to shift toward transitions of shorter duration, an effect precisely opposite to that produced when the syllable was lengthened to the same extent by adding steady-state vowel. We suggest that, in all these cases, the later-occurring information specifies rate of articulation and that the effect on the earlier-occurring cue reflects an appropriate perceptual normalization.     

Acoustic cues for the perception of place of articulation in aphasia

Two experiments assessed the abilities of aphasic patients and nonaphasic controls to perceive place of articulation in stop consonants. Experiment I explored labeling and discrimination of [ba, da, ga] continua varying in formant transitions with or without an appropriate burst onset appended to the transitions. Results showed general difficulty in perceiving place of articulation for the aphasic patients. Regardless of diagnostic category or auditory language comprehension score, discrimination ability was independent of labeling ability, and discrimination functions were similar to normals even in the context of failure to reliably label the stimuli. Further there was less variability in performance for stimuli with bursts than without bursts. Experiment II measured the effects of lengthening the formant transitions on perception of place of articulation in stop consonants and on the perception of auditory analogs to the speech stimuli. Lengthening the transitions failed to improve performance for either the speech or nonspeech stimuli, and in some cases, reduced performance level. No correlation was observed between the patient's ability to perceive the speech and nonspeech stimuli.     

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

When members of a series of synthesized stop consonants varying acoustically inF3 characteristics and varying perceptually from /da/ to /ga/ are preceded by /al/, subjects report hearing more /ga/ syllables relative to when each member is preceded by /ar/ (Mann, 1980). It has been suggested that this result demonstrates the existence of a mechanism that compensates for coarticulation via tacit knowledge of articulatory dynamics and constraints, or through perceptual recovery of vocal-tract dynamics. The present study was designed to assess the degree to which these perceptual effects are specific to qualities of human articulatory sources. In three experiments, series of consonant-vowel (CV) stimuli varying inF3-onset frequency (/da/—/ga/) were preceded by speech versions or nonspeech analogues of /al/ and lav I. The effect of liquid identity on stop consonant labeling remained when the preceding VC was produced by a female speaker and the CV syllable was modeled after a male speaker’s productions. Labeling boundaries also shifted when the CV was preceded by a sine wave glide modeled after F3 characteristics of /al/ and /ar/. Identifications shifted even when the preceding sine wave was of constant frequency equal to the offset frequency ofF3 from a natural production. These results suggest an explanation in terms of general auditory processes as opposed to recovery of or knowledge of specific articulatory dynamics.     

Modification of consonant speech-sound articulation in young children

A series of three experiments was performed in a classroom setting with small groups of young children with severe articulation problems. Variations on a basic token reinforcement procedure were demonstrated in each experiment. A combined multiple baseline/reversal design showed effective experimenter control of rates of correct and incorrect consonant sound articulation in all cases. In addition, the data in each experiment showed the problems of obtaining stimulus generalization of the high rates of correct articulation to non-training settings. The third experiment demonstrated a procedure for producing such generalization by making each child a discriminative stimulus for correct articulaton by the other child, thus maintaining high levels of correct articulation for each child when in the presence of the other.     

Susceptibility of a stop consonant to adaptation on a speech-nonspeech continuum: Further evidence against feature detectors in speech perception

The present experiment uses the perceptual adaptation paradigm to establish the validity of a previous test of the feature detector model of speech perception. In the present study, a synthetic stimulus series varied from a CV syllable, [ba], to a nonspeech buzz. When the endpoint tokens were employed alternatively as adaptors, the category boundary was shifted relative to unadapted identification in each adaptor condition. This result suggests that a prior test which used a vowel as the speech endpoint was legitimate because a stop consonant, an exemplary speech sound, was also susceptible to perceptual adaptation in a speech-nonspeech context. Feature detector models predict, incorrectly, that this outcome is impossible. Therefore, this finding may be taken to undermine the interpretation of adaptation as fatigue in a set of detectors tuned to detect the distinctive features of linguistic analysis.     

Some properties of auditory memory for rapid formant transitions

In the three experiments reported here, subjects indicate whether two sequentially presented syllables, differing in the place of articulation of an initial stop consonant, are phonernically the same or not. The first experiment finds faster |ldsame” responses to acoustically identical pairs than to pairs that are phonemically identical but acoustically distinct. provided that the second syllable is presented within 400 msec of the first. This is interpreted as indicating the persistence of a memory which preserves auditory information about within-category distinctions. The third experiment shows that this advantage remains when a tone is interposed between the two syllables, but is removed when a brief vowel is similarly interposed. The second experiment presents the second syllable of each pair dichotically with a noise burst, and shows that the size of the right-ear advantage for this reaction time task is reduced when the result of comparisons based on this auditory memory is compatible with the required phonemic decision, but that the right-ear advantage is increased when auditory comparisons would contradict the phonemic relationship.     

The role of second formant transitions in the stop-semivowel distinction

An experiment was conducted which assessed the relative contributions of three acoustic cues to the distinction between stop consonant and semivowel in syllable initial position. Subjects identified three series of syllables which varied perceptually from [ba] to [wa]. The stimuli differed only in the extent, duration, and rate of the second formant transition. In each series, one of the variables remained constant while the other two changed. Obtained identification ratings were plotted as a function of each variable. The results indicated that second formant transition duration and extent contribute significantly to perception. Short second formant transition extents and durations signal stops, while long second formant transition extents and durations signal semivowels. It was found that second formant transition rate did not contribute significantly to this distinction. Any particular rate could signal either a stop or semivowel. These results are interpreted as arguing against models that incorporate transition rate as a cue to phonetic distinctions. In addition, these results are related to a previous selective adaptation experiment. It is shown that the “phonetic” interpretation of the obtained adaptation results was not justified.     

Processing of place information in stop consonants

One of the basic questior, s that models of speech perception must answer concerns the conditions under which various cues will be extracted from a stimulus and the nature of the mechanisms which mediate this process. Two selective adaptation experiments were carried out to explore this question for the phonetic feature of place of articulation in both syllableinitial and syllable-final positions. In the first experiment, CV and VC stimuli were constructed with complete overlap in their second- and third-formant transitions. Despite this essentially complete overlap, no adaptation effects were found for a VC adaptor and a CV test series (or vice versa). In the second experiment, various vowel, vowel-like, and VC-like adaptors were used. The VC-like adaptors did have a significant effect on the CV category boundary, while the vowel and vowel-like stimuli did not. These results are interpreted within both one- and twolevel models of selective adaptation. These models are distinguished by whether selective adaptation is assumed to affect a single auditory level of processing or to affect both an auditory level and a later phonetic level. However, both models incorporate detectors at the auditory level which respond whenever particular formant transitions are present. These auditory detectors are not sensitive to the position of the consonant transition information within the syllable.     

Phonological processes and the perception of phonotactically illegal consonant clusters

The perception of consonant clusters that are phonotactically illegal word initially in English (e.g., /tl/, /sr/) was investigated to determine whether listeners’ phonological knowledge of the language influences speech processing. Experiment 1 examined whether the phonotactic context effect (Massaro & Cohen, 1983), a bias toward hearing illegal sequences (e.g., /tl/) as legal (e.g., /tr/), is more likely due to knowledge of the legal phoneme combinations in English or to a frequency effect. In Experiment 2, Experiment 1 was repeated with the clusters occurring word medially to assess whether phonotactic rules of syllabification modulate the phonotactic effect. Experiment 3 examined whether vowel epenthesis, another phonological process, might also affect listeners’ perception of illegal sequences as legal by biasing them to hear a vowel between the consonants of the cluster (e.g., /talee/). Results suggest that knowledge of the phonotactically permissible sequences in English can affect phoneme processing in multiple ways.     

The perception of stop consonants by children

Kindergarten and second-grade children's perception of voicing distinctions among the stop consonants was investigated by assessing their ability to identify and discriminate a series of synthetic speech stimuli varying in voice onset time (VOT). Perception of these sounds was found to be nearly categorical. No differences between the two age groups in either identification or discrimination performance were found; furthermore, the children's performance was comparable to adult performance in other studies using these stimuli. When considered in the context of data on the perception of VOT by infants as well as adults, the results suggest that the differential discriminability of stimuli along the VOT continuum has a biological basis.     

Evidence for phonetic processing of cues to place of articulation: Perceived manner affects perceived place

Using stimuli that could be labeled either as stops [b,d] or as fricatives [f,v,θ,ð], we found that, for a given acoustic stimulus, perceived place of articulation was dependent on perceived manner. This effect appeared for modified natural syllables with a free-identification task and for a synthetic transition continuum with a forced-choice identification task. Since perceived place could be changed by changing manner labels with no change in the acoustic stimulus, it follows that the processing of the place feature depends on the value the listener assigns to the manner feature rather than directly on any of the acoustic cues to manner. We interpret these results as evidence that the identification of place of articulation involves phonetic processing and could not be purely auditor     

Discrimination of intensity differences carried on formant transitions varying in extent and duration

Dorman (1974) found that small-intensity differences carried on the initial portions of consonantvowel syllables were not discriminable. Similar differences carried on steady-state vowels and on isolated formant transitions, however, were readily discriminable. He interpreted the difference between the first and latter conditions as a phonetic effect. Yet Pastore, Ahroon, Wolz, Puleo, and Berger (1975) found similar results using sine-wave analogs to Dorman’s stimuli. They concluded that the effect is not phonetic, and that it is attributable to simple backward masking. The present studies observed the discriminability of intensity differences carried on formant transitions varying in extent and duration. Results support the conclusion of Pastore et al. to the extent that the effect is clearly not phonetic. However, these results and others suggest that simple peripheral backward masking is not a likely cause; instead, recognition masking may be involved. Moreover, the finding that phonetic-like processes occur elsewhere in audition does not necessarily impugn the existence of a speech processor; phonemic and phonological processes remain, as yet, unmatched.     

An auditory evoked potential study of consonant perception in different vowel environments

Auditory evoked responses (AER) to series of consonant—vowel syllables were recorded from temporal and parietal scalp locations from 20 right-handed female college students. Averaged AERs were submitted to principal components analysis and analysis of variance. Seven components of the group's AERs were found to reflect various aspects of the stimulus parameters. One component reflected changes over only the left hemisphere to different consonants independent of the following vowel sound. A second component changed systematically over both hemispheres in response to only consonant changes. A third component systematically changed for the different consonants depending on the following vowel.