Phonetics of intervocalic consonant perception: retrospect and prospect.

This article reviews the production characteristics and perceptual cues of intervocalic consonants as a background for acoustic studies of consonant perception in fluent speech. Data show that in conversation intervocalic consonants occur much more commonly than consonants in initial or final position; all phonetic features are strongly represented. Production characteristics of intervocalic consonants are seen to depend on the tempo and rhythmic conditions of the syllables of which they are components. At a moderate tempo, consonants in syllable-final position combine with the onset consonant of the following syllable. This affects durational characteristics and may be explained by higher energy efficiency of CV units in production. Phonological phenomena are related to the shifts in syllable position and the temporal compensations of intervocalic consonant production. Studies of consonant perception in fluent context have dealt with tempo of utterance, position in word, and rhythmic pattern, as well as phonemic context. Major phenomena are effects of coarticulation, invariance in consonant perception, and cue interaction and masking. Much evidence suggests a dominance of the perceptual cues in the CV portion of VCCV and VCV sequences. We suggest that exploration of perception variables that affect consonants in fluent context would be expedited by reorienting experimental procedures to employ listener adjustment of stimuli, instead of the traditional phoneme identification and discrimination procedures with large sets of constant stimuli. Most of the relevant literature deals with stop consonants. Lateral, rhotic, and nasal consonants also deserve intensive study because of their very frequent occurrence. Theoretical issues of phoneme perceptual invariance and motor vs. auditory theory of perception are discussed in relation to proposed experiments which vary syllable tempo and stress pattern.

Spectral cues to perception of /d, n, l/ by normal- and impaired-hearing listeners.

The alveolar consonants /d, n, l/ occur frequently in intervocalic position in conversational speech but have received little study for differences in their acoustic cues. Impaired- and normal-hearing listeners were investigated for use of consonant-segment versus transition-segment cues to recognition of /d, n, l/ in /aeCae/ tokens extracted from sentences. To examine the cues' contribution to /d, n, l/ recognition, the segments were degraded singly or in combinations in the tokens as follows: [aeC] or [Cae] transitions were replaced by adjacent pitch periods from the respective vowels; the consonant segments were replaced by silence or by a synthetic consonant approximating the summed low-frequency spectra of the /d, n, l/ murmurs. The results with normal-hearing listeners showed that the presence of any one of the three segments, [aeC] transition, [Cae] transition, or natural consonant segment, supported a moderate to high level of /d, n, l/ recognition, depending on the phoneme. In contrast, the severely hearing-impaired listeners' consonant recognition was poor on the basis of transition information, but better in the presence of the natural consonants. The /aeCae/'s with the synthetic consonant yielded chance level performance for the hearing-impaired listeners but good consonant recognition for the normal-hearing listeners--a further indication that cues in the transitions were quite useful for the normal-hearing group but not for the hearing-impaired group.

VCVs vs CVCs for stop/fricative distinctions by hearing-impaired and normal-hearing listeners.

Moderately to profoundly hearing-impaired (n = 30) and normal-hearing (n = 6) listeners identified [p, k, t, f, theta, s] in [symbol; see text], and [symbol; see text]s tokens extracted from spoken sentences. The [symbol; see text]s were also identified in the sentences. The hearing-impaired group distinguished stop/fricative manner more poorly for [symbol; see text] in sentences than when extracted. Further, the group's performance for extracted [symbol; see text] was poorer than for extracted [symbol; see text] and [symbol; see text]. For the normal-hearing group, consonant identification was similar among the syllable and sentence contexts.

Burst and transition cues to voicing perception for spoken initial stops by impaired- and normal-hearing listeners.

The use of cues to voicing perception of initial stop consonants in multiple spoken syllables was studied for moderately/severely hearing-impaired (n = 43) and normal-hearing listeners (n = 12). The test stimuli were ten utterances each of the syllables/baed, gaed, daed, paed, kaed, taed/. The utterances were analyzed acoustically to confirm the presence of certain cues to initial-stop voicing, namely, differences in voice onset time (VOT), aspiration, and vowel-onset values of the first formant and of fundamental frequency (fo). Test conditions were prepared in which different portions of the syllable onsets were either deleted or interchanged for voicing-cognate syllables. Also the fo contour was flattened for syllable pairs via analysis/synthesis using linear predictor code (LPC) processing. The results confirmed that VOT was a strong voicing cue for both the hearing-impaired and normal-hearing listeners. When the aspirations of the voiceless stops were inserted between the release and the vowel of the voiced-stop syllables, the normal-hearing listeners perceived voiceless stops predominantly. The transition portions of the vowel onsets in burstless /baed, gaed, daed/ contained strong cues for voicing perception of /b,g,d/. The hearing-impaired listeners seemed less sensitive than the normal-hearing listeners to the aspiration-presence and the vowel-onset cues. The fo difference at vowel onset appeared to have no cue value for either group of listeners.

Speech-cue enhancement for the hearing impaired: amplification of burst/murmur cues for improved perception of final stop voicing.

A further study is reported from a program of research exploring the improvement of speech perception by hearing-impaired persons via enhancement of acoustic features of consonants (10,11). Enhancements were applied to certain acoustic segments of consonants, segments known to be useful in consonant perception by normal-hearing persons but often not for persons with severe/profound hearing losses. The consonants were /k/, /t/, /g/, and /d/ located as the final phoneme in /baeC/ words; the voicing feature difference of /k/ versus /g/ and /t/ versus /d/ was the focus of study. The results showed that stop voicing perception improved to at least 90 percent for 3/4 of the listeners when the voiced murmur segments during /d/ and /g/ and the release bursts of /t/ and /k/ were amplified above their natural levels. The audibility of the enhanced segments generally explained differences between the listeners who showed large versus minimal improvements. One training session for stop voicing perception with the cue-enhanced words seemed sufficient to effect maximum performance improvement.

Speech cue enhancement for the hearing impaired: I. Altered vowel durations for perception of final fricative voicing.

Hearing-impaired persons with reduced voicing perception for final fricatives were studied for improvement in perception via training or enhancement of the vowel duration cue. Ten utterances each of the syllables /baef, baes, baev, baez/ composed three stimulus sets that were graded for the magnitude of the vowel duration cue. These sets were used differentially among 25 hearing-impaired listeners to train and test voicing perception. When enhanced, the vowels of the utterances were lengthened before voiced fricatives and shortened before voiceless fricatives. Training for the utterances with unmodified vowels yielded a small performance improvement for a subgroup of 14 listeners. In comparison, this subgroup showed considerably more improvement for the utterances with vowel duration enhancements. Another subgroup of 5 listeners displayed good fricative voicing perception independent of training or the vowel duration enhancements. A remaining subgroup of 6 listeners manifested poor use of the maximum unmodified vowel duration cues, even subsequent to training. This group showed a significant improvement in perception only for the voiced fricatives when extra duration enhancement and further training were used. This subgroup also demonstrated the poorest performance for a measure of vowel duration discrimination. Among the auditory variables examined, the listeners' tone thresholds at 250 Hz showed the highest relation to perception of fricative voicing.

Cations decrease specific [3H]-spiroperidol binding in human prefrontal cortex.

Ligand binding at many physiologically relevant receptors is regulated by divalent cations. To determine whether [3H]-spiroperidol binding sites in prefrontal cortex might be physiologically relevant receptors, we examined the effect of ions on the binding of this ligand in postmortem human prefrontal cortex. Our results indicate that several cations decreased [3H]-spiroperidol binding in a dose-dependent fashion. Of these, Cd++ and Zn++ were the most able to decrease [3H]-spiroperidol binding with IC50 of 5.5 +/- 2.4 X 10(-6)M and 5.6 +/- 1.1 X 10(-5)M respectively. These findings indicate that [3H]-spiroperidol may bind at physiologically relevant receptors in human prefrontal cortex.

Some rehabilitative considerations for future speech-processing hearing aids.

Improvements in speech-recognition and speech-processing technology promise eventual ways of assisting the hearing impaired by automatically enhancing the audibility of critical speech segments or distinctive features. Some results of applying enhancement techniques are summarized and procedures are proposed for selecting the speech sounds requiring enhancement, the degree of amplification, and the training that hearing-impaired listeners might need.

Speech communication for the deaf: visual, tactile, and cochlear-implant.

A review is given of current research and development on electronic devices to aid speech communication for the deaf. Visual and tactile displays are compared with stimulation of hearing via electrodes implanted in the cochlea. Specific comparative performance data are given for cochlear electrical implants versus tactile aids.

Perceptual cues to the voiced-voiceless distinction of final fricatives for listeners with impaired or with normal hearing.

Cues to the voicing distinction for final /f,s,v,z/ were assessed for 24 impaired- and 11 normal-hearing listeners. In base-line tests the listeners identified the consonants in recorded /d circumflex C/ syllables. To assess the importance of various cues, tests were conducted of the syllables altered by deletion and/or temporal adjustment of segments containing acoustic patterns related to the voicing distinction for the fricatives. The results showed that decreasing the duration of /circumflex/ preceding /v/ or /z/, and lengthening the /circumflex/ preceding /f/ or /s/, considerably reduced the correctness of voicing perception for the hearing-impaired group, while showing no effect for the normal-hearing group. For the normals, voicing perception deteriorated for /f/ and /s/ when the frications were deleted from the syllables, and for /v/ and /z/ when the vowel offsets were removed from the syllables with duration-adjusted vowels and deleted frications. We conclude that some hearing-impaired listeners rely to a greater extent on vowel duration as a voicing cue than do normal-hearing listeners.

Auditory implants and tactile aids for the profoundly deaf.

This paper reviews data on speech perception via implanted electrodes and via tactile aids. The two approaches are compared in terms of amount and types of aid provided to communication. Related issues discussed are the performance levels with multi-versus single-channel implants, promontory electrical stimulators versus implants, use of minimal residual hearing, implants for children, and the possible design of complementary systems combining auditory implant and tactile information. The diversity of the test methods and subjects used in implant versus tactile research precludes definitive comparisons of speech perception performance. However, it appears from the available data that, at present and for the foreseeable future, neither approach can provide more than a modest aid to lipreading. Speech reception test results from multichannel-implanted subjects are better, on the average, than for single-channel subjects. However, the best single-channel results are comparable to the best multichannel in tests using simple sentences. There is great variation among subjects with the same implant. Tactile aid performance by highly practiced subjects seems comparable to that of the better implant subjects.

Release from first-formant masking in presbyacusis.

Using synthetic vowels, the effects of first-formant masking on discrimination of the presence of a transition in the second formant were measured under conditions of first-formant attenuation and dichotic presentation. The subjects were 26 listeners between the ages of 61 and 91 with borderline to moderate sensorineural hearing losses. Results indicated: (1) dichotic presentation was significantly superior to monotic presentation; (2) attenuation of the first formant did not result in significant improvement of second-formant transition discrimination. Improved performance in the dichotic conditions may be related to the fact that subjects selected lower most comfortable listening levels than for monotic conditions. Results are compared to those of another study using natural speech stimuli.

Acoustic cues to final stop voicing for impaired-and normal hearing listeners.

Voicing perception for final stops was studied for impaired- and for normal-hearing listeners when cues in naturally spoken syllables were progressively neutralized. The syllables were ten different utterances of /daep, daek, daet, daeb, daeg, daed/ spoken in random order by a male. The cue modifications consisted progressively of neutralized vowel duration, equalized occlusion duration, burst deletion, murmur deletion, vowel-transition interchange, and transition deletion. The impaired subjects had moderate-to-severe losses and showed at least 70% correct voicing for the unmodified syllables. For the voiced stops, vowel-duration adjustment and murmur deletion each resulted in significant reductions in voicing perception for more than one-third of the impaired listeners; all normals showed good performance following neutralization of these cues. For the voiceless stops, large percentages of both listener groups showed decreased voicing perception due to the burst deletion, though a majority of both groups performed well above change even after the vowel-duration adjustment and the burst deletion. When the vowel off-going transitions were exchanged between cognate syllables in given pairs, the effect on voicing perception exhibited by many impaired- and all normal-hearing listeners implicated the vowel transitions as an important additional source of cues to final-stop voicing perception.

Differences in speech discrimination in the elderly as a function of type of competing noise: speech-babble or cafeteria.

Speech discrimination of place-loaded Modified Rhyme test words in speech-babble and cafeteria noise by elderly hearing-impaired subjects was measured. The stimuli were presented under the following conditions: (1) monotic: the signal was presented to the preferred ear; (2) low-frequency attenuated: the frequency band below 1,000 Hz was attenuated by 5, 10, and 15 dB relative to the high-frequency band; (3) dichotic: the frequency band above 1,000 Hz was presented to the preferred ear and the low-frequency band to the other ear; (4) diotic: the same signal was presented to both ears. Difference scores between the monotic baseline condition and the other listening modes were calculated and compared. Relationships between listening modes differed as a function of type of competing noise. Low-frequency attenuation improved discrimination in speech-babble but not in cafeteria noise. Dichotic-monotic and diotic-monotic superiority were seen with both noises.

Masking of noise bursts by an adjacent vowel for hearing-impaired listeners.

Some masking effects of synthetic vowels on adjacent noise bursts are reported for hearing-impaired listeners. The 200-ms vowels were similar to [a] and [i]; the 50-ms noise bursts were in one of three frequency bands: .5 to 1.5, 1.5 to 4, or 4 to 6 kHz. With voiceless-stop-like temporal intervals between the stimuli, there was little backward or forward masking of the noise bursts by either vowel. Some forward masking occurred under proximate conditions of vowel and burst in time and frequency.

Effects of dichotic/diotic versus monotic presentation on speech understanding in noise in elderly hearing-impaired listeners.

Hearing-impaired elderly subjects with mild to moderate, bilaterally symmetrical, sensorineural hearing losses received speech stimuli mixed with cafeteria and speech-babble noises in a variety of listening conditions. Open and closed set Rhyme test words and Speech Perception in Noise test sentences were presented at individually determined signal-to-noise ratios. the listening conditions included: (1) monotic against which all other conditions were compared; (2) low frequency attenuation of 5, 10, and 15 dB relative to the high frequencies; (3) dichotic, with the frequency band above 1000 Hz presented to the preferred ear and the band below 1000 Hz to the other ear; and (4) diotic, with identical speech-in-noise signals presented simultaneously to both ears. Results indicated: (1) regardless of type of stimulus, open or closed set presentation, or type of noise, most of the dichotic and/or diotic presentations were superior to monotic at 0.05 or 0.01 significance levels; (2) under most conditions, dichotic/diotic conditions were superior to the low frequency attenuated presentations; (3) subjects with abnormal Synthetic Sentence Inventory functions seemed to use dichotic/diotic input as well as those with normal Synthetic Sentence Inventory performance; (4) subjects with borderline hearing losses showed greater amounts of dichotic/diotic superiority than those with mild-to-moderate losses. Further research with wearable hearing aids is indicated.