Effects of speaker variability and phoneme location on speech perception.

The effects of variation in a speaker's voice and temporal phoneme location were assessed through a series of speeded classification experiments. Listeners monitored speech syllables for target consonants or vowels. The results showed that speaker variability and phoneme-location variability had detrimental effects on classification latencies for target sounds. In addition, an interaction between variables showed that the speaker variability effect was obtained only when temporal phoneme location was fixed across trials. A subadditive decrement in latencies produced by the interaction of the two variables was also obtained, suggesting that perceptual loads may not affect perceptual adjustments to a speaker's voice in the same way that memory loads do.

Selective attention in perceptual adjustments to voice.

The effects of perceptual adjustments to voice information on the perception of isolated spoken words were examined. In two experiments, spoken target words were preceded or followed within a trial by a neutral word spoken in the same voice or in a different voice as the target. Over-all, words were reproduced more accurately on trials on which the voice of the neutral word matched the voice of the spoken target word, suggesting that perceptual adjustments to voice interfere with word processing. This result, however, was mediated by selective attention to voice. The results provide further evidence of a close processing relationship between perceptual adjustments to voice and spoken word recognition.

The persuasiveness of synthetic speech versus human speech.

Is computer-synthesized speech as persuasive as the human voice when presenting an argument? After completing an attitude pretest, 193 participants were randomly assigned to listen to a persuasive appeal under three conditions: a high-quality synthesized speech system (DECtalk Express), a low-quality synthesized speech system (Monologue), and a tape recording of a human voice. Following the appeal, participants completed a posttest attitude survey and a series of questionnaires designed to assess perceptions of speech qualities, perceptions of the speaker, and perceptions of the message. The human voice was generally perceived more favorably than the computer-synthesized voice, and the speaker was perceived more favorably when the voice was a human voice than when it was computer synthesized. There was, however, no evidence that computerized speech, as compared with the human voice, affected persuasion or perceptions of the message. Actual or potential applications of this research include issues that should be considered when designing synthetic speech systems.

The perceptual representation of voice gender.

The perceptual representation of voice gender was examined with two experimental paradigms: identification/discrimination and selective adaptation. The results from the identification and discrimination of a synthetic male-female voice continuum indicated that voice gender perception was not categorical. In addition, results from selective adaptation experiments with natural and synthetic voice stimuli indicated that the perceptual representation of voice adapted is an auditory-based representation. Overall, these findings suggest that the perceptual representation of voice gender is auditory based and is qualitatively different from the representation of phonetic information.

Automaticity and the detection of speech.

The development of automatic perceptual responses to speech stimuli was examined. In the first experiment, phoneme-monitoring performance for speech syllables was examined under conditions in which stimulus-to-response mapping and memory load were manipulated. The results indicated that automaticity develops under consistent-mapping conditions. In the second experiment, a dual-task procedure was combined with mapping and selective attention manipulations in order to examine the development of automaticity across single- and multiple-channel conditions. The results indicated that performance under consistently mapped training conditions was interfered with by dividing attention across multiple channels of input. It is concluded that there may be differences in the way that automaticity develops across visual and auditory modalities and that these differences need to be examined more closely.

Comprehension of synthetic speech produced by rule: word monitoring and sentence-by-sentence listening times.

Previous comprehension studies using postperceptual memory tests have often reported negligible differences in performance between natural speech and several kinds of synthetic speech produced by rule, despite large differences in segmental intelligibility. The present experiments investigated the comprehension of natural and synthetic speech using two different on-line tasks: word monitoring and sentence-by-sentence listening. On-line task performance was slower and less accurate for passages of synthetic speech than for passages of natural speech. Recognition memory performance in both experiments was less accurate following passages of synthetic speech than of natural speech. Monitoring performance, sentence listening times, and recognition memory accuracy all showed moderate correlations with intelligibility scores obtained using the Modified Rhyme Test. The results suggest that poorer comprehension of passages of synthetic speech is attributable in part to the greater encoding demands of synthetic speech. In contrast to earlier studies, the present results demonstrate that on-line tasks can be used to measure differences in comprehension performance between natural and synthetic speech.

Stimulus variability and processing dependencies in speech perception.

Processing dependencies in speech perception between voice and phoneme were investigated using the Garner (1974) speeded classification procedure. Variability in the voice of the talker and in the cues to word-initial consonants were manipulated. The results showed that the processing of a talker's voice and the perception of voicing are asymmetrically dependent. In addition, when stimulus variability was increased in each dimension, the amount of orthogonal interference obtained for each dimension became significantly larger. The processing asymmetry between voice and phoneme was interpreted in terms of a parallel-contingent relationship of talker normalization processes to auditory-to-phonetic coding processes. The processing of voice information appears to be qualitatively different from the encoding of segmental phonetic information, although they are not independent. Implications of these results for current theories of speech perception are discussed.

Effects of talker variability on recall of spoken word lists.

Three experiments were conducted to investigate recall of lists of words containing items spoken by either a single talker or by different talkers. In each experiment, recall of early list items was better for lists spoken by a single talker than for lists of the same words spoken by different talkers. The use of a memory preload procedure demonstrated that recall of visually presented preload digits was superior when the words in a subsequent list were spoken by a single talker than by different talkers. In addition, a retroactive interference task demonstrated that the effects of talker variability on the recall of early list items were not due to use of talker-specific acoustic cues in working memory at the time of recall. Taken together, the results suggest that word lists produced by different talkers require more processing resources in working memory than do lists produced by a single talker. The findings are discussed in terms of the role that active rehearsal plays in the transfer of spoken items into long-term memory and the factors that may affect the efficiency of rehearsal.

Some effects of talker variability on spoken word recognition.

The perceptual consequences of trial-to-trial changes in the voice of the talker on spoken word recognition were examined. The results from a series of experiments using perceptual identification and naming tasks demonstrated that perceptual performance decreases when the voice of the talker changes from trial to trial compared to performance when the voice on each trial remains the same. In addition, the effects of talker variability on word recognition appeared to be more robust and less dependent on task than the effects of word frequency and lexical structure. Possible hypotheses regarding the nature of the processes giving rise to these effects are discussed, with particular attention to the idea that the processing of information about the talker's voice is intimately related to early perceptual processes that extract acoustic-phonetic information from the speech signal.

Integral processing of phonemes: evidence for a phonetic mode of perception.

To investigate the extent and locus of integral processing in speech perception, a speeded classification task was utilized with a set of noise-tone analogs of the fricative-vowel syllables (fae), (integral of ae), (fu), and (integral of u). Unlike the stimuli used in previous studies of selective perception of syllables, these stimuli did not contain consonant-vowel transitions. Subjects were asked to classify on the basis of one of the two syllable components. Some subjects were told that the stimuli were computer generated noise-tone sequences. These subjects processed the noise and tone separably. Irrelevant variation of the noise did not affect reaction times (RTs) for the classification of the tone, and vice versa. Other subjects were instructed to treat the stimuli as speech. For these subjects, irrelevant variation of the fricative increased RTs for the classification of the vowel, and vice versa. A second experiment employed naturally spoken fricative-vowel syllables with the same task. Classification RTs showed a pattern of integrality in that irrelevant variation of either component increased RTs to the other. These results indicate that knowledge of coarticulation (or its acoustic consequences) is a basic element of speech perception. Furthermore, the use of this knowledge in phonetic coding is mandatory, even in situations where the stimuli do not contain coarticulatory information.

When selective adaptation and contrast effects are distinct: a reply to Diehl, Kluender, and Parker.

Two alternative conceptualizations of selective adaptation with speech have recently received attention. The adaptation level theory (AL) outlined by Diehl (1981) and a two-stage model outlined by Sawusch and Jusczyk (1981) can both account for much of the adaptation and paired-comparison data. Recently, Diehl, Kluender, and Parker (1985) proposed that all adaptation and contrast data can be accounted for by AL theory. They reported the results of a study that showed evidence of streaming in selective adaptation and claimed that their results provide a counterdemonstration to recent studies that have argued against the AL approach. In the present article, an outline of how the Diehl et al. results can be accounted for by both the two-stage model and AL theory is presented. In addition, a new set of results comparing adaptation and paired-comparison procedures is presented. These results are precisely as predicted by the two-stage model, but they can not be handled by AL theory.