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.     

Response organization in selective adaptation to speech sounds

Previous experiments in speech perception using the selective adaptation procedure have found a shift in the locus of the category boundary for a series of speech stimuli following repeated exposure to an adapting syllable. The locus of the boundary moves toward the category of the adapting syllable. Most investigators have interpreted these findings in terms of feature detector models in which specific detectors are reduced in sensitivity through repeated adaptation. The present experiment was conducted to determine whether the adaptation results might be due to changes in response organization as a consequence of the labeling instructions presented to subjects in selective adaptation experiments. A perceptually ambiguous speech stimulus was selected from the middle of a [bi]-[di] test series and used as an adaptor under two different sets of instructions. One group of subjects was told that the adapting stimulus was the syllable [bi], while another group was told that the stimulus was the syllable [di]. The acoustically ambiguous adaptor failed to produce a shift in the locus of the category boundary in the direction predicted on the basis of the labeling instructions presented to subjects. These results indicate that the acoustic attributes and perceived quality of the adapting stimulus determine the direction and magnitude of the adaptation effects rather than the labels provided by the experimenter.     

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.     

Vowel-contingent feature detection

Do vowel-contingent selective adaptation effects for place of articulation depend on vowel identity, or on the particular formant frequencies used? An experiment is reported here which tested the adaptation effects of consonants with exactly the same formant transitions before different diphthongs. In this experiment, the phonetic identity of the vowel and the formant frequencies of the consonant are not confounded as they have been in previous studies. In the contingent adaptation condition, no phoneme boundary shifts were observed, and this is interpreted along with previous evidencefor such phoneme boundary shifts when phonetic identity of the vowel and formant frequencies are confounded as indicating that adaptation operates on the spectral representation of the stimulus. Other evidence consistent with this conclusion is that adaptation with alternating adaptors from each end of the test series produced negligible shifts, and that a single adaptor from a diphthong series different from that of the test series produced phoneme boundary shifts as large as those with a single adaptor from the same test series.     

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.     

Audiovisual presentation demonstrates that selective adaptation in speech perception is purely auditory

MRC Institute of Hearing Research, University of Nottingham, Nottingham NG72RD, England Both auditory and phonetic processes have been implicated by previous results from selective adaptation experiments using speech stimuli. It has proved difficult to dissociate their individual contributions because the auditory and phonetic structure of conventional acoustical stimuli are mutually predictive. In the present experiment, the necessary dissociation was achieved by using an audiovisual adaptor consisting of an acoustical [b?] synchronized to a video recording of a talker uttering the syllable [g?]. This stimulus was generally identified as one of the dentals [d?] or [??]. It produced an adaptation effect, measured with an acoustical [b?-d?] test continuum, identical in size and direction to that produced by an acoustical [b?]—an adaptor sharing its acoustical structure—and opposite in direction to that produced by an acoustical [d?]—an adaptor sharing its perceived phonetic identity. Thus, the result strongly suggests that auditory rather than phonetic levels of processing are influenced in selective adaptation.     

Auditory and phonetic processes in place perception for stops

Use of the selective adaptation procedure with speech stimuli has led to a number of theoretical positions with regard to the level or levels of processing affected by adaptation. Recent experiments (i.e., Sawusch & Jusczyk, 1981) have, however, yielded strong evidence that only auditory coding processes are affected by selective adaptation. In the present experiment, a test series that varied along the phonetic dimension of place of articulation for stops ([da]-[ga]) was used in conjunction with a [ska] syllable that shared the phonetic value of velar with the [ga] end of the test series but had a spectral structure that closely matched a stimulus from the [da] end of the series. As an adaptor, the [ska] and Ida] stimuli produced identical effects, whereas in a paired-comparison procedure, the [ska] produced effects consistent with its phonetic label. These results offer further support for the contention that selective adaptation affects only the auditory coding of speech, whereas the paired-comparison procedure affects only the phonetic coding of speech. On the basis of these results and previous place-adaptation results, a process model of speech perception is described.     

Comparison of consonantal and vocalic cues in selective adaptation

The acoustic cues to the phonetic identity of diphthongs normally include both spectral quality and dynamic change. This fact was exploited in a series of selective adaptation experiments examining the possibility of mutual adaptive effects between these two types of acoustic cues. One continuum of syllables varying from [εi] to [εd] and another varying from [ε] to [εi] were synthesized; endpoint stimuli of both series used as adaptors caused identification boundaries to be shifted. Cross-series adaptation was also attempted on the [ε?εi] stimuli, using [?], [∞], and [ai]. Only [ai] proved effective as an adaptor, suggesting the mediation of a rather abstract auditory level of similarity. The results argue strongly against interpretations in terms of feature detectors, but appear compatible with an “auditory contrast” explanation, which might in turn be incorporated within adaptation level theory in the form recently discussed by Restle (1978). The cross-series results further suggest that selective adaptation might be used to quantify the perceptual distance between auditory cues in speech.     

Auditory property detectors and processing place features in stop consonants

The effects of selective adaptation on the perception of consonant-vowel (CV) stimuli varying in place of production was studied under two conditions. In the first condition, repeated presentation of a CV syllable produced an adaptation effect resulting in a shift in the locus of the phonetic boundary between [ba] and [da]. This result replicated previously reported findings. However, in the second condition, an adaptation effect was obtained on this same test series when the critical acoustic information (i.e., formant transitions) was present in final position of a VC speech-like syllable. These latter results support an auditory account of selective adaptation based on the spectral similarity of the adapting stimuli and test series rather than a more abstract linguistic account based on phonetic identity.     

A further parallel between selective adaptation and contrast

It is generally believed that selective adaptation effects in speech perception are due to a reduction in sensitivity of auditory feature detectors. Recent evidence suggest that these effects may derive instead from contrast. In a further test of the contrast hypothesis, we conducted two experiments each involving both adaptation and contrast sessions with matching stimulus sets. During the adaptation sessions of Experiment 1, subjects identified two series of velar stimuli varying in voice onset time, [ga]-[kha] and [gi]-[khi], before and after adaptation with of the following stimuli: [ga], [kha], [gi], and [khi]. In the contrast session, subjects identified either of two ambiguous test items (drawn from near the phonetic boundaries of the [ga]-[kha] and the [gi]-[khi] series) following a single presentation of [ga], [kha], [gi], or [khi]. For both the adaptation and contrast sessions, (a) the [--a] test items were more greatly affected (in a contrast direction) by the [--a] than by the [--i] adaptor/context stimuli, and (b) the [--i] test items were not differentially affected by the [--1] and [--i] adaptor/context stimuli. An analogous design was used in Experiment 2, except that the stimulus sets varied in pitch rather than vowel quality. For both the adaptation and contrast sessions, the test items were not differentially affected by the pitch of the adaptor/context stimulus. These parallel results provide further evidence that adaptation effects are actually a form of contrast.     

Effect of speaking rate on the perceptual structure of a phonetic category

When listeners process temporal properties of speech that convey information about the phonetic segments of the language, they do so in a rate-dependent manner. This is seen as a shift in the location of the phonetic category boundary along a temporal continuum toward longer values of the acoustic property in question, as speech is slowed. In a series of experiments, we found that the adjustment for rate is not confined to the region of the category boundary, but extends throughout the phonetic category. Specifically, a change in rate modified the range of stimuli identified as members of a phonetic category, as well as which stimuli were overtly judged to be good exemplars of the category. These findings suggest that the listener's adjustment for speaking rate entails a comprehensive perceptual remapping between acoustic signal and phonetic structure.     

Evidence for a central representation of instrument timbre

An information processing account of perception seeks to delineate the stages of processing through which a stimulus passes and determine the properties of the representation at each stage. Research in phonetic perception has identified two stages, the second of which is thought to encode abstract acoustic attributes of sounds. The present study provided a further test of this proposal by assessing whether nonphonetic stimuli could yield results similar to those obtained with phonetic stimuli. Five selective adaptation experiments were carried out with a trumpet—piano timbre continuum. Two manipulations were used to measure abstract encoding: cross-ear presentation of adaptor and test series, and the use of adaptors that were acoustically different from the continuum endpoints. The results provide evidence for an abstract representation of timbre. The similarity of the findings to those in the phonetic adaptation literature is discussed.     

Adaptation of central pitch-specific mechanisms

Three previous psychophysical studies have demonstrated that interaural time difference (ITD) coding mechanisms can undergo frequency-specific, selective adaptation. We sought to determine whether this phenomenon extends to the pitch domain, by employing the same psycho-physical paradigm as one used previously, but with harmonic tone complexes lacking energy at the fundamental frequency. Ten normal listeners participated in experiment 1. Psychometric functions for ITDs were obtained for harmonic tone complexes with fundamental frequencies of 110 Hz and 185 Hz, before and after selective adaptation with complexes of the same fundamental frequencies lateralised to opposite sides. In experiment 1, each subject was tested twice. On separate days, subjects were tested with 110 Hz and 185 Hz stimuli that were either partially resolvable complexes or unresolvable ones. Both partially resolved and unresolved stimuli supported adaptation, and at both fundamental frequencies. In experiment 2, which employed nine listeners, the adaptor tone complexes were presented in conjunction with a diotic noise background designed to mask difference tones generated by the adaptor stimuli. The use of the masker had little effect on the mean strength of the adaptation effected by the unresolved adaptor stimuli, and only slightly weakened the adaptation effect found with the partially resolved adaptor stimuli. Taken together, these data constitute the first demonstration of selective adaptation exerted on a central mechanism in the pitch domain.     

Some properties of linguistic feature detectors

A series of experiments, using a selective adaptation procedure, investigated some of the properties of the linguistic feature detectors that mediate the perception of the voiced and voiceless stop consonants. The first experiment showed that these detectors are centrally rather than peripherally located, in that monotic presentation of the adapting stimulus and test stimuli to different ears resulted in large and reliable shifts in the locus of the phonetic boundary. The second experiment revealed that the detectors are part of the specialized speech processor, inasmuch as adaptation of a voicing detector (as measured by a shift in the phonetic boundary) occurred only when the voicing information was presented in a speech context. In the third experiment, the detector mediating perception of the voiced stops was shown to be more resistant to adaptation than the detector mediating perception of the voiceless stops.     

The selective adaptation effects of burst-cued stops

One of the most compelling arguments that selective adaptation affects a phonetic level of processing is the demonstration that adaptation with burst-cued stimuli (which have no formant transitions) affects the perception of transition-cued stimuli (which have formant transitions but no bursts). Experiment 1 showed that adaptation with burst-cued [pi] and [ti] affects the perception of a (transition-cued) [mi-ni] test series, as well as a [bi-di] test series. Inasmuch as nasals never contain bursts, this demonstrates that the adaptation effect of burst-cued stops is not limited to those stimuli which normally contain bursts. If adaptation with burst-cued stops affects the perception of transition-cued stops at a more central, phonetic level of processing, their adaptation effect should transfer interaurally completely. Experiment 2 showed that the adaptation effect of both burst and transition-cued stops transfers interaurally only about one-third. These results suggest that the adaptation effect of burstcued stops is mediated by the fatigue of peripheral auditory detectors which are sensitive to both bursts and formant transitions.     

Context Effects on Musical Chord Categorization: Different Forms of Top‐Down Feedback in Speech and Music?

A critical issue in perception is the manner in which top-down expectancies guide lower level perceptual processes. In speech, a common paradigm is to construct continua ranging between two phonetic endpoints and to determine how higher level lexical context influences the perceived boundary. We applied this approach to music, presenting participants with major/minor triad continua after brief musical contexts. Two experiments yielded results that differed from classic results in speech perception. In speech, context generally expands the category of the expected stimuli. We found the opposite in music: The major/minor boundary shifted toward the expected category, contracting it. Together, these experiments support the hypothesis that musical expectancy can feed back to affect lower-level perceptual processes. However, it may do so in a way that differs fundamentally from what has been seen in other domains.     

Red herring detectors and speech perception: In defense of selective adaptation

The selective adaptation paradigm was used extensively for about 5 years following its introduction to speech research in 1973. During the next few years, its use dropped dramatically; it is now little used. Several reasons for the abandonment of the paradigm are discussed, and theoretical and empirical justification is provided for rejecting these reasons. Experiment 1 demonstrates that “acoustic similarity” of an adapting sound and test items cannot account for the observed results. Experiments 2–4 demonstrate that adaptation effects are not equivalent to simple contrast effects. These experiments indicate that selective adaptation produces robust reaction time effects—items in the adapted category are identified more slowly than unadapted items. The effects found in a simple paired-contrast procedure differ from those found with selective adaptation. Most strikingly, contrast effects are extremely ear dependent—much larger effects occur if testing is conducted in the right ear than in the left; adaptation effects are relatively symmetrical with respect to ear. The empirical and theoretical analyses suggest that the selective adaptation paradigm can be a powerful tool for investigating the perception of complex acoustic stimuli like speech.     

Inferior Frontal Regions Underlie the Perception of Phonetic Category Invariance

ABSTRACT— The problem of mapping differing sensory stimuli onto a common category is fundamental to human cognition. Listeners perceive stable phonetic categories despite many sources of acoustic variability. What are the neural mechanisms that underlie this perceptual stability? In this functional magnetic resonance imaging study, a short-interval habituation paradigm was used to investigate neural sensitivity to acoustic changes within and between phonetic categories. A region in the left inferior frontal sulcus showed a pattern of activation consistent with phonetic invariance: insensitivity to acoustic changes within a phonetic category and sensitivity to changes between phonetic categories. Left superior temporal regions, in contrast, showed graded sensitivity to both within- and between-category changes. These results suggest that perceptual insensitivity to changes within a phonetic category may arise from decision-related mechanisms in the left prefrontal cortex and add to a growing body of literature suggesting that the inferior prefrontal cortex plays a domain-general role in computing category representations.     

Contextual effects in vowel perception I: Anchor-induced contrast effects

Results from recent experiments using a selective adaption paradigm with vowels have been interpreted as the result of the fatigue of a set of feature detectors. These results could also be interpreted, however, as resulting from changes in auditory memory (auditory contrast) or changing response criteria (response bias). In the present studies, subjects listened to vowels under two conditions: an equiprobable control, with each of the stimuli occurring equally often, and an anchoring condition, with one vowel occurring more often than any of the others. Contrast effects were found in that vowel category boundaries tended to shift toward the category of the anchor, relative to the equiprobable control. Results from these experiments were highly similar to previous selective adaptation results and suggest that neither feature detector fatigue nor response criterion changes can adequately account for the adaptation/ anchoring results found with vowels.     

Properties of feature detectors for speech: Evidence from the effects of selective adaptation on dichotic listening

Two selective adaptation experiments were conducted in order to investigate certain properties of feature detector systems sensitive to the information underlying the voicing distinction (Experiment I) and the place of articulation distinction (Experiment II). The experimental paradigm combined binaural adaptation with a dichotic testing procedure. The stimuli were nonboundary, good exemplars of the respective phonetic categories. In both experiments, there was a systematic shift in performance following adaptation on those trials on which the stimulus in one ear had the adapted feature value and the stimulus in the other ear had the unadapted feature value. On these trials, the adapted feature value was relatively less effective in competing for processing with the unadapted value in the opposing ear (compared to preadaptation performance). Since these results were obtained using nonboundary stimuli, it was argued that (1) adaptation affects the relevant detector along its range of operation or sensitivity, and nof simply at the phonetic boundary, and that (2) information regarding the relative output level of the detector, as well as which detector was more strongly excited, must be available at the site of interaction of the two stimuli.