Internal Structure Of Phonetic Categories Research Articles

More than a boundary shift: Perceptual adaptation to foreign-accented speech reshapes the internal structure of phonetic categories.

The literature on perceptual learning for speech shows that listeners use lexical information to disambiguate phonetically ambiguous speech sounds and that they maintain this new mapping for later recognition of ambiguous sounds for a given talker. Evidence for this kind of perceptual reorganization has focused on phonetic category boundary shifts. Here, we asked whether listeners adjust both category boundaries and internal category structure in rapid adaptation to foreign accents. We investigated the perceptual learning of Mandarin-accented productions of word-final voiced stops in English. After exposure to a Mandarin speaker's productions, native-English listeners' adaptation to the talker was tested in 3 ways: a cross-modal priming task to assess spoken word recognition (Experiment 1), a category identification task to assess shifts in the phonetic boundary (Experiment 2), and a goodness rating task to assess internal category structure (Experiment 3). Following exposure, both category boundary and internal category structure were adjusted; moreover, these prelexical changes facilitated subsequent word recognition. Together, the results demonstrate that listeners' sensitivity to acoustic-phonetic detail in the accented input promoted a dynamic, comprehensive reorganization of their perceptual response as a consequence of exposure to the accented input. We suggest that an examination of internal category structure is important for a complete account of the mechanisms of perceptual learning. (PsycINFO Database Record

Visual influences on the internal structure of phonetic categories.

Previous work has demonstrated that the graded internal structure of phonetic categories is sensitive to a variety of contextual factors. One such factor is place of articulation: The best exemplars of voiceless stop consonants along auditory bilabial and velar voice onset time (VOT) continua occur over different ranges of VOTs (Volaitis & Miller, 1992). In the present study, we exploited the McGurk effect to examine whether visual information for place of articulation also shifts the best exemplar range for voiceless consonants, following Green and Kuhl's (1989) demonstration of effects of visual place of articulation on the location of voicing boundaries. In Experiment 1, we established that /p/ and /t/ have different best exemplar ranges along auditory bilabial and alveolar VOT continua. We then found, in Experiment 2, a similar shift in the best-exemplar range for /t/ relative to that for /p/ when there was a change in visual place of articulation, with auditory place of articulation held constant. These findings indicate that the perceptual mechanisms that determine internal phonetic category structure are sensitive to visual, as well as to auditory, information.

Internal structure of phonetic categories: Some characteristics and constraints

A widely held assumption in the speech perception literature for many years was that during the course of processing listeners derive an abstract phonetic representation and, in doing so, discard information about the fine-grained detail of the speech signal. However, more recent research has shown that the representations of speech are much richer than this emphasis on abstract categories would suggest, and that listeners retain in memory a substantial amount of fine-grained acoustic-phonetic information. One line of evidence for the richness of phonetic representations comes from research showing that phonetic categories are internally structured in a graded fashion, with some members of the category perceived as better exemplars (as more ‘‘prototypical’’) than others. In this talk I will describe findings from our research program that highlight some of the characteristics of these internally structured categories, and discuss how these characteristics place constraints on models of phonetic perception. [Work supported by NIDCD.]

Contextual influences on the internal structure of phonetic categories: A distinction between lexical status and speaking rate

Previous research has shown that phonetic categories have a graded internal structure that is highly dependent on acoustic-phonetic contextual factors, such as speaking rate; these factors alter not only the location of phonetic category boundaries, but also the location of a category's best exemplars. The purpose of the present investigation, which focused on the voiceless category as specified by voice onset time (VOT), was to determine whether a higher order linguistic contextual factor, lexical status, which is known to alter the location of the voiced-voiceless phonetic category boundary, also alters the location of the best exemplars of the voiceless category. The results indicated that lexical status has a more limited and qualitatively different effect on the category's best exemplars than does the acoustic-phonetic factor of speaking rate. This dissociation is discussed in terms of a production-based account in which perceived best exemplars of a category track contextual variation in speech production.

Visual influences on internal structure of phonetic categories

We investigated effects of visual place of articulation on internal category structure for voiceless consonants. Volaitis and Miller [J. Acoust. Soc. Am. 92, 723–735 (1992)], measuring category goodness for tokens from extended auditory voice-onset-time (VOT) /bi/-/pi/-*/pi/ and /gi/- /ki/-*/ki/ continua, found that the best exemplars of /ki/ fell at longer VOTs than those of /pi/, consistent with the well-known difference between /b/-/p/ and /g/-/k/ voicing boundaries. To explore the basis of this best-exemplar shift, we changed perceived place of articulation of auditory stimuli by pairing them with an incongruent visual token (i.e., the McGurk effect). First, we established that the best-exemplar region for /t/ (like /k/) falls at longer VOTs than that for /p/ for auditory bilabial and alveolar continua. Second, subjects rated auditory /bi/-/pi/-*/pi/ stimuli with visual /pi/ (congruent) and /ti/ (incongruent) tokens as exemplars of /pi/ or /ti/, respectively. The best-exemplar region fell at longer VOTs for /ti/ than /pi/, consistent with previously reported visually induced voicing boundary shifts [Green and Kuhl, Percept. Psychophys. 45, 34–42 (1989); Brancazio et al., J. Acoust. Soc. Am. 106, 2270 (1999)]. Thus, perceived place of articulation influences internal category structure even when auditory properties are unchanged, suggesting that the structure is not strictly governed by psychoacoustic factors. [Work supported by NIH/NIDCD.]

Contextual influences on the internal structure of phonetic categories: A distinction between lexical status and speaking rate

A series of experiments examined the effects of an acoustic-phonetic contextual factor, speaking rate, and a higher-order linguistic contextual factor, lexical status, on the internal structure of a voicing category, specified by voice-onset-time (VOT). In keeping with previous results, speaking rate fundamentally altered the structure of the voiceless category, not only affecting the perception of stimuli in the voiced–voiceless category boundary region but also altering which tokens were rated as the best exemplars of the voiceless category. In contrast, the effect of lexical status was more limited. Although (as expected) lexical status also affected the perception of stimuli in the category boundary region, this effect disappeared in the region of the best-rated exemplars. This distinction between the effects of speaking rate and lexical status on the internal structure of the voiceless category mirrors the effects of these factors in speech production: It is well known that speaking rate alters the VOT values of voiceless consonants, whereas we confirmed in a speech production experiment that lexical status has no such effect. Based on these findings, we argue that higher-order contextual factors such as lexical status operate differentialy in processing from production-based, acoustic-phonetic factors such as speaking rate. [Work suppported by NIH/NIDCD.]

Internal structure of phonetic categories: effects of speaking rate.

Many studies have shown that listeners process speech in a rate-dependent manner, altering the location of phonetic category boundaries in accord with the acoustic consequences of a change in rate during speech production. In a recent series of papers that focused on a voicing contrast, we reported that the perceptual adjustment for rate is not limited to the region of the category boundary, but extends to well within the category, producing a change in which stimuli are perceived to be the best category exemplars. In the current paper, we provide evidence for the generality of this effect by showing analogous results for a /b/-/w/ contrast, specified by transition duration. The implications of these findings for models of rate-dependent processing are discussed.

Internal structure of phonetic categories: Evidence for within-category trading relations

Phonetically relevant acoustic properties perceptually trade against each other at phonetic category boundaries. The present investigation used a category goodness judgment task to examine whether such properties also trade against each other in specifying the best exemplars of phonetic categories. The experiments focused on the say–stay distinction, specified by F1 onset frequency and silence duration preceding F1 onset. The main experiment demonstrated a within-category trading relation, such that as F1 onset frequency increased from 230 to 430 Hz a longer silence was required for stimuli to be judged the best exemplars of stay. Two follow-up experiments explored the robustness of this effect. Taken together, the findings underscore the importance of multiple acoustic properties in specifying the internal structure of phonetic categories.

The influence of sentential speaking rate on the internal structure of phonetic categories

In the experiments reported in this paper we compared the effects of syllable-level and sentence-level speaking rate on phonetic perception. In an earlier set of experiments, we found that syllable-level rate influences the internal perceptual structure of phonetic categories [Miller and Volaitis, Percept. Psychophys. 46, 505-512 (1989); Volaitis and Miller, J. Acoust. Soc. Am. 92, 723-735 (1992)]. Specifically, a change in target-syllable rate altered the location of the stimuli judged to be the best category exemplars, as well as the width of this best-exemplar range. In the present investigation, it was asked whether sentence-level rate has the same influence. It was found that slowing sentence rate shifted the location of the best-exemplar range, but did not alter its width. These findings are discussed in terms of timing mechanisms that may operate during speech perception.

The internal structure of phonetic categories is robust

Earlier data were presented that underscored the fine-grained internal structure of phonetic categories and the dependence of this structure on the multiple acoustic properties underlying a given phonetic segment [J. Acoust. Soc. Am. 92, 2464(A) (1992)]. Specifically, it was shown that two properties specifying the distinction between stay and sayF1 onset frequency and silence duration preceding F1 onset, enter into a within-category trading relation, as revealed by category goodness judgments of stay: These properties jointly determine which variants are perceived as the best exemplars of stay. The current study examined the robustness of these effects. It was found that both the fine-grained category structure and the within-category trading relation were maintained even when the stimuli were presented in multitalker ‘‘babble’’ noise at an adverse S/N ratio (0 dB). Moreover, we found that these effects held for sinewave replicas of the speech stimuli, for fully half of the subjects tested (the other half yielding a variety of response patterns, as is typical of sinewave speech). These findings attest to the robustness of within-category structure, including its dependence on multiple acoustic properties. Such robustness would be expected if within-category structure plays a central role in the perception of speech. [Work supported by NIH.]

On the internal structure of phonetic categories: a progress report

There is growing evidence that phonetic categories have a rich internal structure, with category members varying systematically in category goodness. Our recent findings on this issue, which are summarized in this paper, underscore the existence and robustness of this structure and indicate further that the mapping between acoustic signal and internal category structure is complex: just as in the case of category boundaries, the best exemplars of a given category are highly dependent on acoustic-phonetic context and are specified by multiple properties of the speech signal. These findings suggest that the listener's representation of phonetic form preserves not only vategorical information, but also fine-grained information about the detailed acoustic-phonetic characteristics of the language.

The influence of sentence articulation rate on the internal structure of phonetic categories

Numerous experiments have shown that listeners perceive phonetic distinctions in a rate-dependent manner. A case in point is the /b/–/p/ voicing distinction, specified by voice onset time (VOT). One standard rate effect is a shift in the/b/–/p/ boundary toward longer VOT values as speech becomes slower. Both a slowing of the target syllable itself and a slowing of the sentence containing the target syllable produce the effect. A recent investigation [J. L. Miller and L. E. Volaitis, Percept. Psychophys. 46, 505–512 (1989)] demonstrated that when speaking rate is specified by the target syllable, it alters not only the location of the category boundary, but also which stimuli within a category are judged to be the best exemplars of that category. In the present investigation, it was asked whether the rate at which a sentence is articulated can have the same comprehensive effect. A series of 215-ms syllables were created that ranged from /bi/ through /pi/ to a breathy, exaggerated version of /pi/; VOT varied from 10 to 205 ms. Each syllable was inserted into a fast and slow version of the sentence frame ‘‘She said she heard −− here.’’ A preliminary two-choice study (with VOT values between 0 and 60 ms), confirmed the standard effect of sentence rate at the /b/–/p/ category boundary. In the main experiment, listeners heard sentences of the same rate in random order. They rated each target syllable for its goodness as /pi/ on a scale of 1 to 10; the higher the number, the better the exemplar. It was expected that for each sentence rate, only a limited range of target stimuli would be given high ratings. At issue was the location of this range of best exemplars. If sentence rate, like target syllable rate, alters the internal structure of the category, then as the sentence becomes slower, the best-exemplar range should shift toward longer VOT values. This is precisely what happened. This finding underscores the comprehensive nature of the listener’s adjustment for speaking rate during phonetic perception. [Work supported by NIH.]

The role of multiple acoustic properties in specifying the internal structure of phonetic categories

The extent to which a phonetic segment may be specified by more than one acoustic property was investigated using the trading relation between vowel duration and closure duration, two properties known to play a role in specifying the voicing contrast for intervocalic bilabial stops. Specifically asked was whether a change in preceding vowel duration results in a comprehensive remapping between closure duration and phonetic category, or whether trading effects are confined to the category boundary. Two series of disyllables were created ranging from /aba/ through /apa/ to * /apa/ (an exaggerated /p/) having initial vowel durations of 153 and 250 ms, respectively. Closure duration in each series varied from 20 to 400 ms. A preliminary experiment revealed a standard trading relation, in that the /b/‐/p/category boundary was located at a longer closure duration for the stimuli with a long, compared to a short, preceding vowel. In the main experiment, listeners were asked to judge each disyllable in each series for the goodness of its consonant as a member of the /p/ category. For both series the /p/ category was perceived as having internal structure, with a limited range of stimuli being judged as the best exemplars. Furthermore, the range of best exemplars for the long vowel series was displaced relative to that for the short vowel series toward longer values of closure duration. These findings indicate that the acoustic properties in question trade against each other not only at the phonetic category boundary but also within the category. This results in a comprehensive remapping of phonetic category structure similar to that observed in earlier research for changes in speaking rate. [Work supported by NIH.]

Human adults and human infants show a “perceptual magnet effect” for the prototypes of speech categories, monkeys do not

Many perceptual categories exhibit internal structure in which category prototypes play an important role. In the four experiments reported here, the internal structure of phonetic categories was explored in studies involving adults, infants, and monkeys. In Experiment 1, adults rated the category goodness of 64 variants of the vowel i parallel on a scale from 1 to 7. The results showed that there was a certain location in vowel space where listeners rated the i parallel vowels as best instances, or prototypes. The perceived goodness of i parallel vowels declined systematically as stimuli were further removed from the prototypic i parallel vowel. Experiment 2 went beyond this initial demonstration and examined the effect of speech prototypes on perception. Either the prototypic or a nonprototypic i parallel vowels was used as the referent stimulus and adults' generalization to other members of the category was examined. Results showed that the typicality of the speech stimulus strongly affected perception. When the prototype of the category served as the referent vowel, there was significantly greater generalization to other i parallel vowels, relative to the situation in which the nonprototype served as the referent. The notion of a perceptual magnet was introduced. The prototype of the category functioned like a perceptual magnet for other category members; it assimilated neighboring stimuli, effectively pulling them toward the prototype. In Experiment 3, the ontogenetic origins of the perceptual magnet effect were explored by testing 6-month-old infants. The results showed that infants' perception of vowels was also strongly affected by speech prototypes. Infants showed significantly greater generalization when the prototype of the vowel category served as the referent; moreover, their responses were highly correlated with those of adults. In Experiment 4, Rhesus monkeys were tested to examine whether or not the prototype's magnet effect was unique to humans. The animals did not provide any evidence of speech prototypes; they did not exhibit the magnet effect. It is suggested that the internal organization of phonetic categories around prototypic members is an ontogenetically early, species-specific, aspect of the speech code.

Effects of speaking rate on the perceived internal structure of phonetic categories

It is well known that when listeners identify consonants as voiced or voiceless on the basis of voice onset time (VOT), they do so in a rate‐dependent manner: As rate of speech decreases such that overall syllable duration increases, the category boundary moves toward a longer VOT value. We have been examining whether this alteration in boundary location is accompanied by a change in the specific stimuli judged to be the best category exemplars. For our experiment we created two /bi/‐/pi/ series with overall syllable durations of 125 and 325 ms, respectively. VOT varied from 10–120 ms for the 125‐ms series and from 10–320 ms for the 325‐ms series. We asked listeners to judge each syllable for its goodness as a member of the /p/ category, using a scale from 1–10; the higher the number, the better the exemplar. For each series, as VOT increased there was an initial increase in rating response, followed by a subsequent decline. Thus for both the short and long syllables, the /p/ category was perceived as having internal structure, with a limited range of stimuli serving as the best exemplars. However, the entire function for the longer syllables was shifted toward longer VOT values. That is, as syllable duration increased, so too did the VOT values of the stimuli judged to be the best category members. Moreover, in a subsequent control experiment we established that the shift in rating funtion was not due to range or frequency effects, but to the alteration in syllable duration, per se. These findings strongly suggest that the adjustment for speaking rate, whatever the underlying mechanism, involves a comprehensive modification in the mapping between acoustical signal and phonetic structure.

A possible auditory basis for internal structure of phonetic categories.

We used a selective adaptation procedure to investigate the possibility that differences in the degree to which stimuli within a phonetic category are considered to be good exemplars of the category--that is, differences in perceived category goodness--have a basis at a prephonetic, auditory level of processing. For three different phonetic contrasts (/b-p/, /d-g/, /b-w/), we assessed the relative magnitude of adaptation along a stimulus continuum produced by a variety of stimuli from the continuum belonging to a given phonetic category. For all three phonetic contrasts, nonmonotonic adaptation functions were obtained: As the adaptor moved away from the category boundary, there was an initial increase in adaptation, followed by a subsequent decrease. On the assumption that selective adaptation taps a prephonetic, auditory level of processing, these findings permit the following conclusions. First, at an auditory level there is a limit on the range of stimuli along a continuum that is treated as relevant to a given contrast; that is, the stimuli along a continuum are effectively grouped into auditory categories. Second, stimuli within an auditory category vary in their effectiveness as category members, providing an internal structure to the categories. Finally, this internal category structure at the auditory level, revealed by the adaptation procedure, may provide a basis for differences in perceived category goodness at the phonetic level.