Randy L. Diehl

The relative perceptual distinctiveness of initial and final consonants in CVC syllables

Among the worlds languages, syllable inventories allowing only initial consonants predominate over those allowing both initial and final consonants. Final consonants may be disfavored because they are less easy to identify and/or more difficult to produce than initial consonants. In this study, two perceptual confusion experiments were conducted in which subjects identified naturally produced consonant-vowel-consonant syllables in different frame sentences. Results indicated that initial consonants were significantly more identifiable than final consonants across all conditions. Acoustic analyses of the test syllables indicated that the relative identifiability of initial and final consonants might be explained in terms of production differences as indicated by the greater acoustic distinctiveness of initial consonants.

The Role of the Insular Cortex in Pitch Pattern Perception: The Effect of Linguistic Contexts

Auditory pitch patterns are significant ecological features to which nervous systems have exquisitely adapted. Pitch patterns are found embedded in many contexts, enabling different information-processing goals. Do the psychological functions of pitch patterns determine the neural mechanisms supporting their perception, or do all pitch patterns, regardless of function, engage the same mechanisms? This issue is pursued in the present study by using 150-water positron emission tomography to study brain activations when two subject groups discriminate pitch patterns in their respective native languages, one of which is a tonal language and the other of which is not. In a tonal language, pitch patterns signal lexical meaning. Native Mandarin-speaking and English-speaking listeners discriminated pitch patterns embedded in Mandarin and English words and also passively listened to the same stimuli. When Mandarin listeners discriminated pitch embedded in Mandarin lexical tones, the left anterior insular cortex was the most active. When they discriminated pitch patterns embedded in English words, the homologous area in the right hemisphere activated as it did in English-speaking listeners discriminating pitch patterns embedded in either Mandarin or English words. These results support the view that neural responses to physical acoustic stimuli depend on the function of those stimuli and implicate anterior insular cortex in auditory processing, with the left insular cortex especially responsive to linguistic stimuli.

Perceptual switching in bilinguals

Three groups of subjects, monolingual English speakers, monolingual Spanish speakers, and English‐Spanish bilinguals, identified a series of naturally‐produced syllables which varied in voice‐onset time (from /ba/ to /pa/). The two monolingual groups differed substantially in their identification performance with English speakers tending to label most of the syllables as /ba/ and Spanish speakers tending to label most of them as /pa/. The bilingual subjects heard the test stimuli in both an English and a Spanish context, each designed to induce a particular language “set”. These subjects perceived a reliably greater number of the test items as /ba/ in the English context than in the Spanish. The magnitude of this perceptual switching effect depends on the listeners degree of bilingualism.

On the Role of Perception in Shaping Phonological Assimilation Rules

Assimilation of nasals to the place of articulation of following consonants is a common and natural process among the worlds languages. Recent phonological theory attributes this naturalness to the postulated geometry of articulatory features and the notion of spreading (McCarthy, 1988). Others view assimilation as a result of perception (Ohala, 1990), or as perceptually tolerated articulatory simplification (Kohler, 1990). Kohler notes that certain consonant classes (such as nasals and stops) are more likely than other classes (such as fricatives) to undergo place assimilation to a following consonant. To explain this pattern, he proposes that assimilation tends not to occur when the members of a consonant class are relatively distinctive perceptually, such that their articulatory reduction would be particularly salient. This explanation, of course, presupposes that the stops and nasals which undergo place assimilation are less distinctive than fricatives, which tend not to assimilate. We report experimental results that confirm Kohlers perceptual assumption: In the context of a following word initial stop, fricatives were less confusable than nasals or unreleased stops. We conclude, in agreement with Ohala and Kohler, that perceptual factors are likely to shape phonological assimilation rules.

A Bayesian approach to the evolution of perceptual and cognitive systems

We describe a formal framework for analyzing how statistical properties of natural environments and the process of natural selection interact to determine the design of perceptual and cognitive systems. The framework consists of two parts: a Bayesian ideal observer with a utility function appropriate for natural selection, and a Bayesian formulation of Darwins theory of natural selection. Simulations of Bayesian natural selection were found to yield new insights, for example, into the co-evolution of camouflage, color vision, and decision criteria. The Bayesian framework captures and generalizes, in a formal way, many of the important ideas of other approaches to perception and cognition.

Trading relations in speech and nonspeech

Two acoustic variables that correlate with the distinction between intervocalic [b] and [p] are closure duration and presence or absence of low-frequency glottal pulsing during the closure interval. These variables may be considered to exhibit a trading relation (Repp, 1982), to the extent that a longer closure is required to perceive the consonant as voiceless when glottal pulsing is present than when it is not. Such trading relations have been interpreted as reflecting a special speech mode of perception. In the present experiments, we demonstrated a trading relation between closure duration and closure pulsing for a set of [aba]-[apa] stimuli. Next we showed that a similar effect could be obtained with square-wave analogue stimuli that mimicked the segment durations and peak amplitudes of the speech stimuli but that were not phonetically categorizable. This nonspeech trading relation depended on the degree of spectral continuity between the low-frequency pulsing and the adjacent portions of the square wave. The implications of these results for the speech mode hypothesis are discussed.

An auditory basis for the stimulus‐length effect in the perception of stops and glides

To investigate possible auditory factors in the perception of stops and glides (e.g., /b/ vs /w/), a two-category labeling performance was compared on several series of /ba/-/wa/ stimuli and on corresponding nonspeech stimulus series that modeled the first-formant trajectories and amplitude rise times of the speech items. In most respects, performance on the speech and nonspeech stimuli was closely parallel. Transition duration proved to be an effective cue for both the stop/glide distinction and the nonspeech distinction between abrupt and gradual onsets, and the category boundaries along the transition-duration dimension did not differ significantly in the two cases. When the stop/glide distinction was signaled by variation in transition duration, there was a reliable stimulus-length effect: A longer vowel shifted the category boundary toward greater transition durations. A similar effect was observed for the corresponding nonspeech stimuli. Variation in rise time had only a small effect in signaling both the stop/glide distinction and the nonspeech distinction between abrupt and gradual onsets. There was, however, one discrepancy between the speech and nonspeech performance. When the stop/glide distinction was cued by rise-time variation, there was a stimulus-length effect, but no such effect occurred for the corresponding nonspeech stimuli. On balance, the results suggest that there are significant auditory commonalities between the perception of stops and glides and the perception of acoustically analogous nonspeech stimuli.

Perception of rhythmic grouping : Testing the iambic/trochaic law

This study was designed to test the iambic/trochaic law, which claims that elements contrasting in duration naturally form rhythmic groupings with final prominence, whereas elements contrasting in intensity form groupings with initial prominence. It was also designed to evaluate whether the iambic/trochaic law describes general auditory biases, or whether rhythmic grouping is speech or language specific. In two experiments, listeners were presented with sequences of alternating /ga/ syllables or square wave segments that varied in either duration or intensity and were asked to indicate whether they heard a trochaic (i.e., strong-weak) or an iambic (i.e., weak-strong) rhythmic pattern. Experiment 1 provided a validation of the iambic/trochaic law in Englishs-peaking listeners; for both speech and nonspeech stimuli, variations in duration resulted in iambic grouping, whereas variations in intensity resulted in trochaic grouping. In Experiment 2, no significant differences were found between the rhythmic-grouping performances of English- and French-speaking listeners. The speech/nonspeech and cross-language parallels suggest that the perception of linguistic rhythm relies largely on general auditory mechanisms. The applicability of the iambic/trochaic law to speech segmentation is discussed.

Feature analyzers for the phonetic dimensionstop vs. continuant

Perception of sounds along the phonetic dimensionstop vs. continuant was studied by means of a selective adaptation procedure. Subjects first identified a series of synthetic consonant-vowel syllables whose formant transitions varied in duration, slope, and amplitude characteristics. They were perceived as either [ba] or [wa]. After the initial identification test, an adapting stimulus was presented repeatedly, and then the subjects again identified the original test series. Adapting with a stop (either [ba] or [da]) led to a decrease in the number of test stimuli identified as [ba], whereas adapting with the continuant sound [wa] led to an increase in the number of [ba] identification responses. Removing the vowel portion of an adapting stimulus greatly reduced the identification shift only when the resulting stimulus was no longer perceived as speech-like. A reduction in the number of [ba] identifications occurred even when a nonspeech “stop” (the sound of a plucked string) was used as the adapting stimulus, suggesting that phonetic processing is not a necessary condition for an adaptation effect.

Contrast effects on stop consonant identification

Changes in the identification of speech sounds following selective adaptation are usually attributed to a reduction in sensitivity of auditory feature detectors. An alternative explanation of these effects is based on the notion of response contrast. In several experiments, subjects identified the initial segment of synthetic consonant-vowel syllables as either the voiced stop [b] or the voiceless stop [ph]. Each test syllable had a value of voice onset time (VOT) that placed it near the English voiced-voiceless boundary. When the test syllables were preceded by a single clear [b] (VOT = -100 msec), subjects tended to identify them as [ph], whereas when they were preceded by an unambiguous [ph] (VOT = 100 msec), the syllables were predominantly labeled [b]. This contrast effect occurred even when the contextual stimuli were velar and the test stimuli were bilabial, which suggests a featural rather than a phonemic basis for the effect. To discount the possibility that these might be instances of single-trial sensory adaptation, we conducted a similar experiment in which the contextual stimuli followed the test items. Reliable contrast effects were still obtained. In view of these results, it appears likely that response contrast accounts for at least some component of the adaptation effects reported in the literature.