Biomedical subjects

C S Crowther

Publications and source records attributed to C S Crowther.

8 recordsLinked to original sources

PubMed2001

Coherence in children's speech perception.

Studies with adults have demonstrated that acoustic cues cohere in speech perception such that two stimuli cannot be discriminated if separate cues bias responses equally, but oppositely, in each. This study examined whether this kind of coherence exists for children's perception of speech signals, a test that first required that a contrast be found for which adults and children show similar cue weightings. Accordingly, experiment 1 demonstrated that adults, 7-, and 5-year-olds weight F2-onset frequency and gap duration similarly in "spa" versus "sa" decisions. In experiment 2, listeners of these same ages made "same" or "not-the-same" judgments for pairs of stimuli in an AX paradigm when only one cue differed, when the two cues were set within a stimulus to bias the phonetic percept towards the same category (relative to the other stimulus in the pair), and when the two cues were set within a stimulus to bias the phonetic percept towards different categories. Unexpectedly, adults' results contradicted earlier studies: They were able to discriminate stimuli when the two cues conflicted in how they biased phonetic percepts. Results for 7-year-olds replicated those of adults, but were not as strong. Only the results of 5-year-olds revealed the kind of perceptual coherence reported by earlier studies for adults. Thus, it is concluded that perceptual coherence for speech signals is present from an early age, and in fact listeners learn to overcome it under certain conditions.

Adult↗

PubMed2000

The effect of segmental order on fricative labeling by children and adults.

We examined whether children modify their perceptual weighting strategies for speech on the basis of the order of segments within a syllable, as adults do. To this end, fricative-vowel (FV) and vowel-fricative (VF) syllables were constructed with synthetic noises from an/[symbol: see text]/-to-/s/continuum combined with natural/a/and/u/portions with transitions appropriate for a preceding or a following /[symbol: see text]/or/s/. Stimuli were played in their original order to adults and children (ages of 7 and 5 years) in Experiment 1 and in reversed order in Experiment 2. The results for adults and, to a lesser extent, those for 7-year-olds replicated earlier results showing that adults assign different perceptual weights to acoustic properties, depending on segmental order. In contrast, results for 5-year-olds suggested that these listeners applied the same strategies during fricative labeling, regardless of segmental order. Thus, the flexibility to modify perceptual weighting strategies for speech according to segmental order apparently emerges with experience.

Adult↗

PubMed1998

Examining the role of auditory sensitivity in the developmental weighting shift.

Studies comparing children's and adults' labeling of speech stimuli have repeatedly shown that children's phonological decisions are more strongly related to portions of the signal that involve rapid spectral change (i.e., formant transitions) and less related to other signal components than are adults' decisions. Such findings have led to a model termed the Developmental Weighting Shift, which suggests that children initially assign particularly strong weight to formant transitions to help delimit individual words in the continuous speech stream but gradually modify these strategies to be more like those of adults as they learn about word-internal structure. The goal of the current study was to test a reasonable alternative: that these apparent age-related differences in perceptual weighting strategies for speech are instead due to age-related differences in auditory sensitivity. To this end, difference limens (DLs) were obtained from children (ages 5 and 7 years) and adults for three types of acoustic properties: dynamic-spectral, static-spectral, and temporal. Two testable hypotheses were offered: Labeling results could reflect either absolute differences in sensitivity between children and adults or relative differences in sensitivity within each group. Empirical support for either hypothesis would indicate that apparent developmental changes in perceptual weighting strategies are actually due to developmental changes in auditory sensitivity to acoustic properties. Results of this study contradicted predictions of both hypotheses, sustaining the suggestion that children's perceptual weighting strategies for speech-relevant acoustic properties change as they gain experience with a native language.

Age Factors↗

PubMed1998

The relative weighting of acoustic properties in the perception of [s] + stop clusters by children and adults.

We examined the perceptual weighting by children and adults of the acoustic properties specifying complete closure of the vocal tract following a syllable-initial [s]. Experiment 1 was a novel manipulation of previously examined acoustic properties (duration of a silent gap and first formant transition) and showed that children weight the first formant transition more than adults. Experiment 2, an acoustic analysis of naturally produced say and stay, revealed that, contrary to expectations, a burst can be present in stay and that first formant transitions do not necessarily distinguish say and stay in natural tokens. Experiment 3 manipulated natural speech portions to create stimuli that varied primarily in the duration of the silent gap and in the presence or absence of a stop burst, and showed that children weight these stop bursts less than adults. Taken together, the perception experiments support claims that children integrate multiple acoustic properties as adults do, but that they weight dynamic properties of the signal more than adults and weight static properties less.

Adult↗

PubMed1995

A measurement-theoretic analysis of the fuzzy logic model of perception.

The fuzzy logic model of perception (FLMP) is analyzed from a measurement-theoretic perspective. FLMP has an impressive history of fitting factorial data, suggesting that its probabilistic form is valid. The authors raise questions about the underlying processing assumptions of FLMP. Although FLMP parameters are interpreted as fuzzy logic truth values, the authors demonstrate that for several factorial designs widely used in choice experiments, most desirable fuzzy truth value properties fail to hold under permissible rescalings, suggesting that the fuzzy logic interpretation may be unwarranted. The authors show that FLMP's choice rule is equivalent to a version of G. Rasch's (1960) item response theory model, and the nature of FLMP measurement scales is transparent when stated in this form. Statistical inference theory exists for the Rasch model and its equivalent forms. In fact, FLMP can be reparameterized as a simple 2-category logit model, thereby facilitating interpretation of its measurement scales and allowing access to commercially available software for performing statistical inference.

Attention↗

PubMed1994

Use of vocalic cues to consonant voicing and native language background: the influence of experimental design.

For native speakers of English and several other languages, preceding vocalic duration and F1 offset frequency are two of the cues that convey the stop consonant voicing distinction in word-final position. For speakers learning English as a second language, there are indications that use of vocalic duration, but not F1 offset frequency, may be hindered by a lack of experience with phonemic (i.e., lexical) vowel length (the "phonemic vowel length account": Crowther & Mann, 1992). In this study, native speakers of Arabic, a language that includes a phonemic vowel length distinction, were tested for their use of vocalic duration and F1 offset in production and perception of the English consonant-vowel-consonant forms pod and pot. The phonemic vowel length hypothesis predicts that Arabic speakers should use vocalic duration extensively in production and perception. On the contrary, Experiment 1 revealed that, consistent with Flege and Port's (1981) findings, they produced only slightly (but significantly) longer vocalic segments in their pod tokens. It further indicated that their productions showed a significant variation in F1 offset as a function of final stop voicing. Perceptual sensitivity to vocalic duration and F1 offset as voicing cues was tested in two experiments. In Experiment 2, we employed a factorial combination of these two cues and a finely spaced vocalic duration continuum. Arabic speakers did not appear to be very sensitive to vocalic duration, but they were about as sensitive as native English speakers to F1 offset frequency. In Experiment 3, we employed a one-dimensional continuum of more widely spaced stimuli that varied only vocalic duration. Arabic speakers showed native-English-like sensitivity to vocalic duration. An explanation based on the perceptual anchor theory of context coding (Braida et al., 1984; Macmillan, 1987; Macmillan, Braida, & Goldberg, 1987) and phoneme perception theory (Schouten & Van Hessen, 1992) is offered to reconcile the apparently contradictory perceptual findings. The explanation does not attribute native-English-like voicing perception to the Arabic subjects. The findings in this study call for a modification of the phonemic vowel length hypothesis.

Adolescent↗

PubMed1992

Native language factors affecting use of vocalic cues to final consonant voicing in English.

Preceding vocalic information can cue final consonant voicing for native English speakers. This study examines subjects' use of two vocalic cues, vocalic duration, and F1 offset frequency, as a function of two native language factors, experience with final stop consonants, and experience with phonemic vowel length. Native speakers of English are compared to native speakers of Japanese and Mandarin Chinese who are learning English as a second language. Experiment 1 measured the F1 offset frequency and vocalic duration in productions of "pod" and "pot." Experiment 2 assessed identification of natural tokens of "pod" and "pot" with and without closure segments and bursts. Experiment 3 assessed categorization of synthetic "pod"-"pot" stimuli that systematically manipulated vocalic duration and F1 offset frequency. Native speakers of English showed the strongest implementation of and sensitivity to vocalic duration, Mandarin speakers showed significantly weaker effects, and Japanese speakers fell in between. Group-related differences in use of F1 offset frequency were smaller, and much clearer in the case of production than perception. It is hypothesized that a lack of experience with final consonants penalizes use of vocalic cues among the Japanese and Mandarin subjects, in general. The advantage of the Japanese subjects for use of vocalic duration further suggests a facilitation effect of experience with phonemic vowel length.

Adolescent↗

PubMed1991

Enhancement of word-recognition performance with a filtering technique.

The NU No. 6 materials spoken by a female speaker were passed through a notch filter centered at 247 Hz with a 34-dB depth. The filtering reduced the amplitude range within the spectrum of the materials by 10 dB that was reflected as a 7.5-vu reduction measured on a true vu meter. Thus, the notch filtering in effect changed the level calibration of the materials. Psychometric functions of the NU No. 6 materials filtered and unfiltered in 60-dB SPL broadband noise were obtained from 12 listeners with normal hearing. Although the slopes of the functions for the two conditions were the same, the functions were displaced by an average of 5.8 dB with the function for the filtered materials located at the lower sound-pressure levels.

Analog-Digital Conversion↗