Auditory enhancement and the perception of concurrent vowels

Listeners identified both constituents of double vowels created by summing the waveforms of pairs of synthetic vowels with the same duration and fundamental frequency. Accuracy of identification was significantly above chance. Effects of introducing such double vowels by visual or acoustical precursor stimuli were examined. Precursors specified the identity of one of the two constituent vowels. Performance was scored as the accuracy with which the other vowel was identified. Visual precursors were standard English spellings of one member of the vowel pair; acoustical precursors were 1-sec segments of one member of the vowel pair. Neither visual precursors nor contralateral acoustical precursors improved performance over the condition with no precursor. Thus, knowledge of the identity of one of the constituents of a double vowel does not help listeners to identify the other constituent. A significant improvement in performance did occur with ipsilateral acoustical precursors, consistent with earlier demonstrations that frequency components which undergo changes in spectral amplitude achieve enhanced auditory prominence relative to unchanging components. This outcome demonstrates the joint but independent operation of auditory and perceptual processes underlying the ability of listeners to understand speech despite adversely peaked frequency responses in communication channels.     

Categories and context in the perception of isolated steady-state vowels

The noncategorical perception of isolated vowels has been attributed to the availability of auditory memory in discrimination. In our first experiment, using vowels from an /i/-/I/epsilon) continuum in a same-different (AX) task and comparing the results with predictions derived from a separate identification test, we demonstrated that vowels are perceived more nearly categorically if auditory memory is degraded by extending the interstimulus interval and/or filling it with irrelevant vowel sounds. In a second experiment, we used a similar paradigm, but in addition to presenting a separate identification test, we elicited labeling responses to the AX pairs used in the discrimination task. We found that AX labeling responses predicted discrimination performance quite well, regardless of whether auditory memory was available, whereas the predictions from the separate identification test were more poorly matched by the obtained data. The AX labeling reponses showed large contrast effects (both proactive and retroactive) that were greatly reduced when auditory memory was interfered with. We conclude from the presence of these contrast effects that vowels are not perceived categorically (that is, absolutely). However, it seems that by taking the effects of context into account properly, discrimination performance can be quite accurately predicted from labeling data, suggesting that vowel discrimination, like consonant discrimination, may be mediated by phonetic labels.     

Detection and resolution of audio-visual incompatibility in the perception of vowels

If audio and video recordings of a talker speaking consonant-vowel syllables containing different consontants are approximately synchronised, observers may fail to detect conflict between the modalities and perceive consonants presented in neither individual modality. The present experiments demonstrate an analogous effect in the perception of vowels. Vision can bias the identity of an acoustical vowel to be more like the vowel presented visually, even when observers detect conflict and are instructed to report only what they hear. The size of the effect is positively related to the size of the physical difference between the visible configuration of the lips and the configuration that would naturally accompany the acoustical vowel. In demonstrating these and other phenomena in audio-visual speech perception, observers behave as if they compute a continuous estimate of the filter function of the vocal tract from both visual and acoustical evidence. If the visual evidence is potent, observers may appear to interpret the acoustical evidence in novel ways. However, these compromises can be predicted from known patterns of acoustical similarity and visual distinctiveness and do not require ad hoc explanations involving categorical levels of perceptual process.     

Vowel perception: A neuroradiological localization of the perception of vowels in the human cortex

Whether the perception of vowels takes place in the right or left cerebral hemisphere, or is dependent on bilateral cortical processes, is of importance in our treatment of patients with language deficiencies. To investigate this problem a phonetic test with a series of vowellike stimuli was administered to 68 right-handed adult patients with cerebral lesions, mainly of vascular origin. The results were compared to the results in a control group of 19 speech therapists. Second, a neuroradiologic method was developed in order to visualize the anatomic site of Wernicke's area in the left hemisphere on CT scans. This method formed the basis for an evaluation of the extent and localization of the patients' lesions in Wernicke's area. Of 46 patients with lesions in the left hemisphere, 19 had no perceptual disturbances and 27 had severe perceptual disturbances with lesions predominantly located in Wernicke's area. Twenty-two patients with lesions in the corresponding area in the right hemisphere showed no perceptual disturbances. The results of this investigation appear to indicate that the perception of vowels in right-handed persons is unilaterally located in Wernicke's area in the left hemisphere.     

The role of distinctive features in the dichotic perception of vowels

A dichotic listening experiment was conducted to determine if vowel perception is based on phonetic feature extraction as is consonant perception. Twenty normal right-handed subjects were given dichotic CV syllables contrasting in final vowels. It was found that, unlike consonants, the perception of dichotic vowels was not significantly lateralized, that the dichotic perception of vowels was not significantly enhanced by the number of phonetic features shared, and that the occurrence of double-blend errors was not greater than chance. However, there was strong evidence for the use of phonetic features at the level of response organization. It is suggested that the differences between vowel and consonant perception reflect the differential availability of the underlying acoustic information from auditory store, rather than differences in processing mechanisms.     

The perception of back vowels: Centre of gravity hypothesis

According to the formant centre of gravity (FCOG) hypothesis, two vowel formants in close proximity are merged during perceptual analysis, and their contribution to vowel quality depends on the centre of gravity of the formant cluster. Findings consistent with this hypothesis are that two formants can be replaced by a single formant of intermediate centre frequency, provided their separation is less than 3-3.5 Bark; and that changes in their relative amplitudes produce systematic shifts in vowel quality. In Experiment 1, listeners adjusted the frequencies of F1 and F2 in a synthesized 6-formant vowel (with the F1-F2 separation fixed at 250 Hz, i.e. less than 3 Bark) to find the best phonetic match to a reference vowel with modified formant amplitudes. Contrary to FCOG predictions, F2 attenuation did not produce lower frequency matches. Raising the amplitude of F2 led to predicted upward shifts in formant frequencies of the matched vowel, but with increased variability of matches for some stimuli. In Experiment 2, listeners identified synthesized vowels with a range of separations of F1 and F2. Formant amplitude manipulations had no effect on listeners' judgements when the fundamental frequency was low (125 Hz). Small shifts in vowel quality appeared for stimuli with a high fundamental (250 Hz), but the shifts were significantly larger for F1-F2 separations greater than 3.5 Bark. These effects of formant amplitude are qualitatively different from those observed with single-formant vowels and are generally incompatible with a formant-averaging mechanism.     

Intrinsicf0 differences in spoken and sung vowels and their perception by listeners

We explore how listeners perceive distinct pieces of phonetic information that are conveyed in parallel by the fundamental frequency (f0) contour of spoken and sung vowels. In a first experiment, we measured differences inf0 of /i/ and /a/ vowels spoken and sung by unselected undergraduate participants. Differences in “intrinsicf0” (withf0 of /i/ higher than of /a/) were present in spoken and sung vowels; however, differences in sung vowels were smaller than those in spoken vowels. Four experiments tested a hypothesis that listeners would not hear the intrinsicf0 differences as differences in pitch on the vowel, because they provide information, instead, for production of a closed or open vowel. The experiments provide clear evidence of “parsing” of intrinsicf0 from thef0 that contributes to perceived vowel pitch. However, only some conditions led to an estimate of the magnitude of parsing that closely matched the magnitude of produced intrinsicf0 differences.     

Adults' perception of native and nonnative vowels: implications for the perceptual magnet effect.

Two experiments were conducted to evaluate the perceptual magnet effect. In Experiment 1, American English speakers representing diverse dialects were presented with a fine-grained set of stimuli (varying in just noticeable differences for F1 and F2) and indicated whether they heard "/i/" or "not/i/," thus delimiting the /i/ portion of the vowel space for individual subjects. Then these same subjects selected their own /i/ prototype with a method-of-adjustment procedure. The data from this experiment were used to synthesize customized prototype and nonprototype stimulus sets for Experiment 2. In Experiment 2, 24 of our original 37 subjects completed a discrimination task for each of three conditions, in which vector stimuli varied from the subject's prototype, the nonprototype, or a foreign vowel (/y/) in 15-mel steps. Subjects displayed higher discrimination, as indexed by d', for the nonprototype condition than they did for both the prototype and the foreign conditions. In addition, discrimination was better for variants further away from the referent in each condition. However, discrimination was not especially poor for stimuli close to subjects' individual prototypes--a result that would have yielded the strongest support for the operation of a magnet effect. This negative finding, together with other aspects of our results, raises problems for any theory of vowel perception that relies solely on "one-size-fits-all" prototype representations.     

The role of duration and rapid temporal processing on the lateral perception of consonants and vowels

This study explored the extent to which rapid temporal processing and duration contribute to the right-ear advantage (REA) and presumably left-hemisphere processing for stop consonants and the lack of clear-cut laterality effects for vowels. Three sets of synthetic stimuli were constructed: consonant vowel stimuli [ba da ga bi di gi bu du gu] of 300 msec duration (full stimuli) and two shortened stimuli consisting either of a noise burst and 40-msec transitions (40-msec stimuli), or a noise burst and 20-msec transitions (20-msec stimuli). Stimuli were presented dichotically for consonant, vowel, and syllable identification. Results indicated a significant REA for consonants in the full and 40-msec conditions and a non-significant REA in the 20-msec condition. Nevertheless, the magnitude of laterality did not change across the three conditions. These results suggest that although transition information including duration contributes to lateralization for stop consonants, it is the presence of abrupt onsets which crucially determines lateralized processing. For vowels, there was a significant REA only in the full stimulus condition, and a significant decrement in the magnitude of the laterality effect in the two shortened stimulus conditions. These results suggest that for vowel perception, it is the nature of the acoustic cue used for phonetic identification and not duration that seems to be the critical determinant of lateralization effects.     

Speech perception by budgerigars (Melopsittacus undulatus): spoken vowels

Discrimination of natural, sustained vowels was studied in 5 budgerigars. The birds were trained using operant conditioning procedures on a same-different task, which was structured so that response latencies would provide a measure of stimulus similarity. These response latencies were used to construct similarity matrices, which were then analyzed by multidimensional scaling (MDS) procedures. MDS produced spatial maps of these speech sounds where perceptual similarity was represented by spatial proximity. The results of the three experiments suggest that budgerigars perceive natural, spoken vowels according to phonetic categories, find the acoustic differences among different talkers less salient than the acoustic differences among vowel categories, and use formant frequencies in making these complex discriminations.     

A method for sensation scaling based on an analogy between perception and judgment

Two techniques for studying judgment are discussed. In the first of these one assumes invariance of perception over different judgment processes and conceives of the properties of one judgment process as known. Then, perceptions may be estimated under this process and utilized in the study of other judgment processes of interest. A second technique disposes of the reliance up on a basic, “known” process and instead suggests that perception be treated as free parameters to be estimated from data. The paper then proceeds to show how this second technique may be used under certain conditions on perception themselves. A concept of sensation is introduced which is thought of as generating percepts in formally the same manner as percepts are thought of as generating judgments. The idea is tried out on the perception of movement time. A simple perception process appears to give a good fit to data.     

Ear differences in the recall of fricatives and vowels

Two experiments on the free recall of dichotically presented synthetic speech sounds are reported. The first shows that the right ear advantage for initial fricative consonants is not simply a function of the recognition response class, but that it is also a function of the particular acoustic cues used to achieve that response. This is true both for the whole response, and for the constituent phonetic features. The second experiment shows that when both the response class and the particular stimuli presented on certain trials are held constant, the right ear advantage for the constant stimuli can be influenced by the range of other stimuli occurring in the experiment. Vowels show a right ear advantage when, within the experiment, there is uncertainty as to vocal tract size, but they show no ear advantage when all the vowels in the experiment are from the same vocal tract. These results are interpreted as demonstrating that there are differences between the ears, and probably between the hemispheres, at some stage between the acoustic analysis of the signal and its identification as a phonetic category.     

The relation between discriminability and memory for vowels,consonants, and silent-center vowels

People remember lists of vowel-contrasting syllables better than lists that vary only in stop consonant identity. Most views suggest that this difference is due to the structure of immediate memory and the greater discriminability of vowels compared with consonants. In all of these views, there is a presumed systematic relationship between discriminability and recall so that the more discriminable an item, the better that item should be recalled. The 11 experiments reported here measured the relative discriminability of and compared serial recall for (1) intact syllables that varied only in the medial vowel, (2) intact syllables that varied only in the initial consonant, and (3) syllables with the center vowel replaced by silence (so-called silent-center vowels). When item discriminability, as measured by identification, was equated for consonant-contrasting and silent-center lists, serial recall performance was also equal. However, even when the vowels were less discriminable than the consonants or silentcenter vowels, serial recall performance for the vowels was still better. These results are problematic for theories based on acoustic discriminability but can be explained parsimoniously by Nairne’s (1990) feature model.     

Selective adaptation of vowels

Two experiments investigating the selective adaptation of vowels examined changes in listeners’ identification functions for the vowel continuum [i-I-∈] as a function of the adapting stimulus. In Experiment I, the adapting stimuli were [i], [I], and [∈]. Both the [i] and [∈] stimuli produced significant shifts in the neighboringand distant phonetic boundaries, whereas [I] did not result in any adaptation effects. In order to explore the phonetic nature of feature adaptation in vowels, a second experiment was conducted using the adapting stimuli [gig] and [g ∈ g], which differed acoustically from the [i] and [∈] vowels on the identification continuum. Only [gig] yielded reliable adaptation effects. The results of these experiments were interpreted as suggesting arelative rather than a stableauditory mode of feature analysis in vowels and a possibly more complex auditory feature analysis for the vowel [i].     

Ethical intuitionism and the linguistic analogy

It is a central tenet of ethical intuitionism as defended by W. D. Ross and others that moral theory should re?ect the convictions of mature moral agents. Hence, intuitionism is plausible to the extent that it corresponds to our well-considered moral judgments. After arguing for this claim, I discuss whether intuitionists o?er an empirically adequate account of our moral obligations. I do this by applying recent empirical research by John Mikhail that is based on the idea of a universal moral grammar to a number of claims implicit in W. D. Ross’s normative theory. I argue that the results at least partly vindicate intuitionism.     

Processing of visually presented vowels in the cerebral hemispheres

The respective efficiency of the cerebral hemispheres in processing visually presented vowels is discussed and examined in a tachistoscopic experiment with normal subjects. It is suggested that results from dichotic listening experiments cannot be used to predict performance on the same material in the visual modality. The physical characteristics of the stimulus, in both modalities, contribute to the pattern of hemispheric advantage and cannot be ignored in the interpretation of results from dichotic listening and tachistoscopic experiments.     

Auditory and phonetic memory codes in the discrimination of consonants and vowels

Recognition memory for consonants and vowels selected from within and between phonetic categories was examined in a delayed comparison discrimination task. Accuracy of discrimination for synthetic vowels selected from both within and between categories was inversely related to the magnitude of the comparison interval. In contrast, discrimination of synthetic stop consonants remained relatively stable both within and between categories. The results indicate that differences in discrimination between consonants and vowels are primarily due to the differential availability of auditory short-term memory for the acoustic cues distinguishing these two classes of speech sounds. The findings provide evidence for distinct auditory and phonetic memory codes in speech perception.