Ability in perceiving nonnative contrasts: performance on natural and synthetic speech stimuli.

The perception of the distinction between /r/ and /l/ by native speakers of American English and of Japanese was studied using natural and synthetic speech. The American subjects were all nearly perfect at recognizing the natural speech sounds, whereas there was substantial variation among the Japanese subjects in their accuracy of recognizing /r/ and /l/ except in syllable-final position. A logit model, which additively combined the acoustic information conveyed by F1-transition duration and by F3-onset frequency, provided a good fit to the perception of synthetic /r/ and /l/ by the American subjects. There was substantial variation among the Japanese subjects in whether the F1 and F3 cues had a significant effect on their classifications of the synthetic speech. This variation was related to variation in accuracy of recognizing natural /r/ and /l/, such that greater use of both the F1 cue and the F3 cue in classifying the synthetic speech sounds was positively related to accuracy in recognizing the natural sounds. However, multiple regression showed that use of the F1 cue did not account for significant variance in natural speech performance beyond that accounted for by the F3 cue, indicating that the F3 cue is more important than the F1 cue for Japanese speakers learning English. The relation between performance on natural and synthetic speech also provides external validation of the logit model by showing that it predicts performance outside of the domain of data to which it was fit.     

Implicit training of nonnative speech stimuli

Learning nonnative speech contrasts in adulthood has proven difficult. Standard training methods have achieved moderate effects using explicit instructions and performance feedback. In this study, the authors question preexisting assumptions by demonstrating a superiority of implicit training procedures. They trained 3 groups of Greek adults on a difficult Hindi contrast (a) explicitly, with feedback (Experiment 1), or (b) implicitly, unaware of the phoneme distinctions, with (Experiment 2) or without (Experiment 3) feedback. Stimuli were natural recordings of consonant-vowel syllables with retroflex and dental unvoiced stops by a native Hindi speaker. On each trial, participants heard pairs of tokens from both categories and had to identify the retroflex sounds (explicit condition) or the sounds differing in intensity (implicit condition). Unbeknownst to participants, in the implicit conditions, target sounds were always retroflex, and distractor sounds were always dental. Post-training identification and discrimination tests showed improved performance of all groups, compared with a baseline of untrained Greek listeners. Learning was most robust for implicit training without feedback. It remains to be investigated whether implicitly trained skills can generalize to linguistically relevant phonetic categories when appropriate variability is introduced. These findings challenge traditional accounts on the role of feedback in phonetic training and highlight the importance of implicit, reward-based mechanisms.     

Training new, nonnative speech contrasts: a comparison of the prototype and perceptual fading techniques

We trained unilingual adult Canadian francophone listeners to identify the English voiceless and voiced linguadental ("th") frictives, /theta/ and /delta/, using synthetic exemplars of each phoneme. Identification training with feedback improved listeners' abilities to identify both natural and synthetic tokens. These results show that training with appropriately selected prototype stimuli can produce a linguistically meaningful improvement in a listener's ability to identify new, nonnative speech sounds--both natural and synthetic. However, it is not yet clear whether such training with a single prototype can improve performance as effectively as the fading technique used by Jamieson and Morosan (1986), since prototype training produced only a moderate improvement in listeners' identifications of synthetic stimuli containing brief frication. Differences between the techniques may reflect the need for listeners to experience the appropriate types of intraphonemic and interphonemic variability during training. Such variability may help to define the category prototype by desensitizing the subjects to differences between exemplars of the same category and by sharpening sensitivity to differences between categories.     

Examination of perceptual reorganization for nonnative speech contrasts: Zulu click discrimination by English-speaking adults and infants

The language environment modifies the speech perception abilities found in early development. In particular, adults have difficulty perceiving many nonnative contrasts that young infants discriminate. The underlying perceptual reorganization apparently occurs by 10-12 months. According to one view, it depends on experiential effects on psychoacoustic mechanisms. Alternatively, phonological development has been held responsible, with perception influenced by whether the nonnative sounds occur allophonically in the native language. We hypothesized that a phonemic process appears around 10-12 months that assimilates speech sounds to native categories whenever possible; otherwise, they are perceived in auditory or phonetic (articulatory) terms. We tested this with English-speaking listeners by using Zulu click contrasts. Adults discriminated the click contrasts; performance on the most difficult (80% correct) was not diminished even when the most obvious acoustic difference was eliminated. Infants showed good discrimination of the acoustically modified contrast even by 12-14 months. Together with earlier reports of developmental change in perception of nonnative contrasts, these findings support a phonological explanation of language-specific reorganization in speech perception.     

Preferences of infants for regular and distorted natural speech stimuli

The schema hypothesis proposed by Kagan (Science, 1970, 170, 826–832) andLewis (Developmental Psychology, 1969, 1, 75–86) was used to make predictions concerning the preferences of infants 3 to 14 months old for speech stimuli. An operant response method was used in determining the infants' preferences for inflected, monotone, and scrambled natural speech stimuli. Although the infants' preferences did not change with age as predicted, the infants produced clear preference orderings for the three stimuli. The speech preferences were interpreted as being based on stimulus variables (e.g., word order, inflection, and speech rate) in addition to the realism variables assumed by the schema hypothesis.     

The fragile nature of the speech-perception deficit in dyslexia: natural vs synthetic speech

A number of studies reported that developmental dyslexics are impaired in speech perception, especially for speech signals consisting of rapid auditory transitions. These studies mostly made use of a categorical-perception task with synthetic-speech samples. In this study, we show that deficits in the perception of synthetic speech do not generalise to the perception of more naturally sounding speech, even if the same experimental paradigm is used. This contrasts with the assumption that dyslexics are impaired in the perception of rapid auditory transitions.     

Selective attention and cerebral dominance in perceiving and responding to speech messages

The effect of attention on cerebral dominance and the asymmetry between left and right ears was investigated using a selective listening task. Right handed subjects were presented with simultaneous dichotic speech messages; they shadowed one message in either the right or left ear and at the same time tapped with either the right or the left hand when they heard a specified target word in either message. The ear asymmetry was shown only when subjects' attention was focused on some other aspect of the task: they tapped to more targets in the right ear, but only when these came in the non-shadowed message; they made more shadowing errors with the left ear message, but chiefly for non-target words. The verbal response of shadowing showed the right ear dominance more clearly than the manual response of tapping. Tapping with the left hand interfered more with shadowing than tapping with the right hand, but there was little correlation between the degree of hand and of ear asymmetry over individual subjects. The results support the idea that the right ear dominance is primarily a quantitative difference in the distribution of attention to left and right ear inputs reaching the left hemisphere speech areas. This affects both the efficiency of speech perception and the degree of response competition between simultaneous verbal and manual responses.     

Critical role of foreground stimuli in perceiving visually induced self-motion (vection)

The effects of a foreground stimulus on vection (illusory perception of self-motion induced by a moving background stimulus) were examined in two experiments. The experiments reveal that the presentation of a foreground pattern with a moving background stimulus may affect vection. The foreground stimulus facilitated vection strength when it remained stationary or moved slowly in the opposite direction to that of the background stimulus. On the other hand, there was a strong inhibition of vection when the foreground stimulus moved slowly with, or quickly against, the background. These results suggest that foreground stimuli, as well as background stimuli, play an important role in perceiving self-motion.     

The role of rhythm in perceiving speech in noise: a comparison of percussionists,vocalists and non-musicians

The natural rhythms of speech help a listener follow what is being said, especially in noisy conditions. There is increasing evidence for links between rhythm abilities and language skills; however, the role of rhythm-related expertise in perceiving speech in noise is unknown. The present study assesses musical competence (rhythmic and melodic discrimination), speech-in-noise perception and auditory working memory in young adult percussionists, vocalists and non-musicians. Outcomes reveal that better ability to discriminate rhythms is associated with better sentence-in-noise (but not words-in-noise) perception across all participants. These outcomes suggest that sensitivity to rhythm helps a listener understand unfolding speech patterns in degraded listening conditions, and that observations of a “musician advantage” for speech-in-noise perception may be mediated in part by superior rhythm skills.     

Selective adaptation effects on end-point stimuli in a speech series

Recent experiments in speech perception using the selective adaptation paradigm have found that the phonetic boundary of a test series shifts following adaptation. However, no changes within the phonetic category have been found. In the present experiment, a series of voiced CV syllables which varied along the feature of place was used in a selective adaptation paradigm. The end-point stimuli trom the test series were used as adaptors. Subjects used a 6-point rating scale to respond to the stimuli instead of the usual two-category identification. The average rating for end-point stimuli from the same category as the adaptor, as well as the boundary stimuli, shifted as a function of adaptation. In all cases, the average rating response shifted toward that of the unadapted category. The average rating for stimuli in the opposite category from that of the adaptor remained relatively unchanged. These results indicate that the entire category of the adapting stimulus changes as a function of selective adaptation and that the effect is not confined to stimuli near the phonetic boundary.     

Perceiving the sequence of speech and non-speech stimuli

The task is to estimate on which syllable of a spoken sentence a click was superimposed. Experiment I confirms Ladefoged and Broadbent's finding of a systematic tendency to prepose the click (negative displacement), but shows also that the tendency is decreased when prior knowledge of the sentence is provided. Experiment II shows that acoustic prior knowledge is not necessary to produce the decrease and that it occurs also with textual prior knowledge. Experiment III shows that the negative displacement is not eliminated by short-term practice on the task, as Fodor and Bever contended. The effect of prior knowledge is inconsistent with the explanation of negative displacement in terms of attention demands suggested by Ladefoged and Broadbent. It is argued that this explanation was unnecessary, and that negative displacement can be expected in a system which analyses speech by discrete units.     

Spotting animals in natural scenes: efficiency of humans and monkeys at very low contrasts

The ability of monkeys to categorize objects in visual stimuli such as natural scenes might rely on sets of low-level visual cues without any underlying conceptual abilities. Using a go/no-go rapid animal/non-animal categorization task with briefly flashed achromatic natural scenes, we show that both human and monkey performance is very robust to large variations of stimulus luminance and contrast. When mean luminance was increased or decreased by 25–50%, accuracy and speed impairments were small. The largest impairment was found at the highest luminance value with monkeys being mainly impaired in accuracy (drop of 6% correct vs. <1.5% in humans), whereas humans were mainly impaired in reaction time (20 ms increase in median reaction time vs. 4 ms in monkeys). Contrast reductions induced a large deterioration of image definition, but performance was again remarkably robust. Subjects scored well above chance level, even when the contrast was only 12% of the original photographs (≈81% correct in monkeys; ≈79% correct in humans). Accuracy decreased with contrast reduction but only reached chance level -in both species- for the most extreme condition, when only 3% of the original contrast remained. A progressive reaction time increase was observed that reached 72 ms in monkeys and 66 ms in humans. These results demonstrate the remarkable robustness of the primate visual system in processing objects in natural scenes with large random variations in luminance and contrast. They illustrate the similarity with which performance is impaired in monkeys and humans with such stimulus manipulations. They finally show that in an animal categorization task, the performance of both monkeys and humans is largely independent of cues relying on global luminance or the fine definition of stimuli.     

Development and experimentation with synthetic visible speech

We have implemented software for development of synthetic visual speech and perceptual experimentation on a UNIX workstation. We describe recent improvements in the speech synthesis and the capabilities of the development system. We also show how a typical experiment is programmed and describe our solutions for real-time experimental control under the UNIX operating system.     

Perception of temporally competing speech stimuli in preschool children

The aim of this study was to investigate the perception of competing speech stimuli in 3-, 4-, and 5-year-old normally developing children. A dichotic listening paradigm was used in which the temporal alignment between the two stimuli was varied to represent three levels of competition. Minimal, moderate, and maximal levels of temporal competition were represented by a Separation, Lag, and Simultaneous test condition, respectively. The subjects were behaviorally set to listen for and to report the two stimuli on each trial. The incidence of double correct responses in the test conditions was the measure of interest. The results show a sharp and linear drop in double correct scores from the Separation, to the Lag, and to the Simultaneous condition. There were no age-related differences in the Separation and the Simultaneous conditions. In the Lag condition, the performance of the 3-year-olds was significantly lower than the 4- and 5-year-olds. The findings were interpreted to be indicative of limited auditory processing ability in preschoolers for moderately and maximally competing speech stimuli.     

Lateral asymmetry in the perceived sequence of speech and nonspeech stimuli

The task was to estimate the position where a click had been superimposed in a spoken sentence. Experiment 1 confirmed Fodor and Bever’s observation of an ear-asymmetry effect: the click is located earlier when it is presented to the left ear and the sentence to the right ear than with the opposite arrangement. In Experiment 2, combinations of monaural and binaural presentations were considered. They made it possible to eliminate interpretations which link the laterality effect to the fact of reaching or not reaching a particular ear and showed that the relevant factor is the relative position of the stimuli in acoustic space. Experiments 3 and 4 explored the relation between spatial separation and perceived sequence in greater detail. The relation involves a plateau: when the click comes to the left of the speech, it is preposed to a degree which depends on the amount of spatial separation; but, when it comes to the right of the speech, separation is irrelevant and the mean error is of the same order of magnitude as in a control condition without separation.     

Brain potentials to native and non-native speech contrasts in 7- and 11-month-old American infants

Abstract Behavioral data establish a dramatic change in infants' phonetic perception between 6 and 12 months of age. Foreign-language phonetic discrimination significantly declines with increasing age. Using a longitudinal design, we examined the electrophysiological responses of 7- and 11-month-old American infants to native and non-native consonant contrasts. Analyses of the event-related potentials (ERP) of the group data at 7 and at 11 months of age demonstrated that infants' discriminatory ERP responses to the non-native contrast are present at 7 months of age but disappear by 11 months of age, consistent with the behavioral data reported in the literature. However, when the same infants were divided into subgroups based on individual ERP components, we found evidence that the infant brain remains sensitive to the non-native contrast at 11 months of age, showing differences in either the P150-250 or the N250-550 time window, depending upon the subgroup. Moreover, we observed an increase in infants' responsiveness to native language consonant contrasts over time. We describe distinct neural patterns in two groups of infants and suggest that their developmental differences may have an impact on language development.