Detection of sounds in the auditory stream: event-related fMRI evidence for differential activation to speech and nonspeech.

The detection of speech in an auditory stream is a requisite first step in processing spoken language. In this study, we used event-related fMRI to investigate the neural substrates mediating detection of speech compared with that of nonspeech auditory stimuli. Unlike previous studies addressing this issue, we contrasted speech with nonspeech analogues that were matched along key temporal and spectral dimensions. In an oddball detection task, listeners heard nonsense speech sounds, matched sine wave analogues (complex nonspeech), or single tones (simple nonspeech). Speech stimuli elicited significantly greater activation than both complex and simple nonspeech stimuli in classic receptive language areas, namely the middle temporal gyri bilaterally and in a locus lateralized to the left posterior superior temporal gyrus. In addition, speech activated a small cluster of the right inferior frontal gyrus. The activation of these areas in a simple detection task, which requires neither identification nor linguistic analysis, suggests they play a fundamental role in speech processing.

Six-month-old infants' preference for lexical words.

Previous work has shown that newborn infants categorically discriminate the fundamental syntactic category distinction between lexical and grammatical words. In this article, we show that by the age of 6 months, infants prefer to listen to lexical over grammatical words. In Experiment 1, infants were habituated to a list of either lexical or grammatical words, and then tested on new lists of words from the same and the contrasting categories. The infants showed recovery to lexical words after habituation to grammatical words but not vice versa. This asymmetry indicates a possible preference for lexical words. In Experiments 2 and 3, preference was assessed directly by presenting infants with alternating trials of lexical and grammatical words, in the central-fixation preference procedure. The infants looked significantly longer during lexical-word than grammatical-word trials. These results show that by 6 months, infants attend preferentially to lexical words. The implications of this emerging attentional preference for subsequent language acquisition are discussed.

Newborn infants' sensitivity to perceptual cues to lexical and grammatical words.

In our study newborn infants were presented with lists of lexical and grammatical words prepared from natural maternal speech. The results show that newborns are able to categorically discriminate these sets of words based on a constellation of perceptual cues that distinguish them. This general ability to detect and categorically discriminate sets of words on the basis of multiple acoustic and phonological cues may provide a perceptual base that can help older infants bootstrap into the acquisition of grammatical categories and syntactic structure.

Influences on infant speech processing: toward a new synthesis.

To comprehend and produce language, we must be able to recognize the sound patterns of our language and the rules for how these sounds "map on" to meaning. Human infants are born with a remarkable array of perceptual sensitivities that allow them to detect the basic properties that are common to the world's languages. During the first year of life, these sensitivities undergo modification reflecting an exquisite tuning to just that phonological information that is needed to map sound to meaning in the native language. We review this transition from language-general to language-specific perceptual sensitivity that occurs during the first year of life and consider whether the changes propel the child into word learning. To account for the broad-based initial sensitivities and subsequent reorganizations, we offer an integrated transactional framework based on the notion of a specialized perceptual-motor system that has evolved to serve human speech, but which functions in concert with other developing abilities. In so doing, we highlight the links between infant speech perception, babbling, and word learning.

Acquisition of word-object associations by 14-month-old infants.

The following experiments were designed to determine the age at which infants can first readily learn word--object pairings with only minimal exposure and without social or contextual support. To address this question, 8- to 14-month-old infants were tested on their ability to form word--object associations in a "switch" design. Infants were habituated to 2 word--object pairings and then tested with 1 trial that maintained a familiar word--object pairing and 1 that involved a familiar word and object in a new combination. Across 6 experiments, only 14-month-old infants formed word--object associations under these controlled testing conditions but appeared to do so only when the objects were moving. Although 8- to 12-month-olds did not form the associations, they appeared to process both the word and the object information. These studies provide strong evidence that 14-month-old infants can rapidly learn arbitrary associations between words and objects, that this ability appears to develop at about 14 months of age, and that the Switch design is a useful method for assessing word--object learning in infancy.

Infants listen for more phonetic detail in speech perception than in word-learning tasks.

Infants aged 4-6 months discriminate the fine phonetic differences that distinguish syllables in both their native and unfamiliar languages, but by 10-12 months their perceptual sensitivities are reorganized so that they discriminate only the phonetic variations that are used to distinguish meaning in their native language. It would seem, then, that infants apply their well honed phonetic sensitivities as they advance and begin to associate words with objects, but the question of how speech perception sensitivities are used in early word learning has not yet been answered. Here we use a recently developed technique to show that when they are required to pair words with objects, infants of 14 months fail to use the fine phonetic detail they detect in syllable discrimination tasks. In contrast, infants of 8 months--who are not yet readily learning words--successfully discriminate phonetic detail in the same task in which infants aged 14 months fail. Taken together, these results suggest a second reorganization in infants's use of phonetic detail as they move from listening to syllables to learning words.

An exploration of why preschoolers perform differently than do adults in audiovisual speech perception tasks.

Preschoolers' perception of audiovisual speech is considerably less influenced by visual information than adults'. We test the hypothesis that experience correctly producing consonants plays a role in developing the underlying representation which mediates the perception of visible speech. We divided preschoolers into two groups: those who made substitution errors and those who did not. Using a newly developed methodology, we tested substituters, nonsubstituters, and adults in an auditory-only condition, a visual-only condition, and an audiovisual condition. There were no differences among groups in the auditory-only condition. Overall, children still showed less visual influence than adults. Among the children, substituters were poorer at lip-reading in the visual-only condition and showed less visual influence on the incongruent audiovisual tokens than did nonsubstituters. These results support our hypothesis that experience correctly producing consonants plays a role in the elaboration of the underlying representation.

Adult and infant perception of two English phones.

Previous research has shown that young infants easily discriminate both native and non-native consonant contrasts, but by 10-12 months of age infants perform like adults and easily discriminate only native consonant contrasts. The present study was designed to determine what kind of experience is required to maintain discrimination of native consonants. To address this question, English listeners of three ages (6-8 months, 10-12 months, and adults) were presented with the phonetic difference, [da] vs [t = a]. This distinction occurs in English but is not phonemic: [t = a] occurs when it follows an [s] (as in /sta/). If passive exposure is sufficient to maintain discrimination, all age groups should discriminate [da] vs [t = a]. However, if phonological status plays a role, then older infants and adults should fail. In experiment 1, English adults judged [da] and [t = a] to be equally good instances of the same phonemic category /da/. In an AX procedure in experiment 2, English adults discriminated [da] vs [t = a] better than chance but worse than native phonemic levels. In the Conditioned Head Turn procedure in experiment 3, adults and 6- to 8-month-old infants discriminated [da] vs [t = a], but 10- to 12-month-old infants did not. Taken together, these results are most consistent with the hypothesis that phonological status plays a role in maintaining discrimination of phonetic information.

Developmental changes in perception of nonnative vowel contrasts.

Discrimination of 2 German vowel contrasts was examined in English-learning infants of 6-8 and 10-12 months of age using a head turn procedure. The younger infants were better able than the older infants to discriminate the nonnative contrasts, but performance at 6-8 months was below levels that have been reported for nonnative consonant contrasts. A 2nd experiment using a habituation looking procedure showed that 4-month-old infants discriminated both German vowel contrasts, but the 6-month-olds could not. The findings are consistent with previous consonant work, revealing a shift from a language-general toward a language-specific pattern during the 1st year of life. However, that shift begins earlier in development for vowels than for consonants.

La langue et les Lèvres: cross-language influences on bimodal speech perception.

Previous research in speech perception has yielded two sets of findings which are brought together in the present study. First, it has been shown that normal hearing listeners use visible as well as acoustical information when processing speech. Second, it has been shown that there is an effect of specific language experience on speech perception such that adults often have difficulty identifying and discriminating non-native phones. The present investigation was designed to extend and combine these two sets of findings. Two studies were conducted using six consonant-vowel syllables (/ba/, /va/, /alpha a/, /da/, /3a/, and /ga/ five of which occur in French and English, and one (the interdental fricative /alpha a/) which occurs only in English. In Experiment 1, an effect of specific linguistic experience was evident for the auditory identification of the non-native interdental stimulus by French-speakers. In Experiment 2, it was shown that the effect of specific language experience extends to the perception of the visible information in speech. These findings are discussed in terms of their implications for our understanding of cross-language processes in speech perception and for our understanding of the development of bimodal speech perception.

Infant preference for both male and female infant-directed talk: a developmental study of attentional and affective responsiveness.

This research increases our understanding of infants' preference for "motherese" by demonstrating that this preference extends to infant-directed talk (IDT) delivered by males as well as females and that infants show both more attentional responsiveness and more affective responsiveness to IDT than to adult-directed talk (ADT). Infants aged 4-5.5 and 7.5-9 months were shown video recordings of male and female adults reciting identical scripts in either IDT or ADT. Attentional preference was measured by the amount of time the infants watched in each condition, and affective responsiveness was measured by two trained raters. Overall, it was found that infants of both ages show greater attentional and affective responsiveness to IDT than to ADT when spoken by either a male or a female. The younger group was also found to be more responsive, on both measures, than the older group. Of perhaps greater significance, it was shown that the behaviour infants displayed in response to IDT makes them more attractive to naive adult judges. This suggests that IDT may facilitate and maintain positive adult-infant interactions.

Phonemic and phonetic factors in adult cross-language speech perception.

Previous research has indicated that young infants can discriminate speech sounds across phonetic boundaries regardless of specific relevant experience, and that there is a modification in this ability during ontogeny such that adults often have difficulty discriminating phonetic contrasts which are not used contrastively in their native language. This pattern of findings has often been interpreted as suggesting that humans are endowed with innate auditory sensitivities which enable them to discriminate speech sounds according to universal phonetic boundaries and that there is a decline or loss in this ability after being exposed to a language which contrasts only a subset of those distinctions. The present experiments were designed to determine whether this modification represents a loss of sensorineural response capabilities or whether it shows a shift in attentional focus and/or processing strategies. In experiment 1, adult English-speaking subjects were tested on their ability to discriminate two non-English speech contrasts in a category-change discrimination task after first being predisposed to adopt one of four perceptual sets. In experiments 2, 3, and 4 subjects were tested in an AX (same/different) procedure, and the effects of both limited training and duration of the interstimulus interval were assessed. Results suggest that the previously observed ontogenetic modification in the perception of non-native phonetic contrasts involves a change in processing strategies rather than a sensorineural loss. Adult listeners can discriminate sounds across non-native phonetic categories in some testing conditions, but are not able to use that ability in testing conditions which have demands similar to those required in natural language processing.

Developmental aspects of cross-language speech perception.

Previous research has suggested that infants discriminate many speech sounds according to phonemic category regardless of language exposure, while adults of one language group may have difficulty discriminating nonnative linguistic contrasts. Our study attempted to address directly questions about infant perceptual ability and the possibility of its decline as a function of development in the absence of specific experience by comparing English-speaking adults, Hindi-speaking adults, and 7-month-old infants on their ability to discriminate 2 pairs of natural Hindi (non-English) speech contrasts. To do this, infants were tested in a "visually reinforced infant speech discrimination" paradigm, while a variant of this paradigm was used to test adults. Support was obtained for the above hypotheses. Infants were shown to be able to discriminate both Hindi sound pairs, and support for the idea of a decrease in speech perceptual abilities wih age and experience was clearly evident with the rarer of the 2 non-English contrasts. The results were then discussed with respect to the possible nature and purpose of these abilities.