Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> The effect of the frequency of occurrence of words upon their intelligibility in noise was examined under two conditions: (1) in unknown message sets where the specific words under test were initially unknown to the listener; and (2) in known message sets where the specific words under test were known to the listener. Substantial effects of word frequency are observed with unknown message sets, but not with known message sets. In known message sets, the prime factor determining intelligibility is the phonemic interconfusability among the words. In unknown message sets, it is suggested that the important determinant of the intelligibility of a word is its frequency of occurrence relative to the frequencies of the words with which it might be confused. <s> BIB001 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> The following interpretation of the word frequency effect—the tendency for common words to be perceived correctly at much lower speech to‐noise ratios than uncommon words—was proposed and verified experimentally. When a stimulus word is only a few decibels below its intelligibility threshold, many of its features—the number of syllables, for example—are still perceived correctly. If enough such features are heard, then only a small number of English words will be consistent with them. Subjects' incorrect responses will be taken from that small set of words, and furthermore they will usually be the relatively common words in the set. If the stimulus is an uncommon word and there are common English words that are phonetically quite similar to it, then these common words will usually be given as (incorrect) responses except at quite high speech‐to noise ratios. This interpretation is confirmed by an analysis of errors in an articulation test. The results show that all subjects tend to give the same incorrect... <s> BIB002 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> When an extraneous sound (such as a cough or tone) completely replaces a speech sound in a recorded sentence, listeners believe they hear the missing sound. The extraneous sound seems to occur during another portion of the sentence without interfering with the intelligibility of any phoneme. If silence replaces a speech sound, the gap is correctly localized and the absence of the speech sound detected. <s> BIB003 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> Lombard noted in 1911 that a speaker changes his voice level similarly when the ambient noise level increases, on the one hand, and when the level at which he hears his own voice (his sidetone) dec... <s> BIB004 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> Cohort theory, developed by Marslen-Wilson and Welsh (1978), proposes that a "cohort" of all the words beginning with a particular sound sequence will be activated during the initial stage of the word recognition process. We used a priming technique to test specific predictions regarding cohort activation in three experiments. In each experiment, subjects identified target words embedded in noise at different signal-to-noise ratios. The target words were either presented in isolation or preceded by a prime item that shared phonological information with the target. In Experiment 1, primes and targets were English words that shared zero, one, two, three, or all phonemes from the beginning of the word. In Experiment 2, nonword primes preceded word targets and shared initial phonemes. In Experiment 3, word primes and word targets shared phonemes from the end of a word. Evidence of reliable phonological priming was observed in all three experiments. The results of the first two experiments support the assumption of activation of lexical candidates based on word-initial information, as proposed in cohort theory. However, the results of the third experiment, which showed increased probability of correctly identifying targets that shared phonemes from the end of words, did not support the predictions derived from the theory. The findings are discussed in terms of current models of auditory word recognition and recent approaches to spoken-language understanding. <s> BIB005 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> Addition of noise to gaps produced by deletion of speech segments can result in both illusory continuity and increased intelligibility. Earlier findings suggest that the perceptual restoration of speech may represent a linguistic adaptation of a nonverbal phenomenon. “Auditory induction” permits perceptual synthesis of contextually appropriate sounds when deleted segments of a signal are replaced by a potential masker. In Experiment 1, when intelligible narrowband discourse was periodically interrupted by silence and by one of a number of bands of noise having different center frequencies, the illusory restoration of continuity was found to follow the spectral rules for nonverbal auditory induction. In Experiment 2, the durational limit of illusory continuity was determined for several types of broadband speech interrupted by broadband noise. The limiting duration was longest for normal discourse interrupted by noise and was equal to the average word duration of that stimulus. The results suggest that auditory induction becomes coupled with special linguistic mechanisms permitting perceptual synthesis of verbal components from noise, with the size of the segments dependent upon the extent of contextual information. <s> BIB006 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> Recent models of speech perception emphasize the possibility of interactions among different processing levels. There is evidence that the lexical status of an utterance (i.e., whether it is a meaningful word or not) may influence earlier stages of perceptual analysis. To test how far down such ‘‘top‐down’’ influences might penetrate, an investigation was conducted to determine whether there is a difference in detectability of words and nonwords masked by amplitude‐modulated or unmodulated broadband noise. The results were negative, suggesting either that the stages of perceptual analysis engaged in the detection task are impermeable to lexical top‐down effects, or that the lexical level was not sufficiently activated to have any facilitative effect on perception. <s> BIB007 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> Deleted segments of speech can be restored perceptually if they are replaced by a louder noise. An earlier study of this "phonemic restoration effect" found that, when recorded discourse was interrupted periodically by noise, the durational limit for illusory continuity corresponded to the average word duration. The present study employed a different passage of discourse recorded by a different speaker. Durational limits for apparent continuity of discourse interrupted by noise were measured at the normal (original) playback speed, as well as at rates that were 15% greater and 15% less. At the normal playback rate, once again the limit of continuity approximated the average word duration--but of especial interest was the finding that changes in playback rate produced proportional changes in continuity limits. These results, together with other evidence, suggest that phonemic restorations represent a special linguistic application of a general auditory mechanism (auditory induction) producing appropriate syntheses of obliterated sounds, and that for discourse the limits of illusory continuity correspond to a fixed amount of verbal information, and not a fixed temporal value. <s> BIB008 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> The perceptual consequences of trial‐to‐trial changes in the voice of the talker on spoken word recognition were examined. The results from a series of experiments using perceptual identification and naming tasks demonstrated that perceptual performance decreases when the voice of the talker changes from trial to trial compared to performance when the voice on each trial remains the same. In addition, the effects of talker variability on word recognition appeared to be more robust and less dependent on task than the effects of word frequency and lexical structure. Possible hypotheses regarding the nature of the processes giving rise to these effects are discussed, with particular attention to the idea that the processing of information about the talker’s voice is intimately related to early perceptual processes that extract acoustic–phonetic information from the speech signal. <s> BIB009 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> To determine how familiarity with a talker's voice affects perception of spoken words, we trained two groups of subjects to recognize a set of voices over a 9-day period One group then identified novel words produced by the same set of talkers at four signal-to-noise ratios Control subjects identified the same words produced by a different set of talkers The results showed that the ability to identify a talker's voice improved intelligibility of novel words produced by that talker The results suggest that speech perception may involve talker-contingent processes whereby perceptual learning of aspects of the vocal source facilitates the subsequent phonetic analysis of the acoustic signal <s> BIB010 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> Since the publication of Oldfield's (1966) seminal article, “Things, Words and the Brain,” a great deal of attention has been devoted to the structural organization of words in the mental lexicon. Most of this research, however, has focused on the structure of higher level aspects of lexical representations, namely the semantic and conceptual organization of lexical items in memory (e.g., Miller & Johnson-Laird, 1976; Smith, 1978). As a consequence, little attention has been directed to the structural organization of the representations of sensory and perceptual information used to gain access to these higher level sources of information. The goal of the present investigation was to explore in detail this structure and its implications for perception of spoken words by normal and hearing-impaired listeners. ::: ::: In the present set of studies, structure will be defined specifically in terms of similarity relations among the sound patterns of words. Similarity will serve as the primary means by which the organization of acoustic-phonetic representations in memory will be investigated. We assume that similarity relations among the sound patterns of spoken words represent one of the earliest stages at which the structural organization of the lexicon comes into play. The precise aim of the present investigation was to gain a detailed understanding of the lower level relations between stimulus input, activation of phonetic representations, and, subsequently, recognition of spoken words. ::: ::: The identification of structure with similarity relations among sound patterns of words raises the difficult problem of defining similarity. Similarity, although crucial to the present investigation, is an ill-defined concept in research on speech perception and spoken word recognition, and one that deserves considerably more work in these areas of research (see Mermelstein, Reference Note 7). However, similarity can be approximated by both computational and behavioral predictors of confusion, the approach taken here. Thus, similarity will be defined in terms of a specific computational metric for predicting confusions among phonetic patterns as well as a behavioral, or operational, metric based on the results of a series of perceptual experiments. ::: ::: Having defined structure as the similarity relations among the sound patterns of words, the question arises: Should the structural organization of representations in memory have consequences for spoken word recognition? Consider a content-addressable memory system in which there is no noise either in the signal or the listener (Kohonan, 1980). In such a system, encoding the acoustic-phonetic information in the stimulus word is tantamount to locating the word in memory. In this case, the structural organization of acoustic-phonetic representations in memory would have no consequences for word identification. Instead, the task of spoken word recognition would be identical to phonetic perception, and one need only study phonetic perception to understand how words are recognized. (By phonetic perception we are referring to the perception of individual segments such as consonants and vowels.) ::: ::: It is undeniable that phonetic perception is important in spoken word recognition. It is not undeniable, however, that the human word recognition system operates as a noiseless content-addressable system or that the acoustic-phonetic signal itself is devoid of noise. To begin, the signal is very often less than ideal for the purposes of the listener. Words are typically perceived against a background of considerable ambient noise, reverberation, and the voices of other talkers. In addition, coarticulatory effects and segmental reductions and deletions substantially restructure the phonetic information in a myriad of ways (Luce & Pisoni, 1987). Although such effects may indeed be useful to the listener (Church, Reference Note 3; Elman & McClelland, 1986), they also undoubtedly produce considerable ambiguities in the speech signal, making a strictly content-addressable word recognition system based on phonetic encoding unrealistic. In short, both the noise inherent in the signal as well as the noise against which the signal is perceived make it unlikely that word recognition is accomplished by direct access, based solely on phonetic encoding, to acoustic-phonetic representations in memory. ::: ::: Not only is the speech signal noisy, so too is the recognition system of the listener. Although the human is clearly well adapted for the perception of spoken language, the system by which language is perceived is by no means a perfect one. In normal listeners, encoding, attentional, and memory demands frequently result in the distortion, degradation, or loss of important acoustic-phonetic information. The data on misperceptions alone attest to the fact that spoken word recognition is less than perfect (Bond & Garnes, 1980). In hearing-impaired listeners, the problems faced by normal listeners are exacerbated by impoverished input representations. Thus, again, a strictly content-addressable system does not suffice as a model of human spoken word recognition. ::: ::: The alternative to a noiseless content-addressable system is one in which the stimulus input activates a number of similar acoustic-phonetic representations or candidates in memory, among which the system must choose (Marslen-Wilson, 1989; Marslen-Wilson & Welsh, 1978; Triesman, 1978a, b). In this system, a considerable amount of processing activity involves discriminating among the lexical items activated in memory. Indeed, many current models of word recognition subscribe to the view that word recognition is to a great degree a process of discriminating among competing lexical items (Forster, 1979; Luce, Pisoni, & Goldinger, 1990; Marslen-Wilson, 1989; Marslen-Wilson & Welsh, 1978; McClelland & Rumelhart, 1981; McQueen, Cutler, Briscoe, & Norris, 1995; Morton, 1979; Norris, 1994). ::: ::: Given that one of the primary tasks of the word recognition system involves discrimination among lexical items, the study of the structural organization of words in memory takes on considerable importance, especially if structural relations can be shown to influence the ease or difficulty of lexical discrimination, and, subsequently, word recognition and lexical access. By the same token, under the assumption that word recognition involves discrimination and selection among competing lexical items, variations in the ease or difficulty of discriminating among items in memory can enlighten us as to the structural organization of the sound patterns of words. In short, lexical discrimination and structure are so inextricably tied together that the study of one leads to a further understanding of the other. ::: ::: Assuming, then, that structural relations among words should influence spoken word recognition via the process of discrimination, it is important to determine that structural differences among words actually exist. Previous research by Landauer and Streeter (1973) has demonstrated that words vary substantially not only in the number of words to which they are similar, but also in the frequencies of these similar words. These findings suggest that both structural and frequency relations among words may mediate lexical discrimination. Investigation of the behavioral effects of these relations should help us to understand further not only the process of lexical discrimination, but also the organization of the sound patterns of spoken words in memory. ::: ::: The issue of word frequency takes on an important role in the investigation of the structural organization of the sound patterns of words. Numerous studies over the years (Howes, 1954, 1957; Newbigging, 1961; Savin, 1963; Soloman & Postman, 1952) have demonstrated that the ease with which spoken words are recognized is monotonically related to experienced frequency, as measured by some objective count of words in the language. However, little work has been devoted to detailing the interaction of word frequency and structural relations among words (see, however, Triesman, 1978a, b). If word frequency influences the perceptibility of the stimulus word, it should also influence the degree of activation of similar words in memory. Frequency is important, then, in further specifying the relative competition among activated items that are to be discriminated among. ::: ::: The goal of the present investigation was, therefore, to examine the effects of the number and nature of words activated in memory on auditory word recognition. Throughout the ensuing discussion, the term similarity neighborhood will be employed. A similarity neighborhood is defined here as a collection of words that are phonetically similar to a given stimulus word. (The term stimulus word will be used to refer to the word for which a neighborhood is computed.) Similarity neighborhood structure refers to two factors: 1) the number and degree of confusability of words in the neighborhood, and 2) the frequencies of the neighbors. This first factor will be referred to as neighborhood density or neighborhood confusability; the second factor will be called neighborhood frequency. In addition to neighborhood structure, the frequency of the stimulus word itself will be of interest. <s> BIB011 </s> Non-native speech perception in adverse conditions: A review <s> The effects of adverse conditions on spoken-word recognition <s> Recent research has shown that listeners can classify talkers based on their regional dialect of American English (AE). The present experiment explored the effects of regional dialect variation on speech intelligibility. Participants from various AE dialects transcribed meaningful English sentences mixed with noise. The sentences were produced by male and female talkers from four different AE regional dialects. At a favorable signal‐to‐noise ratio (−2 dB), intelligibility of General American and Southern talkers was significantly better than Northern and Mid‐Atlantic talkers, and was significantly better for the Northern than the Mid‐Atlantic talkers. At a more difficult signal‐to‐noise ratio (−6 dB), intelligibility was equally poor for the Southern, Northern, and Mid‐Atlantic talkers, but remained significantly better for the General American talkers. No effects of listener dialect were observed. Thus, speech intelligibility in noise was generally poorer for more phonologically marked dialects (e.g., No... <s> BIB012
The processes described in Section 2 are affected by the presence of noise; evidence on this issue is available from the many studies of human speech recognition by native listeners that have used noise as a diagnostic tool, to examine the distribution of information in the speech signal, or to test between alternative models of speech processing. The neighborhood activation model of spoken-word recognition proposed by BIB011 , for instance, was based inter alia on data showing that the relative accuracy and the range of responses for noise-masked spoken words reflected the density of their lexical neighborhood, and the relative frequency of the word itself and its neighbours. It has long been known that noise masking can produce a greater advantage for frequent over uncommon words  BIB002 , but this effect disappears if listeners are choosing from a known set of words rather than an open set BIB001 . Thus there are strong effects of guessing under uncertainty. Similarly, voice familiarity affects recognition under noise; speech by a single speaker is better recognised than speech by multiple speakers BIB009 , speech in previously learned voices better than speech in new voices BIB010 , and speech in common dialects better than speech in less familiar dialects BIB012 . Speech is a temporal signal, so that the beginnings of words are heard before the ends; but this does not mean that stored representations of words in memory can only be successfully contacted if speech information arrives in this canonical order. There is substantial evidence that listeners are able to exploit whatever information is on offerfor instance, that they can extract useful information from the ends of words if they did not hear the beginnings because of noise interference. Van der Vlugt and Nooteboom (1986) found that beginnings and ends of synthesised words were equally effective prompts for correct recognition responses when the rest of the word was masked with noise. BIB005 measured word recognition in noise as a function of prior primes. They found similar effects of primes which overlapped with either the beginning or the end of the target, whether by a single phoneme (e.g. flock preceded by fret or steak), two phonemes (flap, stock) or three (flop, block). The usefulness of all relevant information fits well with our current understanding of spoken-word recognition as described in the preceding section. Effects of informational masking have been argued to depend in part on competition from lexical items activated by the masking speech  . Native listener adjustment to noise is in fact very sensitive, and also accords with our understanding of the spoken-word recognition process. Consider the Lombard effect, whereby people automatically start to talk louder when there is masking noise (see BIB004 . They can also adjust their speech to maintain intelligibility against differing kinds of noise, e.g., by adapting the formant frequencies of their vowels  . Listeners can likewise modulate the probability of words in the competition process as a function of masking noise. If AM radio crackle is presented as a masker, so that it occasionally masks a phoneme here or there in the input, listeners reduce their certainty about the words they hear, including the words actually unaffected; in an experiment in which looks to a display of objects or printed words are recorded, the word onset usually determines where listeners' looks go, but with the radio crackle there were more looks than otherwise to words with a different onset (e.g., to a tent when the input was cent; McQueen and Huettig, submitted for publication). Many studies, finally, have used temporary or complete noise-masking to address the question of whether spokenword recognition models should allow higher-level influence on lower-level processing. The first outcome of this type of work was the "phoneme restoration" phenomenon discovered by BIB003 . When part of an utterance is excised and replaced with a noise that sounds like a cough, listeners report hearing a cough simultaneously with an intact speech signal -something which, after all, they have probably often experienced hearing. That is, listeners "restore" the sound, which was actually completely excised, to its rightful place in the utterance. The listeners are also not very good at judging exactly where in the utterance the cough occurred. An extension of the technique was introduced by Samuel (1981), who compared listeners' judgements of stimuli in which the noise actually replaced a phoneme (as it did in Warren's stimuli), versus stimuli in which the noise was overlaid on an intact speech signal (as Warren's listeners reported hearing). Samuel argued that the phoneme restoration effect might arise due to direct influence from the lexical level on perceptual judgements about phonemes; in this case, the replaced and overlaid versions of a word should sound the same. He therefore examined the discriminability of the two types of stimuli, using signal detection techniques. The discriminability between replaced versus overlaid phonemes turned out to be significantly worse when the phonemes were in real words rather than in nonwords. Samuel argued that these results indeed indicated lexical influence; nonwords show no reduction of discriminability because nonwords have no lexical representations which can exercise influence. However, note that discriminability here concerns an individual masked phoneme, not the word as a whole. BIB007 compared the detectability of whole real words and nonsense words under complete noise masking. Although words are more likely to be correctly identified in noise than nonwords are, this could be because in uncertainty listeners tend to guess real words, as pointed out above. Repp and Frost argued, like Samuel, that direct lexical influence on the earliest stages of speech perception should make real words more detectable than nonwords when the listeners' task is simply to decide as to the presence of some spoken signal underneath the noise. However, they found no difference at all: nonwords and words were equally detectable. Thus the discriminability advantage is limited to phonemes within words. Warren's original explanation of phoneme restoration was that judgements of noise location are based on the signal itself, not on a derived representation of it; after a word has been recognised, the acoustic trace of it in the speech signal is no longer needed and is then no longer available for the performance of precise location determination. By studying the characteristics of restoration effects when rate of speech was varied, BIB006 and BIB008 showed that the limits for illusory continuity corresponded to the average word duration in any given utterance. Once a lexical representation has emerged as the winner of the competition process, it is thenceforth the only representation available to the processor. For native listeners, in other words, noisy or other adverse conditions do exert a detrimental effect on the processes of speech perception, but recovery is efficient and relatively rapid.