MEG detects neural consequences of anomalous nasalization in vowel–consonant pairs

Sensorimotor adaptation affects perceptual compensation for coarticulation

A given speech sound will be realized differently depending on the context in which it is produced. Listeners have been found to compensate perceptually for these coarticulatory effects, yet it is unclear to what extent this effect depends on actual production experience. In this study, whether changes in motor-to-sound mappings induced by adaptation to altered auditory feedback can affect perceptual compensation for coarticulation is investigated. Specifically, whether altering how the vowel [i] is produced can affect the categorization of a stimulus continuum between an alveolar and a palatal fricative whose interpretation is dependent on vocalic context is tested. It was found that participants could be sorted into three groups based on whether they tended to oppose the direction of the shifted auditory feedback, to follow it, or a mixture of the two, and that these articulatory responses, not the shifted feedback the participants heard, correlated with changes in perception. These results indicate that sensorimotor adaptation to altered feedback can affect the perception of unaltered yet coarticulatorily-dependent speech sounds, suggesting a modulatory role of sensorimotor experience on speech perception.

Co-articulatory Cues for Communication: An Investigation of Five Environments

We hypothesized that speakers adjust co-articulation in vowel–consonant (VC) sequences in order to provide listeners with enhanced perceptual cues to C, and that they do so specifically in those situations where primary cues to C place of articulation tend to be diminished. We tested this hypothesis in a speech production study of American English, measuring the duration and extent of VC formant transitions in five conditioning environments – consonant voicing, phrasal position, sentence accent, vowel quality, and consonant place – that modulate primary cues to C place in different ways. Results partially support our hypothesis. Although speakers did not exhibit greater temporal co-articulation in contexts that tend to diminish place cues, they did exhibit greater spatial co-articulation. This finding suggests that co-articulation serves specific communicative goals.

Repair or Violation Detection? Pre-Attentive Processing Strategies of Phonotactic Illegality Demonstrated on the Constraint of g-Deletion in German

PURPOSE

Effects of categorical phonotactic knowledge on pre-attentive speech processing were investigated by presenting illegal speech input that violated a phonotactic constraint in German called "g-deletion." The present study aimed to extend previous findings of automatic processing of phonotactic violations and to investigate the role of stimulus context in triggering either an automatic phonotactic repair or a detection of the violation.

METHOD

The mismatch negativity event-related potential component was obtained in 2 identical cross-sectional experiments with speaker variation and 16 healthy adult participants each. Four pseudowords were used as stimuli, 3 of them phonotactically legal and 1 illegal. Stimuli were contrasted pairwise in passive oddball conditions and presented binaurally via headphones. Results were analyzed by means of mixed design analyses of variance.

RESULTS

Phonotactically illegal stimuli were found to be processed differently compared to legal ones. Results indicate evidence for both automatic repair and detection of the phonotactic violation depending on the linguistic context the illegal stimulus was embedded in.

CONCLUSIONS

These findings corroborate notions that categorical phonotactic knowledge is activated and applied even in the absence of attention. Thus, our findings contribute to the general understanding of sublexical phonological processing and may be of use for further developing speech recognition models.

Differences in coda voicing trigger changes in gestural timing: A test case from the American English diphthong /aɪ/

We investigate the hypothesis that duration and spectral differences in vowels before voiceless versus voiced codas originate from a single source, namely the reorganization of articulatory gestures relative to one another in time. As a test case, we examine the American English diphthong /aɪ/, in which the acoustic manifestations of the nucleus /a/ and offglide /ɪ/ gestures are relatively easy to identify, and we use the ratio of nucleus-to-offglide duration as an index of the temporal distance between these gestures. Experiment 1 demonstrates that, in production, the ratio is smaller before voiceless codas than before voiced codas; this effect is consistent across speakers as well as changes in speech rate and phrasal position. Experiment 2 demonstrates that, in perception, diphthongs with contextually incongruent ratios delay listeners' identification of target words containing voiceless codas, even when the other durational and spectral correlates of voicing remain intact. This, we argue, is evidence that listeners are sensitive to the gestural origins of voicing differences. Both sets of results support the idea that the voicing contrast triggers changes in timing: gestures are close to one another in time before voiceless codas, but separated from one another before voiced codas.

Neurobiological evidence for voicing underspecification in English

In long-term memory, the phoneme units that make up words are coded for the distinctive features and feature values that are necessary to distinguish between words in the mental lexicon. Underspecification theory says that the phonemes that have unmarked feature values are even more abstract in that the feature is omitted from the representation altogether. This makes phoneme representations in words more sparse than the fully specified phonetic representations of the same words. Eulitz and Lahiri (2004) demonstrated that this theory predicts certain asymmetries in the Mismatch Negativity (MMN) response to phoneme contrasts. We expand on this research by demonstrating underspecification-driven asymmetry in the brain response to laryngeal feature contrasts in English (i.e. what makes /d/ and /t/ different). We add a new test by showing that the asymmetry disappears if the MMN paradigm is modified to encourage the formation of phonetic memory traces instead of phonemic memory traces. This result adds further neurobiological evidence that long-term phonological representations are more sparsely represented than phonetic representations.

Complex linguistic rules modulate early auditory brain responses

During speech perception, listeners compensate for phonological rules of their language. For instance, English place assimilation causes green boat to be typically pronounced as greem boat; English listeners, however, perceptually compensate for this rule and retrieve the intended sound (n). Previous research using EEG has focused on rules with clear phonetic underpinnings, showing that perceptual compensation occurs at an early stage of speech perception. We tested whether this early mechanism also accounts for the compensation for more complex rules. We examined compensation for French voicing assimilation, a rule with abstract phonological restrictions on the contexts in which it applies. Our results reveal that perceptual compensation for this rule by French listeners modulates an early ERP component. This is evidence that early stages of speech sound categorization are sensitive to complex phonological rules of the native language.

Evidence for the role of German final devoicing in pre-attentive speech processing: a mismatch negativity study

Results of a mismatch negativity experiment are reported in which the pre-attentive relevance of the German phonological alternation of final devoicing (FD) is shown in two ways. The experiment employs pseudowords. (1) A deviant [vus] paired with standard /vuzə/ did not show a mismatch effect for the voicing change in /z/ versus [s] because the two can be related by FD. When standard and deviant were reversed, the two could not be related by FD and a mismatch effect for the voicing difference occurred. (2) An ill-formed deviant that violates FD, *[vuz], triggered mismatch effects that were plausibly attributed to its ill-formedness. The results show that a syllable-related process like FD is already taken into account by the processing system in early pre-attentive processing.

Positive expectation in the processing of allophones

Does native knowledge introduce a perceptual bias against allophones that mismatch their context? In German, [x] only occurs after back vowels, while [ç] occurs elsewhere. German and English listeners heard "allophonic" ([ç-x]) and "non-allophonic" ([ç-f], [x-f]) continua after front and back vowels. Vowel affected German responses to [ç-x] and [ç-f], but not [x-f]. Vowel affected English responses to all continua. The asymmetric effect on German responses is explained as a perceptual expectation of [ç] after [y]. The effect on English responses is explained by acoustic misparsing, which causes some of the vowel's spectrum to cue a spectrally similar fricative.

The time course of perception of coarticulation

The perception of coarticulated speech as it unfolds over time was investigated by monitoring eye movements of participants as they listened to words with oral vowels or with late or early onset of anticipatory vowel nasalization. When listeners heard [CṼNC] and had visual choices of images of CVNC (e.g., send) and CVC (said) words, they fixated more quickly and more often on the CVNC image when onset of nasalization began early in the vowel compared to when the coarticulatory information occurred later. Moreover, when a standard eye movement programming delay is factored in, fixations on the CVNC image began to occur before listeners heard the nasal consonant. Listeners' attention to coarticulatory cues for velum lowering was selective in two respects: (a) listeners assigned greater perceptual weight to coarticulatory information in phonetic contexts in which [Ṽ] but not N is an especially robust property, and (b) individual listeners differed in their perceptual weights. Overall, the time course of perception of velum lowering in American English indicates that the dynamics of perception parallel the dynamics of the gestural information encoded in the acoustic signal. In real-time processing, listeners closely track unfolding coarticulatory information in ways that speed lexical activation.

Processing Nasals with and without Consecutive Context Phonemes: Evidence from Explicit Categorization and the N100

With neurophysiological (N100) and explicit behavioral measures (two-alternative forced-choice categorization), we investigated how the processing of nasal segments of German is affected by following context phonemes and their place of articulation. We investigated pre-lexical processing, with speech stimuli excised from naturally spoken utterances. Participants heard nasals (/n/, /m/, and place-assimilated /n′/), both with and without a subsequent context phoneme. Context phonemes were voiced or voiceless, and either shared or did not share their place of articulation with the nasals. The explicit forced-choice categorization of the isolated nasals showed /n′/ to be in-between the clear categorizations for /n/ and /m/. In early, implicit processing, /m/ had a significantly higher N100 amplitude than both /n/ and /n′/, with, most importantly, no difference between the latter two. When presented in context (e.g., /nb/, /mt/), explicit categorizations were affected by both the nasal and the context phoneme: a consecutive labial led to more M-categorizations, a following alveolar to more N-categorizations. The early processing of the nasal/+context stimuli in the N100 showed strong effects of context, modulated by the type of preceding nasal. Crucially, the context effects on assimilated nasals /n′/ were clearly different to effects on /m/, and indistinguishable from effects on /n/. The grouping of the isolated nasals in the N100 replicates previous findings, using magnetoencephalography and a different set of stimuli. Importantly, the same grouping was observed in the nasal/+context stimuli. Most models that deal with assimilation are either challenged by the mere existence of phonemic context effects, and/or use mechanisms that rely on lexical information. Our results support the existence, and early activation, of pre-lexical categories for phonemic segments. We suggest that due to experience with assimilation, specific speech-sound categories are flexible enough to accept (or even ignore) inappropriate place cues, in particular when the appropriate place information is still present in the signal.

The role of stimulus cross-splicing in an event-related potentials study. Misleading formant transitions hinder automatic phonological processing

The mental organization of linguistic knowledge and its involvement in speech processing can be investigated using the mismatch negativity (MMN) component of the auditory event-related potential. A contradiction arises, however, between the technical need for strict control of acoustic stimulus properties and the quest for naturalness and acoustic variability of the stimuli. Here, two methods of preparing speech stimulus material were compared. Focussing on the automatic processing of a phonotactic restriction in German, two corresponding sets of various vowel-fricative syllables were used as stimuli. The former syllables were naturally spoken while the latter ones were created by means of cross-splicing. Phonetically, natural and spliced syllables differed with respect to the appropriateness of coarticulatory information about the forthcoming fricative within the vowels. Spliced syllables containing clearly misleading phonetic information were found to elicit larger N2 responses compared to their natural counterparts. Furthermore, MMN results found for the natural syllables could not be replicated with these spliced stimuli. These findings indicate that the automatic processing of the stimuli was considerably affected by the stimulus preparation method. Thus, in spite of its unquestioned benefits for MMN experiments, the splicing technique may lead to interference effects on the linguistic factors under investigation.

Preattentive Phonotactic Processing as Indexed by the Mismatch Negativity

Processing of an obligatory phonotactic restriction outside the focus of the participants' attention was investigated by means of ERPs using (reversed) experimental oddball blocks. Dorsal fricative assimilation (DFA) is a phonotactic constraint in German grammar that is violated in *[ɛx] but not in [ɔx], [ɛ∫], and [ɔ∫]. These stimulus sequences engage the auditory deviance detection mechanism as reflected by the MMN component of the ERP. In Experiment 1 (n = 16), stimuli were contrasted pairwise such that they shared the initial vowel but differed with regard to the fricative. Phonotactically ill-formed deviants elicited stronger MMN responses than well-formed deviants that differed acoustically in the same way from the standard stimulation but did not contain a phonotactic violation. In Experiment 2 (n = 16), stimuli were contrasted such that they differed with regard to the vowel but shared the fricative. MMN was elicited by the vowel change. An additional, later MMN response was observed for the phonotactically ill-formed syllable only. This MMN cannot be attributed to any phonetic or segmental difference between standard and deviant. These findings suggest that implicit phonotactic knowledge is activated and applied in preattentive speech processing.

Impairments in phonological processing and nonverbal intellectual function in parents of children with autism

Language difficulties have been implicated to be a part of the broad autism phenotype in first-degree relatives of individuals with autism. Phonological processing difficulties in particular have been reported by some, but not all groups studying parents or siblings of probands with autism. In the present study, we examined a broad battery of language tasks and general cognitive abilities in parents of children with autistic disorder. Parents of individuals with autism (n = 22) were compared to matched adult controls on a series of cognitive and language measures. Parents of children with autism exhibited lower performance on the matrix reasoning subtest and total performance IQ than did controls, but did not show differences in verbal IQ measures, when tested with the Wechsler Abbreviated Scale of Intelligence (WASI). In addition, parents of children with autism had lower performance on a nonword repetition task, but did not show differences on tests of figurative language, receptive language, expressive language, and verbal fluency and on a questionnaire assessing history of reading difficulties. Results from this study are generally consistent with the cognitive profiles reported for parents of children with autism. Our finding of nonword repetition difficulties, along with others' previous findings for nonword reading in autism families, suggests that problems in phonological processing might be characteristic of the broad autism phenotype.