The structural correlates of statistical information processing during speech perception

The processing of continuous and complex auditory signals such as speech relies on the ability to use statistical cues (e.g. transitional probabilities). In this study, participants heard short auditory sequences composed either of Italian syllables or bird songs and completed a regularity-rating task. Behaviorally, participants were better at differentiating between levels of regularity in the syllable sequences than in the bird song sequences. Inter-individual differences in sensitivity to regularity for speech stimuli were correlated with variations in surface-based cortical thickness (CT). These correlations were found in several cortical areas including regions previously associated with statistical structure processing (e.g. bilateral superior temporal sulcus, left precentral sulcus and inferior frontal gyrus), as well other regions (e.g. left insula, bilateral superior frontal gyrus/sulcus and supramarginal gyrus). In all regions, this correlation was positive suggesting that thicker cortex is related to higher sensitivity to variations in the statistical structure of auditory sequences. Overall, these results suggest that inter-individual differences in CT within a distributed network of cortical regions involved in statistical structure processing, attention and memory is predictive of the ability to detect structural structure in auditory speech sequences

Activation of the left planum temporale in pitch processing is shaped by language experience

Implicit, abstract knowledge acquired through language experience can alter cortical processing of complex auditory signals. To isolate prelexical processing of linguistic tones (i.e., pitch variations that convey part of word meaning), a novel design was used in which hybrid stimuli were created by superimposing Thai tones onto Chinese syllables (tonal chimeras) and Chinese tones onto the same syllables (Chinese words). Native speakers of tone languages (Chinese, Thai) underwent fMRI scans as they judged tones from both stimulus sets. In a comparison of native vs. non‐native tones, overlapping activity was identified in the left planum temporale (PT). In this area a double dissociation between language experience and neural representation of pitch occurred such that stronger activity was elicited in response to native as compared to non‐native tones. This finding suggests that cortical processing of pitch information can be shaped by language experience and, moreover, that lateralized PT activation can be driven by top‐down cognitive processing

Representation of Sound Categories in Auditory Cortical Maps

We used functional magnetic resonance imaging (fMRI) to investigate the representation of sound categories in human auditory cortex. Experiment 1 investigated the representation of prototypical and non-prototypical examples of a vowel sound. Listening to prototypical examples of a vowel resulted in less auditory cortical activation than listening to nonprototypical examples. Experiments 2 and 3 investigated the effects of categorization training and discrimination training with novel non-speech sounds on auditory cortical representations. The two training tasks were shown to have opposite effects on the auditory cortical representation of sounds experienced during training: discrimination training led to an increase in the amount of activation caused by the training stimuli, whereas categorization training led to decreased activation. These results indicate that the brain efficiently shifts neural resources away from regions of acoustic space where discrimination between sounds is not behaviorally important (e.g., near the center of a sound category) and toward regions where accurate discrimination is needed. The results also provide a straightforward neural account of learned aspects of categorical perception: sounds from the center of a category are more difficult to discriminate from each other than sounds near category boundaries because they are represented by fewer cells in the auditory cortical areas.National Institute on Deafness and Other Communication Disorders (R01 DC02852

N400 evidence for musical facilitation of word boundary identification in second language exposure

Lexical acquisition requires the ability to identify word boundaries in a continuous auditory speech stream. This complex task is even more challenging when learning a new language in adulthood. Previous studies have shown that word boundary identification can be enhanced by pairing musical tones with native language phonemes. The objective of this dissertation study was to investigate whether musical tones also have this effect in a novel pseudo-language that uses non-native speech sounds. The N400, a brain event-related potential that has been linked with familiarity responses and detection of statistical regularities during exposure to pseudowords, provides an index of brain activation associated with semantico-lexical processing. In this study, language-like stimuli incorporating a French phoneme (a high, front, rounded vowel that is not part of the English phonetic inventory) were presented to typically developing English monolingual adults. Participants were presented to one of two types of exposure conditions for 7 minutes: monotone presentation of the concatenated language-like stimuli; or the same speech stream with a musical tone associated with each syllable. The exposure protocol was based on Schön, Boyer, Moreno et. al. (2008). Exposure was followed by a lexical decision task, requiring participants to distinguish “words” (heard during the exposure in a concatenated speech stream) from “part words” (end of one word and the beginning of another, crossing word boundaries). High-density EEG was recorded during the lexical decision and analyzed offline to determine N400 event-related responses to the stimuli in each condition. Although behavioral measures did not reveal any significant differences between groups or conditions, we found a N4 significantly different response to “partword” in the tone-exposed group, compared to the monotone. This difference only occurred in a frontal region with a right-hemisphere bias, and was not found to be significant over the left hemisphere. This difference suggests that participants in the tone group were supported in differentiating “words” from “partwords”, supporting the view that the inclusion of tonal information is beneficial in the early stages of L2 lexical learning

Models of Speech Processing

One of the fundamental questions about language is how listeners map the acoustic signal onto syllables, words, and sentences, resulting in understanding of speech. For normal listeners, this mapping is so effortless that one rarely stops to consider just how it takes place. However, studies of speech have shown that this acoustic signal contains a great deal of underlying complexity. A number of competing models seek to explain how these intricate processes work. Such models have often narrowed the problem to mapping the speech signal onto isolated words, setting aside the complexity of segmenting continuous speech. Continuous speech has presented a significant challenge for many models because of the high variability of the signal and the difficulties involved in resolving the signal into individual words. The importance of understanding speech becomes particularly apparent when neurological disease affects this seemingly basic ability. Lesion studies have explored impairments of speech sound processing to determine whether deficits occur in perceptual analysis of acoustic-phonetic information or in stored abstract phonological representations (e.g., Basso, Casati,& Vignolo, 1977; Blumstein, Cooper, Zurif,& Caramazza, 1977). Furthermore, researchers have attempted to determine in what ways underlying phonological/phonetic impairments may contribute to auditory comprehension deficits (Blumstein, Baker, & Goodglass, 1977). In this chapter, we discuss several psycholinguistic models of word recognition (the process of mapping the speech signal onto the lexicon), and outline how components of such models might correspond to the functional anatomy of the brain. We will also relate evidence from brain lesion and brain activation studies to components of such models. We then present some approaches that deal with speech perception more generally, and touch on a few current topics of debate.National Institutes of Health under grant NIH DC R01–3378 to the senior author (SLS

Stimulus Complexity and Categorical Effects in Human Auditory Cortex: An Activation Likelihood Estimation Meta-Analysis

Investigations of the functional organization of human auditory cortex typically examine responses to different sound categories. An alternative approach is to characterize sounds with respect to their amount of variation in the time and frequency domains (i.e., spectral and temporal complexity). Although the vast majority of published studies examine contrasts between discrete sound categories, an alternative complexity-based taxonomy can be evaluated through meta-analysis. In a quantitative meta-analysis of 58 auditory neuroimaging studies, we examined the evidence supporting current models of functional specialization for auditory processing using grouping criteria based on either categories or spectro-temporal complexity. Consistent with current models, analyses based on typical sound categories revealed hierarchical auditory organization and left-lateralized responses to speech sounds, with high speech sensitivity in the left anterior superior temporal cortex. Classification of contrasts based on spectro-temporal complexity, on the other hand, revealed a striking within-hemisphere dissociation in which caudo-lateral temporal regions in auditory cortex showed greater sensitivity to spectral changes, while anterior superior temporal cortical areas were more sensitive to temporal variation, consistent with recent findings in animal models. The meta-analysis thus suggests that spectro-temporal acoustic complexity represents a useful alternative taxonomy to investigate the functional organization of human auditory cortex

Design choices in imaging speech comprehension: An Activation Likelihood Estimation (ALE) meta-analysis

The localisation of spoken language comprehension is debated extensively: is processing located anterior or posterior on the left temporal lobe, and is it left- or bilaterally organised? An Activation Likelihood Estimation (ALE) analysis was conducted on functional MRI and PET studies investigating speech comprehension to identify the neural network involved in comprehension processing. Furthermore, the analysis aimed to establish the effect of four design choices (scanning paradigm, non-speech baseline, the presence of a task, and the type of stimulus material) on this comprehension network. The analysis included 57 experiments contrasting intelligible with less intelligible or unintelligible stimuli. A large comprehension network was found across bilateral Superior Temporal Sulcus (STS), Middle Temporal Gyrus (MTG) and Superior Temporal (STS) bilaterally, in left Inferior Frontal Gyrus (IFG), left Precentral Gyrus, and Supplementary Motor Area (SMA) and pre-SMA. The core network for post-lexical processing was restricted to the temporal lobes bilaterally with the highest ALE values located anterior to Heschl's Gyrus. Activations in the ALE comprehension network outside the temporal lobes (left IFG, SMA/pre-SMA, and Precentral Gyrus) were driven by the use of sentences instead of words, the scanning paradigm, or the type of non-speech baseline

The emergence of structure from continuous speech: Multiple cues and constraints for speech segmentation and its neural bases

This thesis studies learning mechanisms and cognitive biases present from birth involved in language acquisition, in particular in speech segmentation and the extraction of linguistic regularities. Due to the sequential nature of speech, uncovering language structure is closely related with how infants segment speech. We investigated infant abilities to track distributional properties on the stimuli, and the role of prosodic cues and of memory constraints. In two experiments we investigated neonates\u2019 capacities to segment and extract words from continuous speech by using fNIRS. Experiment 1 demonstrates that neonates can segment and extract words from continuous speech based on distributional cues alone; whereas Experiment 2 shows that newborns can extract words when they are marked only by prosodic contours. Additionally we implemented a method for the study of the dynamics of the functional connectivity of the neonatal brain during speech segmentation tasks. We identi\ufb01ed stable and reproducible functional networks with small-world properties that were task independent. Moreover, we observed periods of high global and low global connectivity, which remarkably, were task dependent, with stronger values when neonates listen to speech with structure. In another set of experiments we studied memory constraints on the encoding of six-syllabic words in newborns using fNIRS. Experiment 4 demonstrates that the edge syllables of a sequence are better encoded, and Experiment 5 goes beyond by showing that a subtle pause enhances the encoding of intermediate syllables, which evidences the role of prosodic cues in speech processing. A \ufb01nal group of experiments explore how information is encoded when it is presented continuously across different modalities; speci\ufb01cally if an abstract encoding of the sequences\u2019 constituents is generated. Experiments 6-9 suggest that adults form an abstract representation of words based on the position of the syllables, but only in the speech modality. In Experiments 10 and 11 we used pupillometry to test the same in 5-month-old infants. Nevertheless results were not conclusive, we did not \ufb01nd evidence of an abstract encoding