Complexity of STG signals and linguistic rhythm: a methodological study for EEG data.

The superior temporal and the Heschl's gyri of the human brain play a fundamental role in speech processing. Neurons synchronize their activity to the amplitude envelope of the speech signal to extract acoustic and linguistic features, a process known as neural tracking/entrainment. Electroencephalography has been extensively used in language-related research due to its high temporal resolution and reduced cost, but it does not allow for a precise source localization. Motivated by the lack of a unified methodology for the interpretation of source reconstructed signals, we propose a method based on modularity and signal complexity. The procedure was tested on data from an experiment in which we investigated the impact of native language on tracking to linguistic rhythms in two groups: English natives and Spanish natives. In the experiment, we found no effect of native language but an effect of language rhythm. Here, we compare source projected signals in the auditory areas of both hemispheres for the different conditions using nonparametric permutation tests, modularity, and a dynamical complexity measure. We found increasing values of complexity for decreased regularity in the stimuli, giving us the possibility to conclude that languages with less complex rhythms are easier to track by the auditory cortex.

Traces of statistical learning in the brain's functional connectivity after artificial language exposure.

Our environment is full of statistical regularities, and we are attuned to learn about these regularities by employing Statistical Learning (SL), a domain-general ability that enables the implicit detection of probabilistic regularities in our surrounding environment. The role of brain connectivity on SL has been previously explored, highlighting the relevance of structural and functional connections between frontal, parietal, and temporal cortices. However, whether SL can induce changes in the functional connections of the resting state brain has yet to be investigated. To address this question, we applied a pre-post design where participants (n = 38) were submitted to resting-state fMRI acquisition before and after in-scanner exposure to either an artificial language stream (formed by 4 concatenated words) or a random audio stream. Our results showed that exposure to an artificial language stream significantly changed (corrected p < 0.05) the functional connectivity between Right Posterior Cingulum and Left Superior Parietal Lobule. This suggests that functional connectivity between brain networks supporting attentional and working memory processes may play an important role in statistical learning.

Bilingualism at the core of the brain. Structural differences between bilinguals and monolinguals revealed by subcortical shape analysis.

Naturally acquiring a language shapes the human brain through a long-lasting learning and practice process. This is supported by previous studies showing that managing more than one language from early childhood has an impact on brain structure and function. However, to what extent bilingual individuals present neuroanatomical peculiarities at the subcortical level with respect to monolinguals is yet not well understood, despite the key role of subcortical gray matter for a number of language functions, including monitoring of speech production and language control - two processes especially solicited by bilinguals. Here we addressed this issue by performing a subcortical surface-based analysis in a sample of monolinguals and simultaneous bilinguals (N=88) that only differed in their language experience from birth. This analysis allowed us to study with great anatomical precision the potential differences in morphology of key subcortical structures, namely, the caudate, accumbens, putamen, globus pallidus and thalamus. Vertexwise analyses revealed significantly expanded subcortical structures for bilinguals compared to monolinguals, localized in bilateral putamen and thalamus, as well as in the left globus pallidus and right caudate nucleus. A topographical interpretation of our results suggests that a more complex phonological system in bilinguals may lead to a greater development of a subcortical brain network involved in monitoring articulatory processes.

The acquisition of phonetic categories in bilingual infants: new data from an anticipatory eye movement paradigm.

Contrasting results have been reported regarding the phonetic acquisition of bilinguals. A lack of discrimination has been observed for certain native contrasts in 8-month-old Catalan-Spanish bilingual infants (Bosch & Sebastián-Gallés, 2003a), though not in French-English bilingual infants (Burns, Yoshida, Hill & Werker, 2007; Sundara, Polka & Molnar, 2008). At present, the data for Catalan-Spanish bilinguals constitute an exception in the early language acquisition literature. This study contributes new findings that show that Catalan-Spanish bilingual infants do not lose the capacity to discriminate native contrasts. We used an adaptation of the anticipatory eye movement paradigm (AEM; McMurray & Aslin, 2004) to explore this question. In two experiments we tested the ability of infants from Catalan and Spanish monolingual families and from Catalan-Spanish bilingual families to discriminate a Spanish-Catalan common and a Catalan-specific vowel contrast. Results from both experiments revealed that Catalan-Spanish bilingual infants showed the same discrimination abilities as those shown by their monolingual peers, even in a phonetic contrast that had not been discriminated in previous studies. Our results demonstrate that discrimination can be observed in 8-month-old bilingual infants when tested with a measure not based on recovery of attention. The high ratio of cognates in Spanish and Catalan may underlie the reason why bilinguals failed to discriminate the native vowels when tested with the familiarization-preference procedure but succeeded with the AEM paradigm.

Time course and functional neuroanatomy of speech segmentation in adults.

The present investigation was devoted to unraveling the time-course and brain regions involved in speech segmentation, which is one of the first processes necessary for learning a new language in adults and infants. A specific brain electrical pattern resembling the N400 language component was identified as an indicator of speech segmentation of candidate words. This N400 trace was clearly elicited after a short exposure to the words of the new language and showed a decrease in amplitude with longer exposure. Two brain regions were observed to be active during this process: the posterior superior temporal gyrus and the superior part of the ventral premotor cortex. We interpret these findings as evidence for the existence of an auditory-motor interface that is responsible for isolating possible candidate words when learning a new language in adults.

Lexical plasticity in early bilinguals does not alter phoneme categories: II. Experimental evidence.

When listening to modified speech, either naturally or artificially altered, the human perceptual system rapidly adapts to it. There is some debate about the nature of the mechanisms underlying this adaptation. Although some authors propose that listeners modify their prelexical representations, others assume changes at the lexical level. Recently, Larsson, Vera, Sebastian-Galles, and Deco [Lexical plasticity in early bilinguals does not alter phoneme categories: I. Neurodynamical modelling. Journal of Cognitive Neuroscience, 20, 76-94, 2008] proposed a biologically plausible computational model to account for some existing data, one which successfully modeled how long-term exposure to a dialect triggers the creation of new lexical entries. One specific prediction of the model was that prelexical (phoneme) representations should not be affected by dialectal exposure (as long as the listener is exposed to both standard and dialectal pronunciations). Here we present a series of experiments testing the predictions of the model. Native listeners of Catalan, with extended exposure to Spanish-accented Catalan, were tested on different auditory lexical decision tasks and phoneme discrimination tasks. Behavioral and electrophysiological recordings were obtained. The results supported the predictions of our model. On the one hand, both error rates and N400 measurements indicated the existence of alternative lexical entries for dialectal varieties. On the other hand, no evidence of alterations at the phoneme level, either in the behavioral discrimination task or in the electrophysiological measurement (MMN), could be detected. The results of the present study are compared with those obtained in short-term laboratory exposures in an attempt to provide an integrative account.

First- and second-language phonological representations in the mental lexicon.

Performance-based studies on the psychological nature of linguistic competence can conceal significant differences in the brain processes that underlie native versus nonnative knowledge of language. Here we report results from the brain activity of very proficient early bilinguals making a lexical decision task that illustrates this point. Two groups of Spanish-Catalan early bilinguals (Spanish-dominant and Catalan-dominant) were asked to decide whether a given form was a Catalan word or not. The nonwords were based on real words, with one vowel changed. In the experimental stimuli, the vowel change involved a Catalan-specific contrast that previous research had shown to be difficult for Spanish natives to perceive. In the control stimuli, the vowel switch involved contrasts common to Spanish and Catalan. The results indicated that the groups of bilinguals did not differ in their behavioral and event-related brain potential measurements for the control stimuli; both groups made very few errors and showed a larger N400 component for control nonwords than for control words. However, significant differences were observed for the experimental stimuli across groups: Specifically, Spanish-dominant bilinguals showed great difficulty in rejecting experimental nonwords. Indeed, these participants not only showed very high error rates for these stimuli, but also did not show an error-related negativity effect in their erroneous nonword decisions. However, both groups of bilinguals showed a larger correct-related negativity when making correct decisions about the experimental nonwords. The results suggest that although some aspects of a second language system may show a remarkable lack of plasticity (like the acquisition of some foreign contrasts), first-language representations seem to be more dynamic in their capacity of adapting and incorporating new information.