Begoña Díaz

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.

An effect of bilingualism on the auditory cortex

Two studies (Golestani et al., 2007; Wong et al., 2008) have reported a positive correlation between the ability to perceive foreign speech sounds and the volume of Heschls gyrus (HG), the structure that houses the auditory cortex. More precisely, participants with larger left Heschls gyri learned consonantal or tonal contrasts faster than those with smaller HG. These studies leave open the question of the impact of experience on HG volumes. In the current research, we investigated the effect of early language exposure on Heschls gyrus by comparing Spanish–Catalan bilinguals who have been exposed to two languages since childhood, to a group of Spanish monolinguals matched in education, socio-economic status, and musical experience. Manual volumetric measurements of HG revealed that bilinguals have, on average, larger Heschls gyri than monolinguals. This was corroborated, for the left Heschls gyrus, by a voxel-based morphometry analysis showing larger gray matter volumes in bilinguals than in monolinguals. Since the bilinguals in this study were not a self-selected group, this observation provides a clear demonstration that learning a second language is a causal factor in the increased size of the auditory cortex.

Dysfunction of the auditory thalamus in developmental dyslexia

Developmental dyslexia, a severe and persistent reading and spelling impairment, is characterized by difficulties in processing speech sounds (i.e., phonemes). Here, we test the hypothesis that these phonological difficulties are associated with a dysfunction of the auditory sensory thalamus, the medial geniculate body (MGB). By using functional MRI, we found that, in dyslexic adults, the MGB responded abnormally when the task required attending to phonemes compared with other speech features. No other structure in the auditory pathway showed distinct functional neural patterns between the two tasks for dyslexic and control participants. Furthermore, MGB activity correlated with dyslexia diagnostic scores, indicating that the task modulation of the MGB is critical for performance in dyslexics. These results suggest that deficits in dyslexia are associated with a failure of the neural mechanism that dynamically tunes MGB according to predictions from cortical areas to optimize speech processing. This view on task-related MGB dysfunction in dyslexics has the potential to reconcile influential theories of dyslexia within a predictive coding framework of brain function.

Brain structure is related to speech perception abilities in bilinguals

Morphology of the human brain predicts the speed at which individuals learn to distinguish novel foreign speech sounds after laboratory training. However, little is known about the neuroanatomical basis of individual differences in speech perception when a second language (L2) has been learned in natural environments for extended periods of time. In the present study, two samples of highly proficient bilinguals were selected according to their ability to distinguish between very similar L2 sounds, either isolated (prelexical) or within words (lexical). Structural MRI was acquired and processed to estimate vertex-wise indices of cortical thickness (CT) and surface area (CSA), and the association between cortical morphology and behavioral performance was inspected. Results revealed that performance in the lexical task was negatively associated with the thickness of the left temporal cortex and angular gyrus, as well as with the surface area of the left precuneus. Our findings, consistently with previous fMRI studies, demonstrate that morphology of the reported areas is relevant for word recognition based on phonological information. Further, we discuss the possibility that increased CT and CSA in sound-to-meaning mapping regions, found for poor non-native speech sounds perceivers, would have plastically arisen after extended periods of increased functional activity during L2 exposure.

Oscillation encoding of individual differences in speech perception.

Individual differences in second language (L2) phoneme perception (within the normal population) have been related to speech perception abilities, also observed in the native language, in studies assessing the electrophysiological response mismatch negativity (MMN). Here, we investigate the brain oscillatory dynamics in the theta band, the spectral correlate of the MMN, that underpin success in phoneme learning. Using previous data obtained in an MMN paradigm, the dynamics of cortical oscillations while perceiving native and unknown phonemes and nonlinguistic stimuli were studied in two groups of participants classified as good and poor perceivers (GPs and PPs), according to their L2 phoneme discrimination abilities. The results showed that for GPs, as compared to PPs, processing of a native phoneme change produced a significant increase in theta power. Stimulus time-locked analysis event-related spectral perturbation (ERSP) showed differences for the theta band within the MMN time window (between 70 and 240 ms) for the native deviant phoneme. No other significant difference between the two groups was observed for the other phoneme or nonlinguistic stimuli. The dynamic patterns in the theta-band may reflect early automatic change detection for familiar speech sounds in the brain. The behavioral differences between the two groups may reflect individual variations in activating brain circuits at a perceptual level.

Variability in L2 phonemic learning originates from speech-specific capabilities: An MMN study on late bilinguals

People differ in their ability to perceive second language (L2) sounds. In early bilinguals the variability in learning L2 phonemes stems from speech-specific capabilities (Diaz, Baus, Escera, Costa & Sebastian-Galles, 2008 ). The present study addresses whether speech-specific capabilities similarly explain variability in late bilinguals. Event-related potentials were recorded (using a design similar to Diaz et al., 2008 ) in two groups of late Dutch–English bilinguals who were good or poor in overtly discriminating the L2 English vowels /e-ae/. The mismatch negativity, an index of discrimination sensitivity, was similar between the groups in conditions involving pure tones (of different length, frequency, and presentation order) but was attenuated in poor L2 perceivers for native, unknown, and L2 phonemes. These results suggest that variability in L2 phonemic learning originates from speech-specific capabilities and imply a continuity of L2 phonemic learning mechanisms throughout the lifespan.

Exploring the relationship between speech perception and production across phonological processes, language familiarity, and sensory modalities

ABSTRACT Current speech perception models disagree over the role of speech production in speech perception. In the current study we aimed to characterise the relationship between speech perception and production by testing a large sample of early and highly proficient Spanish-Catalan bilinguals in a variety of speech perception and production tasks. Speech perception was measured for different phonological processes (sub-lexical and phono-lexical), different language familiarities (native, second, and unknown language), and different sensory modalities (auditory and audio-visual). Speech production ability was assessed in the second language. Non-linguistic auditory and sensory motor abilities were also measured. We used factor analysis to look at the relations between the variables. Results showed a tight relationship between speech perception and production measurements, which was present across phonological processes and language familiarities but was independent of audio-visual and non-linguistic (auditory and sensory-motor) skills.

Task-dependent modulation of the visual sensory thalamus assists visual-speech recognition

Abstract The cerebral cortex modulates early sensory processing via feed‐back connections to sensory pathway nuclei. The functions of this top‐down modulation for human behavior are poorly understood. Here, we show that top‐down modulation of the visual sensory thalamus (the lateral geniculate body, LGN) is involved in visual‐speech recognition. In two independent functional magnetic resonance imaging (fMRI) studies, LGN response increased when participants processed fast‐varying features of articulatory movements required for visual‐speech recognition, as compared to temporally more stable features required for face identification with the same stimulus material. The LGN response during the visual‐speech task correlated positively with the visual‐speech recognition scores across participants. In addition, the task‐dependent modulation was present for speech movements and did not occur for control conditions involving non‐speech biological movements. In face‐to‐face communication, visual speech recognition is used to enhance or even enable understanding what is said. Speech recognition is commonly explained in frameworks focusing on cerebral cortex areas. Our findings suggest that task‐dependent modulation at subcortical sensory stages has an important role for communication: Together with similar findings in the auditory modality the findings imply that task‐dependent modulation of the sensory thalami is a general mechanism to optimize speech recognition.

Attention modulates somatosensory influences in passive speech listening

ABSTRACT Previous studies showed that manipulating the speech production system influenced speech perception. This influence was mediated by task difficulty, listening conditions, and attention. In the present study we investigated the specificity of a somatosensory manipulation – a spoon over the tongue – in passive listening. We measured the mismatch negativity (MMN) while participants listened to vowels that differ in their articulation – the tongue height – and familiarity – native and unknown vowels. The same participants heard the vowels in a spoon and no-spoon block. The order of the blocks was counterbalanced across participants. Results showed no effect of the spoon. Instead, starting with the spoon enhanced the MMN amplitude. A second experiment showed the same MMN enhancement for starting with a somatosensory manipulation applied to a non-articulator – the hand. This result suggests that starting a study with a somatosensory manipulation raises attention to the task.