Cortical specialization for music in preverbal infants

Audition is perhaps the most developed and acute sense available to infants at birth. One theory supported by speech and music researchers alike proposes that the auditory system is biased to salient properties such as pitch and allocates processing of such stimuli to specialized areas. In the current study, we sought to investigate whether infants would show similar patterns for processing music and language, as they both contain predictable changes in pitch. In a previous study, we established that language processing is lateralized to the left temporal region in the infant brain. We hypothesized music would be processed in the right temporal area. Although it contains a rule-based structure somewhat akin to language, it is heavily dependent on fine distinctions in pitch. Preverbal infants watched a video of animated shapes (visual stimuli) coupled with either speech (1 of 10 different stories in infant-direct speech) or music (Scriabbin's Ballade No. 3 in A flat) while hemodynamic activity in bilateral temporal sites was recording using near-infrared spectroscopy. Results indicated significant right temporal decreases in HbO2 concentration in comparison with baseline measures during music trials relative to the left temporal area. These results suggest that even at the preverbal stage, infants process speech differently than other similarly structured auditory stimuli

Tracking Language Tuning across the First Year of Life using Near-infrared Spectroscopy

Both behavioral and neurophysiological data indicate that many factors contribute to how infants tune to their native language(s) in early infancy. However, substantial debate remains regarding the neural mechanisms that underlie this tuning process. This study was designed to determine whether the behavioral changes in infants' processing of native and non-native speech during the second half of the first year correspond to qualitative neural processing changes that can be measured using near-infrared spectroscopy (NIRS). Specifically, we used NIRS to examine changes in hemodynamic activity in monolingually-exposed infants between the ages of 3 and 14 months while they were exposed to native (English) and non-native (Spanish) speech. In all infants, measurements were taken from the bilateral temporal regions of the cerebral cortex. Three age groups were tested: pre-tuned infants, who should show no sensitivity to phonological differences between the native and non-native speech samples (3-to-6-month-olds), actively tuning infants, who should be beginning to differentiate between the phonology of the native and non-native speech samples (7-to-10-month-olds), and tuned infants, who should readily distinguish between the phonologies of the native and non-native speech samples (11-to-14-month-olds). Results demonstrated significant differences in hemodynamic activity during the processing of native speech compared to non-native speech in each of the three age groups, with qualitatively different patterns of hemispheric lateralization emerging in response to the two types of speech in each of the three groups. These findings point to a potential neural marker of infants' sensitivity to the phonology of their native language as it emerges with increasing age that will be useful in future research

A Mechanistic Approach to Cross-Domain Perceptual Narrowing in the First Year of Life

Language and face processing develop in similar ways during the first year of life. Early in the first year of life, infants demonstrate broad abilities for discriminating among faces and speech. These discrimination abilities then become tuned to frequently experienced groups of people or languages. This process of perceptual development occurs between approximately 6 and 12 months of age and is largely shaped by experience. However, the mechanisms underlying perceptual development during this time, and whether they are shared across domains, remain largely unknown. Here, we highlight research findings across domains and propose a top-down/bottom-up processing approach as a guide for future research. It is hypothesized that perceptual narrowing and tuning in development is the result of a shift from primarily bottom-up processing to a combination of bottom-up and top-down influences. In addition, we propose word learning as an important top-down factor that shapes tuning in both the speech and face domains, leading to similar observed developmental trajectories across modalities. Importantly, we suggest that perceptual narrowing/tuning is the result of multiple interacting factors and not explained by the development of a single mechanism

The Developmental Trajectory of Brain-Scalp Distance from Birth through Childhood: Implications for Functional Neuroimaging

Measurements of human brain function in children are of increasing interest in cognitive neuroscience. Many techniques for brain mapping used in children, including functional near-infrared spectroscopy (fNIRS), electroencephalography (EEG), magnetoencephalography (MEG) and transcranial magnetic stimulation (TMS), use probes placed on or near the scalp. The distance between the scalp and the brain is a key variable for these techniques because optical, electrical and magnetic signals are attenuated by distance. However, little is known about how scalp-brain distance differs between different cortical regions in children or how it changes with development. We investigated scalp-brain distance in 71 children, from newborn to age 12 years, using structural T1-weighted MRI scans of the whole head. Three-dimensional reconstructions were created from the scalp surface to allow for accurate calculation of brain-scalp distance. Nine brain landmarks in different cortical regions were manually selected in each subject based on the published fNIRS literature. Significant effects were found for age, cortical region and hemisphere. Brain-scalp distances were lowest in young children, and increased with age to up to double the newborn distance. There were also dramatic differences between brain regions, with up to 50% differences between landmarks. In frontal and temporal regions, scalp-brain distances were significantly greater in the right hemisphere than in the left hemisphere. The largest contributors to developmental changes in brain-scalp distance were increases in the corticospinal fluid (CSF) and inner table of the cranium. These results have important implications for functional imaging studies of children: age and brain-region related differences in fNIRS signals could be due to the confounding factor of brain-scalp distance and not true differences in brain activity

Dissociating Cortical Activity during Processing of Native and Non-Native Audiovisual Speech from Early to Late Infancy

Initially, infants are capable of discriminating phonetic contrasts across the world’s languages. Starting between seven and ten months of age, they gradually lose this ability through a process of perceptual narrowing. Although traditionally investigated with isolated speech sounds, such narrowing occurs in a variety of perceptual domains (e.g., faces, visual speech). Thus far, tracking the developmental trajectory of this tuning process has been focused primarily on auditory speech alone, and generally using isolated sounds. But infants learn from speech produced by people talking to them, meaning they learn from a complex audiovisual signal. Here, we use near-infrared spectroscopy to measure blood concentration changes in the bilateral temporal cortices of infants in three different age groups: 3-to-6 months, 7-to-10 months, and 11-to-14-months. Critically, all three groups of infants were tested with continuous audiovisual speech in both their native and another, unfamiliar language. We found that at each age range, infants showed different patterns of cortical activity in response to the native and non-native stimuli. Infants in the youngest group showed bilateral cortical activity that was greater overall in response to non-native relative to native speech; the oldest group showed left lateralized activity in response to native relative to non-native speech. These results highlight perceptual tuning as a dynamic process that happens across modalities and at different levels of stimulus complexity

Linking behavioral and neurophysiological indicators of perceptual tuning to language.

Little is known about the neural mechanisms that underlie tuning to the native language(s) in early infancy. Here we review language tuning through the lens of type and amount of language experience and introduce a new manner in which to conceptualize the phenomenon of language tuning: the relative speed of tuning hypothesis. This hypothesis has as its goal a characterization of the unique time course of the tuning process, given the different components (e.g., phonology, prosody, syntax, semantics) of one or more languages as they become available to infants, and biologically based maturational constraints. In this review, we first examine the established behavioral findings and integrate more recent neurophysiological data on neonatal development, which together demonstrate evidence of early language tuning given differential language exposure even in utero. Next, we examine traditional accounts of sensitive and critical periods to determine how these constructs complement current data on the neural mechanisms underlying language tuning. We then synthesize the extant infant behavioral and neurophysiological data on monolingual, bilingual, and sensory deprived tuning, thereby scrutinizing the effect of these three different language profiles on the specific timing, progression, and outcome of language tuning. Finally, we discuss future directions researchers might pursue to further understand this aspect of language development, advocating our relative speed of tuning hypothesis as a useful framework for conceptualizing the complex process by which language experience works together with biological constraints to shape language development