The Development of Cortical Responses to the Integration of Audiovisual Speech in Infancy

In adults, the integration of audiovisual speech elicits specific higher (super-additive) or lower (sub-additive) cortical responses when compared to the responses to unisensory stimuli. Although there is evidence that the fronto-temporal network is active during perception of audiovisual speech in infancy, the development of fronto-temporal responses to audiovisual integration remains unknown. In the current study, 5-month-olds and 10-month-olds watched bimodal (audiovisual) and alternating unimodal (auditory + visual) syllables. In this context we use alternating unimodal to denote alternating auditory and visual syllables that are perceived as separate syllables by adults. Using fNIRS we measured responses over large cortical areas including the inferior frontal and superior temporal regions. We identified channels showing different responses to bimodal than alternating unimodal condition and used multivariate pattern analysis (MVPA) to decode patterns of cortical responses to bimodal (audiovisual) and alternating unimodal (auditory + visual) speech. Results showed that in both age groups integration elicits cortical responses consistent with both super- and sub-additive responses in the fronto-temporal cortex. The univariate analyses revealed that between 5 and 10 months spatial distribution of these responses becomes increasingly focal. MVPA correctly classified responses at 5 months, with key input from channels located in the inferior frontal and superior temporal channels of the right hemisphere. However, MVPA classification was not successful at 10 months, suggesting a potential cortical re-organisation of audiovisual speech perception at this age. These results show the complex and non-gradual development of the cortical responses to integration of congruent audiovisual speech in infancy

Measuring brain activity when exposed to infant-directed speech: A meta-analysis

This meta-analysis synthesizes findings of research done using fNIRS technology measuring brain responses when infants were exposed to infant-directed speech

The Development of Social Perception Networks in Low- and Middle-Income Infants: Longitudinal Assessments of fNIRS Background Functional Connectivity

Shortly after birth, human infants demonstrate behavioural selectivity to social stimuli. However, the neural underpinnings of this selectivity are largely unknown. Here we examine patterns of functional connectivity to determine how regions of the brain interact while processing social stimuli and how these interactions change during the first two years of life. Using functional near-infrared spectroscopy (fNIRS), we measured functional connectivity at 6 (n = 183) and 24 (n = 123) months of age in infants from Bangladesh who were exposed to varying levels of environmental adversity (i.e., low- and middle-income cohorts). We employed a background functional connectivity approach that regresses out the effects of stimulus-specific variables known to affect functional connectivity. At 6 months, the two cohorts had similar fNIRS patterns, with moderate connectivity estimates for regions within and between hemispheres. At 24 months, the patterns diverged for the two cohorts. Global (brain-wide) connectivity estimates increased from 6 to 24 months for the low-income cohort and decreased for the middle-income cohort. In particular, connectivity estimates among regions of interest (ROIs) within the right hemisphere decreased for the middle-income cohort, providing evidence of neural specialization by two years of age. These findings provide insights about the impact of early environmental influences on functional brain development relevant to the processing of social stimuli