Abstract
Although anatomical brain hemispheric asymmetries have been clearly documented in the infant brain, findings concerning functional hemispheric specialization have been inconsistent. The present report aims to assess whether bilaterally symmetric synchronous activity between the two hemispheres is a characteristic of the infant brain. To asses cortical bilateral synchronicity, we used decomposition by independent component analysis (ICA) of high-density electroencephalographic (EEG) data collected in an auditory passive oddball paradigm. Decompositions of concatenated 64-channel EEG data epochs from each of 34 typically developing 6-month-old infants and from 18 healthy young adults participating in the same passive auditory oddball protocol were compared to characterize differences in functional brain organization between early life and adulthood. Our results show that infant EEG decompositions comprised a larger number of independent component (IC) effective source processes compatible with a cortical origin and having bilaterally near-symmetric scalp projections (13.8% of the infant data ICs presented a bilateral pattern vs. 4.3% of the adult data ICs). These IC projections could be modeled as the sum of potentials volume-conducted to the scalp from synchronous locally coherent field activities in corresponding left and right cortical source areas. To conclude, in this paradigm, source-resolved infant brain EEG exhibited more bilateral synchronicity than EEG produced by the adult brain, supporting the hypothesis that more strongly unilateral and likely more functionally specialized unihemispheric cortical field activities are concomitants of brain maturation.
Highlights
The existence of structural and functional hemispheric asymmetries supporting differential specialization of the left and right hemispheres has been widely documented in the adult brain (Toga and Thompson, 2003)
We suggest that the synchronous activity identified by independent component analysis (ICA) decomposition was related both to ongoing, non-stimulus locked EEG activity (Figure 3, bottom left) and to the auditory stimulus-locked response activity (Figure 3, bottom right)
The left/right dipole moment ratios for the dual-dipolar independent component (IC) matched the left/right distribution of unilateral ICs in the same clusters. This is the first study that compares ICA decompositions of infant and adult EEG data collected in the same paradigm
Summary
The existence of structural and functional hemispheric asymmetries supporting differential specialization of the left and right hemispheres has been widely documented in the adult brain (Toga and Thompson, 2003). This hemispheric specialization supports execution of cognitive, behavioral, emotional, and motor functions and deficiencies in this brain asymmetry are suspected to be involved in various neurodevelopmental disorders (e.g., autism spectrum disorder and language disorders; Herbert et al, 2005; Bishop, 2013). Some studies have supported the hypothesis that early structural asymmetry is associated with functional hemispheric asymmetry, and have identified asymmetric sensory activation patterns similar to those identified in adults, but overall these reports have been inconsistent
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