The developing human brain exhibits remarkable functional specialization, with hemispheric lateralization emerging as a fundamental organizational principle that begins during fetal development. This review synthesizes current knowledge about the neural underpinnings of language and emotion processing in infants (ages 0~12 months) and toddlers (ages 1~3 years), focusing on the establishment of hemispheric asymmetry and its implications for both typical and atypical neurodevelopment.

Structural and functional hemispheric asymmetries manifest early in ontogeny, preceding the contralateral organization of sensorimotor systems. Unlike the strict contralaterality observed in primary sensory and motor pathways, cognitive lateralization represents a relative dominance where one hemisphere assumes primary responsibility while maintaining bilateral involvement. This specialized organization enhances neural efficiency by reducing redundant processing and enabling the parallel computation of complex cognitive functions. Crucially, the developmental trajectory of lateralization serves as a sensitive biomarker of neurodevelopmental integrity, with atypical patterns predicting subsequent emergence of conditions including autism spectrum disorder (ASD), schizophrenia, and obsessive-compulsive disorder.

Our comprehensive analysis reveals robust evidence of left-hemisphere dominance in infant language processing at multiple neurobiological levels. Structural MRI studies demonstrate early leftward asymmetries in perisylvian regions and white matter tracts, particularly the arcuate fasciculus connecting Broca's and Wernicke's areas. Functional neuroimaging confirms this specialization, with left temporal regions showing preferential activation for speech stimuli as early as the neonatal period. Notably, this typical pattern appears disrupted in infants at high risk for ASD, who exhibit either reduced leftward lateralization or paradoxical right-hemisphere dominance - a finding with significant implications for early identification.

The neural substrates of emotion processing present a more complex picture. While the right-hemisphere hypothesis receives substantial support from studies on emotional prosody and facial expression processing, competing evidence for valence-specific lateralization (the left hemisphere for positive affect, right for negative) complicates theoretical accounts. Developmental studies suggest this dichotomy may reflect different processing stages, with initial right-hemisphere bias for rapid emotion detection followed by valence-dependent lateralization during conscious evaluation. Methodological variations in stimulus selection and experimental paradigms likely contribute to these discrepant findings.

Three critical directions emerge for advancing this field:

First, refined experimental paradigms must address the inherent confounds of current emotion research. The prevalent use of emotional speech stimuli inadvertently engages both language and emotion networks, potentially obscuring emotion-specific lateralization. We recommend developmentally appropriate alternatives: non-linguistic auditory stimuli (e.g., emotional music) for younger infants and dynamic visual stimuli (e.g., facial/bodily expressions) for older infants. These approaches would provide a clearer characterization of emotion processing networks.

Second, the field urgently requires longitudinal investigations to map the dynamic development of functional asymmetry. Existing cross-sectional designs cannot adequately capture the potentially non-linear trajectories of lateralization establishment. Prospective studies with dense sampling intervals could identify critical periods and developmental milestones in hemispheric specialization, while clarifying the relationship between early atypical patterns and subsequent neurodevelopmental outcomes.

Third, integrative research designs simultaneously assessing language and emotion processing could yield transformative insights. The known behavioral coupling of these domains suggests their neural substrates may develop interactively. Combined assessment protocols would not only reveal potential synergies in typical development but also enhance early identification of neurodevelopmental risk through multi-modal neural signatures.

The exceptional plasticity of the infant brain underscores the clinical importance of this research. By elucidating both normative and atypical patterns of functional lateralization, we move closer to developing sensitive, objective markers for early intervention. Future work should prioritize: (1) Standardized protocols for assessing lateralization across development, (2) Mechanistic studies examining genetic and environmental influences on asymmetry development, and (3) Translational applications linking neural markers to behavioral outcomes.

This synthesis highlights how the study of early hemispheric specialization bridges fundamental neuroscience with clinical applications. As neuroimaging technologies advance, refined characterization of lateralization patterns promises to transform our understanding of both typical brain development and the origins of neurodevelopmental disorders.