Introduction

The ability to attend to social information represents a cornerstone of typical human development, facilitating the acquisition of language, culture, and interpersonal skills. While behavioural studies have documented clear developmental gains in social attention across infancy, the neural mechanisms supporting these changes remain poorly characterized. Recent technological advances in high-density EEG systems designed for infant populations now permit the safe examination of neural oscillations during naturalistic viewing paradigms.

Neurobiological theories of social development suggest that progressive tuning of sensory systems to social information reflects both maturational changes in neural circuits and experience-dependent refinement. However, longitudinal neuroimaging data from typical populations are sparse, and links between early neural specialization and later behavioural outcomes are underexplored. This study addresses these gaps by tracking neural responses to biological motion across a critical period of social-perceptual development, testing whether individual differences in early neural specialization predict social competence in the second year of life.

Method

Participants

Forty-eight typically developing infants (Mage at baseline = 6.1 months, SD = 0.8; 52% female) were recruited from the greater Ottawa area through hospital birth records and parenting groups. All infants were born full-term (≥37 weeks), had no significant perinatal complications, and had no reported hearing loss or neurological conditions. Thirty-eight infants completed all four assessment points (6, 12, 18, and 24 months; 79% retention). Families provided informed written consent, and the study was approved by the local research ethics board.

Procedure

At each visit, infants viewed a series of point-light biological motion (PLM) stimuli paired with phase-scrambled control stimuli, presented on a monitor positioned 60 cm from the infant's eyes. Stimuli (4-second duration) alternated between biological and control conditions in randomized blocks. High-density EEG was recorded using a 128-channel geodesic net amplifier (EGI Systems) at 500 Hz sampling rate. Impedances were kept below 50 kΩ. Data were referenced to the vertex and filtered offline (0.1–100 Hz). Behavioural gaze was simultaneously recorded using an infrared eye tracker (Tobii T60 XL) at 60 Hz.

Concurrently, parents completed the Social Skills Index from the Bayley Scales of Infant and Toddler Development (Bayley-III) at 24 months, assessing social engagement, imitation, and interaction quality. We also administered the Mullen Scales of Early Learning (cognitive and motor domains) to evaluate developmental progress across modalities.

Results

Time-frequency decomposition of the EEG data revealed significant developmental increases in gamma-band power (30–50 Hz) over centro-parietal regions during biological motion viewing. A mixed-model ANOVA with age (6, 12, 18, 24 months) and stimulus type (biological vs. control) revealed significant main effects of age, F(3, 114) = 8.34, p < .001, and stimulus type, F(1, 38) = 24.17, p < .001, with a significant interaction, F(3, 114) = 5.21, p < .01. Post-hoc contrasts showed linear increases in the biological motion effect size from 6 to 24 months (γ = 1.12, p = .002, 95% CI [0.34, 1.90]).

Gaze fixation duration to biological motion showed parallel developmental gains, increasing from M = 2.8 seconds at 6 months (SD = 1.2) to M = 3.9 seconds at 24 months (SD = 0.9), t(37) = 5.43, p < .001, d = 0.88. Critically, individual differences in gamma-band specialization (mean biological-control difference at 12 months) significantly predicted 24-month Social Skills Index scores (r = 0.52, p < .001, 95% CI [0.27, 0.72]), even after controlling for cognitive (Mullen ELC) and motor development (Mullen motor), ΔR² = 0.18, F(1, 34) = 8.91, p < .01.

Discussion

These findings provide longitudinal neuroimaging evidence for progressive neural specialization to social information during infancy, visible at the level of oscillatory dynamics. The association between early gamma-band differentiation and later social competence suggests that individual variation in neural efficiency for social perception represents a meaningful marker of social-developmental trajectories. This work aligns with theoretical models emphasizing the role of neural tuning in driving behavioural development.

Future research combining this approach with genetic and environmental moderation analyses could illuminate the relative contributions of maturation and experience to social-perceptual specialization. Longitudinal extension into the preschool years would clarify whether early neural markers predict clinical or subclinical variation in social development, with implications for early identification of atypical social trajectories. The integration of high-density EEG with eye tracking provides a powerful methodology for understanding brain-behaviour relationships during this critical developmental window.

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