Author Type

Graduate Student

Date of Award

Spring 3-25-2026

Document Type

Dissertation

Publication Status

Version of Record

Submission Date

April 2026

Department

Psychology

College Granting Degree

Charles E. Schmidt College of Science

Department Granting Degree

Psychology

Degree Name

Doctor of Philosophy (PhD)

Thesis/Dissertation Advisor [Chair]

Teresa Wilcox

Abstract

Interbrain synchrony (IBS), or coordinated brain activation, between parents and infants is thought to support emerging social and cognitive development. Few studies have examined how neural coupling unfolds during the second year of life; a period marked by growing autonomy and complexity during play. Using functional near-infrared spectroscopy (fNIRS) hyperscanning, this study investigated mother-infant brain-to-brain synchrony during naturalistic free play interactions with a sample of 31 mother-infant dyads (12-24 months). Interbrain synchrony was examined in six cortical regions of interest (ROIs), with a focus on phase alignment. Rigorous behavioral coding captured maternal sensitivity, intrusiveness, child involvement, and dyadic relational qualities. True dyads showed significantly greater interbrain synchrony than the control group (shuffled pseudo-dyads) in the left anterior and middle temporal cortices (ROIs 5 and 6), regions implicated in social perception and affective mentalizing. Contrary to expectations, no significant synchrony emerged in prefrontal regions, suggesting that spontaneous dyadic interaction during this developmental window may rely more on emotionally grounded temporal networks than on top-down frontal coordination. Phase-specific analyses revealed that maternal intrusiveness predicted increased time in in-phase synchrony, suggesting that over directive or rigid parenting may constrain reciprocal neural exchange. Post-hoc regressions further demonstrated that dyadic reciprocity, dyadic negative affect, and child involvement were each associated with decreased time in in-phase synchrony in the left anterior region, while higher child involvement predicted greater time in anti-phase synchrony. These findings challenge assumptions that in-phase synchrony is always optimal, highlighting that anti-phase dynamics may reflect flexible, co-regulated interaction when infants take an active role. Together, this study provides the first evidence that phase-aligned neural coupling in the second year of life is associated with behavior and regionally specific. It expands current models of IBS by distinguishing between forms of synchrony that reflect either attuned responsiveness or relational rigidity. Methodologically, it advances the field by combining detailed behavioral coding with phase-sensitive neural metrics; an approach never before applied to infant populations. These findings contribute to a more dynamic understanding of how the infant social brain is shaped through interpersonal exchange and the temporal structure of neural coordination with the mother.

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