State-dependent neural signatures of autistic traits: evidence from fNIRS-based fractional amplitude of low-frequency fluctuation analysis during natural social interaction

Autistic traits exist on a continuum within the general population, yet how these traits influence regional neural activity during real-world social interactions remains unclear. Using functional near-infrared spectroscopy (fNIRS), we investigated how autistic traits relate to the fractional amplitude of low-frequency fluctuations (fALFF) during both resting-state and real-time conversations in 62 high autistic trait and 58 low autistic trait neurotypical adults. While resting-state differences were minimal, significant group differences emerged during conversation. Specifically, individuals with high autistic traits showed lower ΔfALFF in the right superior temporal gyrus, with the largest differences observed during emotionally positive topics. Interestingly, although fALFF strongly correlated with functional connectivity across both states, only ΔfALFF demonstrated sensitivity to autistic traits. These findings reveal state-dependent neural differences linked to autistic traits, emphasizing the importance of studying brain activity during naturalistic social interactions. Our results provide new insights into how autistic traits modulate neural processing during dynamic social contexts and suggest fALFF change as a sensitive marker for studying social processing differences.