Interoception refers to sensations from inside the body,
such as the internal organs, and it has been considered to be regulated by the principle
of predictive processing in the brain, through which the brain actively maintains
homeostasis. However, it is still unknown when and how the predictive processing of
interoception develops. To address this issue, we used a method to examine
interoception of infants. Visual stimuli synchronized or asynchronized with the
heartbeat of infants were presented, and the duration of gazing to each stimulus was
measured. Since asynchronous stimuli should generate greater interoceptive
prediction errors, it was hypothesized that if the mechanism of interoception in infants
had developed, they would pay more attention to asynchronous stimuli and look at
them longer. With 3-8-month-old infants, we found that interoception emerges during
the first year of life: the older infants exhibited the longer duration of gazing for
asynchronous over synchronous stimuli. Importantly, this phenomenon occurred only
when stimuli coincide with the baroreceptor-active period of systole, supporting a role
of neural representation of cardiac signals in interoception. Further, infants' pupils
which should reflect attention, showed age-dependent dilation to systole-locked
asynchronous stimuli, illuminating the underlying mechanisms within the predictive
processing framework. Moreover, maternal interoceptive sensibility correlated with
the sizes of effects of interoception in infants, suggesting the caregiver-infant
interplay should be critical in development of infants' interoception. Previously, it
was claimed that the primary aspects of selfhood are shaped by embodied interactions
with caregivers in early infancy, and such embodied interactions allow the
development of abilities of mentalizing. Our findings shed new light on the concrete
processes of such early development of interoception and mentalizing.