Project Summary Early caregiver availability fosters the development of self-regulation capabilities that protect against numerous risks, including poor health behaviors, impulsivity, and mental illness. However, the brain processes and caregiver behaviors that account for this are unknown. In adults, self-regulation is largely mediated through the prefrontal cortex (PFC), which inhibits threat-sensitive subcortical systems such as the amygdala. Work with adults and rodents suggests that during childhood, caregivers scaffold the growth of these neural systems by reducing amygdala activity while increasing PFC activity, but this has not yet been tested in human infancy. Further, caregiver availability alone may not be sufficient for effective scaffolding—caregiver behaviors during availability likely play a crucial role. This proposal tests the hypotheses that (1) during threat, caregiver availability increases infant PFC response while decreasing amygdala responses, (2) these effects are moderated by caregiver sensitivity and predictability, and (3) the effects of caregiver availability and behavior differ depending on the sensitive period in which they occur. Infant-caregiver dyads will visit the lab at 4 and 8 months. At each visit, infant PFC and amygdala will be measured using functional near infrared spectroscopy (fNIRS) and a novel proxy measure of amygdala activity, the eyeblink startle reflex. During measurement, infants will view threatening and non- threatening stimuli in each of two contexts: sitting on their caregiver’s lap and sitting in a high chair, away from their caregiver. They will then complete a free-play session from which caregiver behavior will be coded. Results of this study may reveal the effects of caregivers on the development of self-regulatory brain function and associated sensitive periods.