PROJECT SUMMARY/ABSTRACT Social interaction is essential to the cognitive development and wellbeing of many species including humans. Social withdrawal is an early risk factor and predictor of mental illness onset and often begins during adolescence. Adolescence is a period of increased plasticity during which environmental input and brain activity guide circuitry maturation, including thalamo-prefrontal circuitry. Adolescent social interaction is necessary for the establishment of adult cognitive behaviors, which rely on the prefrontal cortex (PFC). Our lab has previously shown that adolescent inhibition of the medial thalamus (mTH) impairs PFC maturation and development of adult cognitive flexibility in mice. I have observed that adolescent social isolation (ASI) impairs cognitive flexibility selectively in females, who have been traditionally understudied in this field. Therefore, I hypothesize that ASI induces cognitive impairments by reducing adolescent mTH activity and thalamo-prefrontal connectivity in a sexually dimorphic manner. Furthermore, I have observed that some ASI-treated individuals do not show deficits in cognitive flexibility. Indeed, many studies of ASI and other stressful paradigms have reported large variability in behavioral adaptations, but little work has been done to uncover the underlying mechanisms of this variation. I hypothesize that variation in the level of mTh activity and connectivity deficits will explain behavioral variation, with higher activity and connectivity conferring resilience to ASI-induced cognitive deficits. With this in mind, I have designed my study with the statistical power to detect sex differences and resilience. I propose two periods for intervention to rescue and prevent ASI-induced cognitive deficits. Our lab has shown that excitation of the mTH in adulthood transiently rescues cognitive flexibility in mice who underwent adolescent mTH inhibition. Similarly, I hypothesize that excitation of the mTH during a task requiring cognitive flexibility will rescue ASI-induced cognitive deficits. Based on the necessity of adolescent mTH activity for proper PFC maturation, and the role of thalamo-prefrontal circuitry in adult cognitive flexibility, I hypothesize that adolescent thalamic excitation during social isolation will prevent formation of long-term cognitive deficits. I will excite all PFC-projecting neurons in a similar manner. To test these hypotheses, I will be trained in and employ a combination of, optogenetics, chemogenetics transgenic mouse lines, and targeted viral strategies. Additionally, I will be trained on machine learning tools for histology registration and segmentation, decoding and clustering of cell distribution patterns, and unbiased behavioral scoring. The identification of an early critical period of socially guided brain development may provide a window for intervention to prevent or slow development of cognitive symptoms in adolescence. If proven, my hypotheses point t...