Loneliness, or the perceived discrepancy of ones relationships in terms of quality, is known as Perceived Social Isolation (PSI). Studies have shown that PSI is both increasing and is correlated with health risks. Specifically, PSI is not only related with risks of mortality but is also linked with variations in the brain. Having few social contacts, or being Objectively Socially Isolated (OSI) does not qualify as PSI. The aim of this thesis is to investigate the neural correlates of PSI, as distinguished from OSI. The true role of PSI is still unknown, however, arguments can be made that PSI serves an important role in survival. The social brain, which allows for social cognition is used as a basis for understanding PSI in this thesis. In this thesis, I found that individuals suffering from PSI have increased attention towards social threat, and a preference to engage in positive social stimuli. Further, PSI affects both social cognition and the social brain. However, regional brain activity was not confined to the social brain. The results showed that PSI may be related to both affective and attentional networks of the brain. PSI also affects activity in the ventral striatum. Further, PSI is related to varied regional brain size. I argue that PSI can be reduced by mainly fixing maladaptive cognitive patterns.