Facebook addicts who frequently check the app on their smartphones tend to have less gray matter in the "reward centers" of their brains, according to a study published in medical journal Behavioral Brain Research. The nucleus accumbens, located in one's basal forebrain, is part of the organ's mesolimbic pathway, which is activated during pleasurable experiences. Each cerebral hemisphere has its own nucleus accumbens, which is comprised of a shell and a core. During a five-week study, researchers from Germany's Ulm University and the University of Bonn monitored 46 male and 39 female Facebook users, tracking the length of time each subject spent on the popular social media site and number of times they "checked in." MRI (magnetic resonance imaging) scans were used to survey participants' brain structures. "It appeared, that...higher daily frequency of checking Facebook on the smartphone was robustly linked with smaller gray matter volumes of the nucleus accumbens," the study concluded. The trial is one of many that examines the dark side of social media and the negative psychological repercussions it may have on users. A Yale University study from earlier this year found that Facebook usage can leave people feeling sad or unfulfilled. According to research, for every one percent increase in the number of "likes" a person gave, status updates and link clicks, self-reported happiness levels decreased by five to eight percent. A similar experiment from the University of Pittsburgh found that using multiple social media platforms could be emotionally and mentally harmful to adults. People who reported using seven to 11 social media sites were more than three times as likely to experience depression and anxiety than those who used zero to two different platforms.