Global Journal of Health Science; Vol. 8, No. 11; 2016 ISSN 1916-9736 E-ISSN 1916-9744 Published by Canadian Center of Science and Education 249 Quantitative Electroencephalography for Objective and Differential Diagnosis of Depression: A Comprehensive Review Ali Yadollahpour 1 & Hadi Nasrollahi 1 1 Department of Medical Physics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran Correspondence: Ali Yadollahpour, Assistant Professor of Medical Physics, Department of Medical Physics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Golestan Blvd., Ahvaz 61357-33118, Iran. Tel: 98-613-333-0074. E-mail: yadollahpour.a@gmail.com Received: February 3, 2016 Accepted: March 21, 2016 Online Published: March 31, 2016 doi:10.5539/gjhs.v8n11p249 URL: http://dx.doi.org/10.5539/gjhs.v8n11p249 Abstract Quantitative electroencephalography (QEEG) has been dramatically developed during recent years in cognitive neurosciences. It has shown significant potential in the diagnosis of cognitive neurological disorders as well as in the evaluation of treatment outcomes and response. Early diagnosis of depression, differential diagnosis, and assessing the treatment outcomes and response are currently the main research fields of QEEG in depression. Identifying reliable disorder-specific EEG-based biomarkers that have strong correlations with the depression specific cognitive functions is one of the major challenges in these fields. Such biomarkers not only allow early and cost-effective diagnosis of depression, but also may have differential diagnostic and predictive values for treatment response of a variety of treatments. This paper aims at a comprehensive review on the main principles of QEEG in developing biomarkers for MDD. The databases of PubMed (1985-2015), Web of Sciences (1985-2015), and Google Scholar (1980-2015) were searched using the set terms. The obtained results were screened for the title and abstract by two authors and they came to consensus whether the studies are related to the review. The main advantages of QEEG for mood disorders are also reviewed. In addition, different QEEG-based measures for objective diagnosis of MDD as well as for distinguishing depressed patients from healthy subjects are discussed. Keywords: major depressive disorder, depression, EEG biomarker index, QEEG, objective diagnosis, differential diagnosis 1. Introduction Depression is among the most common psychiatric disorders worldwide, affected more than 350 million people (Lépine & Briley, 2011). Its symptoms include fatigue, boredom, sleeplessness, suicidal thoughts, suicide attempts, digestive problems, frustration, anger, loss of sexual desire, isolation, lack of concentration, and memory loss (Lépine, 2000; Marcus, Yasamy, van Ommeren, Chisholm, & Saxena, 2012). In addition to these symptoms, recent studies and functional imaging techniques have shown that different regions of the brain of depression patients undergo significant physiological and functional changes (Anderson et al., 2002; Koolschijn et al., 2009; Kwon, Youn, & Jung, 1996). These symptoms affect various physiological and psychological, personal, and social aspects of human life so that making the treatment of depressive disorders as one of the world's most expensive medical treatments (Ali & Mahmud, 2014; Health, 2010; Lépine & Briley, 2011; Von Korff et al., 1998). Effective pharmaceutical or alternative treatments such as electrical and magnetic stimulations for depression require early diagnosis of depression. On one hand, depressive disorders have different subcategories of disorders including bipolar, major, and seasonal depressions with similar clinical and behavioral symptoms, so that conventional psychometric questionnaires such as Beck depression inventory (BDI) and Hamilton Depression Rating Scale (HDRS) are not capable of a specific differential diagnosis (Beck, Steer, & Carbin, 1988). On the other hand, depression is also accompanied with other psychological and physiological disorders, which makes disease-specific and differential diagnosis of the depression against other psychological disorders difficult. Therefore, developing objective, sensitive, disease-specific, and differential diagnostic techniques are necessary for more efficient management of depressive disorders. Psychometric questionnaires and interviews with the patients are mostly used for diagnosis and classification of