Walden University College of Social and Behavioral Sciences This is to certify that the doctoral dissertation by Octavious Bishop has been found to be complete and satisfactory in all respects, and that any and all revisions required by the review committee have been made. Review Committee Dr. Thomas Edman, Committee Chairperson, Psychology Faculty Dr. Ricardo Thomas, Committee Member, Psychology Faculty Dr. Joanna Bissell-Havran, University Reviewer, Psychology Faculty Chief Academic Officer Eric Riedel, Ph.D. Quantifying the Spectrum of Depression by Octavious Bishop MA, University of Texas at Austin, 2008 BS, University of Texas at Austin, 2001 Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Psychology Walden University May 2018 Depression is a medically serious and widespread mood disorder that is difficult to diagnose in an objective manner. Dopamine irregularities have been strongly implicated in depression studies, and drug therapy based on dopamine is in wide use. However, the same neurological abnormalities associated with depression also affect other neural systems, including the vestibular system, in which involuntary muscle movements involved with the contralateral acoustic reflex are located. Using nigrostriatal pathways that transmit dopamine as a framework, this study investigated the biological and physiological links between depression and acoustic reflexes, and their potential usefulness for objectively assessing depression. Records of 52 randomly-selected patients who presented symptoms of depression were assessed to determine the relationship between depression and the contralateral acoustic reflex. The patients were both male and female, ranging in age from 23 to 84. Acoustic reflex threshold testing was assessed through ranges of frequencies using a tympanogram. The resulting individual average scores for the right ear and the left ear were then statistically tested against the medically accepted normal score using one-sample t tests. Evidence indicated that acoustic reflex abnormality may be concomitant with depression. These findings offer promising possibilities to researchers looking to develop a functional quantifiable assessment of patients who present with symptoms of depression. Addressing the wide variance of symptoms in patients may help mental health professionals determine which antidepressants to prescribe or if a patient is ready for a therapeutic process.