Schizophrenia and bipolar disorder are two mental health conditions known for the disruptive, dysfunctional changes they can make in everyday human behavior. Mental health professionals view these two conditions as distinct from one another, and the American Psychiatric Association officially places them in separate categories of mental illness. However, according to the authors of a September 2013 study in the journal Biological Psychiatry, schizophrenia and bipolar disorder affect some of the same circuits inside the brain and differ fundamentally only in certain respects. Schizophrenia Basics Schizophrenia is the quintessential psychotic disorder defined by the presence of sensory hallucinations, delusional thinking and unusually disjointed thought processes, as well as non-psychosis-related symptoms such as a reduced ability to express emotion, a lack of personal motivation or an interest in pleasurable pursuits, and an inability or unwillingness to communicate verbally. The American Psychiatric Association places schizophrenia in a group of conditions called \u201cschizophrenia spectrum and other psychotic disorders,\u201d which also includes brief psychotic disorder, delusional disorder, schizoaffective disorder, schizotypal personality disorder and schizophreniform disorder. Bipolar Disorder Basics The name bipolar disorder is actually shared by two distinct conditions\u2014bipolar I disorder and bipolar II disorder\u2014that feature episodes of relatively extreme or relatively subdued mania and episodes of severe depression. Another condition, called cyclothymic disorder, combines less intense episodes of depression with relatively mild manic episodes. Together, these three conditions form the core of a category of ailments called \u201cbipolar and related disorders.\u201d While the most unique feature of bipolar illness is the presence of manic episodes, affected individuals actually commonly spend substantial amounts of time affected by depression, not mania. Points of Overlap Despite the official placement of schizophrenia and bipolar disorder in separate categories of mental illness, doctors have known for a long time that people diagnosed with one of these disorders often exhibit symptoms normally attributed to the other disorder. For example, an individual diagnosed with a bipolar illness may develop the delusions and hallucinations usually associated with schizophrenia or other psychotic disorders closely related to schizophrenia. In turn, an individual diagnosed with schizophrenia can develop agitated states of mind that strongly resemble mania, and can also develop emotionally flat states of mind that bear some resemblance to depression. In fact, in any given person, the symptoms of the two disorders can overlap to such a degree that professionals in the field may have problems telling them apart, even after extensive observation and review. New Findings In the study in Biological Psychiatry, a team of researchers sought to deepen the understanding of the connection between schizophrenia and bipolar illness. All of the participating researchers belonged to a multi-institutional project called the Bipolar & Schizophrenia Network on Intermediate Phenotypes (BSNIP). During the current study, they examined the brain functions of large numbers of people affected by bipolar disorder or schizophrenia, then compared those functions to the brain functions of healthy relatives of the affected individuals, as well as to the brain functions of unrelated people in good health with no history of mental illness. After completing their comparisons, the researchers came to several conclusions. First, they determined that malfunctions in a total of seven separate brain circuits account for all of the symptoms associated with both schizophrenia and bipolar illness. They also concluded that schizophrenia and bipolar illness produce similar malfunctions in the brain circuits responsible for providing control over all conscious mental functions. In addition, the researchers concluded that schizophrenia and bipolar illness manifest in different ways for two basic reasons. First, generally speaking, the problems in conscious brain function found in people with schizophrenia are more severe than the same types of problems found in people with bipolar illness. Also, people with schizophrenia have problems in a greater overall number of brain circuits than people with various forms of bipolar disorder. Considerations After examining the brain functions of the healthy relatives of bipolar and schizophrenic individuals, the authors of the study in Biological Psychiatry concluded that these people have substantially less severe, but still detectable, malfunctions in the brain circuits responsible for producing schizophrenia and bipolar disorder. For this and other reasons, the authors believe that it\u2019s possible that schizophrenia and bipolar illness sit on a single spectrum of brain circuit function that extends all the way from complete health to severe mental illness. They also believe that this possibility underscores the limitations of classifying mental illnesses in separate, supposedly unrelated categories.