In one of the early studies of the phenomenology of depressed states, Dunner and colleagues [17] found that in the midst of a depressive phase, inpatients with bipolar I disorder showed significantly less attention to personal appearance and exhibited greater psychomotor slowing than inpatients with major depressive disorder (MDD). Similarly, in a study looking at melancholic depression in patients who met criteria for melancholic depression based on three different definitions, including DSM-III, the rate of bipolar disorder was significantly higher than unipolar depression, regardless of the definition of melancholia employed [18]. They also found that melancholia was most clearly distinguished by psychomotor disturbance. Likewise, in a study by Mitchell and colleagues [19] that compared bipolar I disorder patients with patients with MDD, patients with bipolar disorder were found to be more likely to have psychomotor retardation and atypical features (such as hypersomnia and leaden paralysis) than depressed patients with MDD.
The generally held belief that patients with bipolar disorder (in particular bipolar I disorder) are more likely to experience a melancholic depression characterized by psychomotor retardation than patients with MDD, as well as the research showing that melancholic depression is more common in bipolar I disorder, has led to the notion, among clinicians, that there is a corresponding mental slowing as well [20,21]. However, there have been no studies to suggest that patients with bipolar disorder are less mentally active. The notion that there is mental slowing in bipolar depression may also be, in part, a contrast to the large body of evidence that points to an active mind in major depressive disorder, in the form of rumination.