While people with Type I and the less-severe Type II bipolar disorder share some of the same symptoms, there are significant differences in the physical structure of their brains. Type I sufferers have somewhat smaller brain volume, researchers report in the Journal of Affective Disorders, while those with Type II appear to have less robust white matter.

As brain imaging technologies have advanced and matured over the past few decades, there’s been considerable interest in understanding whether and how there are differences between the brains of people with mental illness and those without. In particular, neuroscientists studying depression have been interested in structural variation, such as differences in total brain volume. Still, the various forms of bipolar disorder have received somewhat less attention than others, such as major depression, schizophrenia, or autism.

That led Jerome Maller and colleagues at Monash University in Melbourne, Australia, to look into whether there were structural differences among the brains of people with different sorts of bipolar disorder. Using standard MRI scans—much the same as you would get if you’d had a concussion or bleeding in the brain—on 16 Type I and 15 Type II bipolar patients along with 31 healthy control subjects, the team examined whether there were differences in gray matter, white matter, and cerebrospinal fluid. The team also used a relatively new technique called diffusion tensor imaging (DTI) to measure the integrity of the brains’ white matter, the long nerves called axons that connect different brain regions to each other.

Overall, there was less total brain volume—gray and white matter volume added together—and more cerebrospinal fluid volume in bipolar patients than in healthy controls, consistent with other recent studies suggesting a connection between brain volume and depression. After controlling for total brain volume, however, Type II patients’ brains were essentially the same as controls’ brains, while Type I patients had relatively higher volume in the caudate nucleus and other areas associated with reward processing and decision making. DTI studies, meanwhile, revealed that while patients with Type I and II bipolar disorder had reduced white matter integrity relative to controls, the effect was stronger among those with Type II, particularly in the frontal and prefrontal cortex, suggesting that Type II bipolar disorder is in some way a cognitive dysfunction.