For many years, researchers and especially the National Institute of Mental Health have pushed to investigate the connection between mood disorders and now two recent studies dive deeper into the further links between bipolar disorder and depression. 

How are Depression and Bipolar Linked?

It's important to not that the two conditions have very different symptoms. Bipolar disorder more commonly known for the fluctuations in mood, doesn't necessarily have "mania" that major depressive disorder comes with. 

The study, lead by Kelly Ryan, PhD, from the University of Michigan, is a neuropsychologist and clinical assistant professor in the U-M Department Psychiatry. Her team recently published their latest study in the journal, BRAIN, with new evidence regarding the link between depression and those with bipolar disorder. The study's researchers found that the fuzzy thinking or worsening concentration before the onset of both sets of symptoms were common. 

Over 612 women with depression or bipolar disorder and those without any mental condition were studied. To test this, each participant was administered a specific test, which involved reacting quickly to letters sequenced on a computer screen. The test required extended concentration to react quickly to letters. 

Surprisingly, those without a mood disorder performed poorly on the exam, but those with depression or bipolar generally performed worse. 

To take the test even further, researchers put 52 women under a brain scanner while the tests were in progress. Women with depression or bipolar disorder had different levels of cognitive activity in a particular area of the brain, which is called the posterior parietal cortex. This controls all important function in the brain.

Those with depression tended to have a higher brain activity than healthy test-takers, while those with bipolar disorder had lower, a finding they would've never found without the brain scans. 

"Those with major depression had over activity, whereas those with bipolar disorder had under activity, yet they showed the same pattern of activation," said Ryan.

From these findings, we can conclude that these two conditions may fall along a spectrum instead of two distinct disorders. 

Second Study: Depressed State v. Diagnosed Depression

Another recent study published in the JAMA Psychiatry also explored the similarities and differences between these disorders. Clinicians often have difficulty distinguishing those with bipolar and unipolar depression during the low times. What they found was that people with bipolar disorder, while in a depressed state, experienced symptoms of depression differently than people who actually have been diagnosed with a condition of it.

They examined how medication-free patients with conditions regulated their emotions in depressed or remitted states. 42 people with major depressive disorder, 35 with bipolar disorder and 36 healthy patients each rated their emotions after looking at emotional photos, some negative and sad, others positive and upbeat.

In a normal state, bipolar patients had difficulty regulating both happy and sad emotions, while depressed patients were better at it. Both groups were consistent in regulating sad emotions. Also performing brain scans on all subjects, researchers found those with major depressive disorder and bipolar disorder may appear the same while in a depressed state. The differences in brain activity shows they're experiencing and controlling emotions in a different way.

Depression Quick Facts

Over 19 million Americans have some form of depression. A key sign of depression is either depressed mood or loss of interest in activities you once enjoyed.

Major Types of Depression:

• Major depression
• Chronic Depression (Dysthymia)
• Atypical depression
• Postpartum depression 
• Bipolar depression (manic depression)
• Seasonal depression (SAD)
• Psychotic depression
• Serotonin syndrome

Common symptoms include: 

• A depressed mood for most of the day and in the morning
• Fatigue or low energy almost every day
• Feelings of worthlessness or guilt almost every day
• Impaired concentration, indecisiveness
• Insomnia 
• Low to no interest in almost all activities nearly every day
• Significant weight loss or weight gain

Bipolar Quick Facts

There are over 3 million cases of diagnosed bipolar disorder in the United State each year. Also called manic depression, this disorder consists of mood swings with periods of elevated mood (mania) and then other periods of a depressed mood. It typically affects those between the ages of 19-60, which is obviously a large range. Those ages 14-18 and even 13 and under can be diagnosed with this. 

Common symptoms include:

• Overly good mood
• Irritability
• High energy
• Excessive activity
• Restlessness
• Fast talking
• Inability to concentrate
• Little need to sleep
• Poor judgement
• Aggression

While there is a lot we know and understand between these various mood disorders, there's still much to be learned when it comes to the connections in the brain that initiate these symptoms. As always, understanding the risk factors for both depression and bipolar disorder could lead us towards a stronger prevention path.