Van Gogh once described being depressed as ​‘lying bound hand and foot at the bottom of a deep dark well, utterly helpless’ in a letter to his brother. He couldn’t have summed it up better – patients suffering from depression often report feeling despondent, having disturbed sleep schedules, irritability, suicidal tendencies, and difficulties in learning and concentration. 

Now a recent study led by Bhaktee Dongaonkar and Sumantra Chattarji at the National Center for Biological Sciences (NCBS), Bangalore, has revealed how patients suffering from unipolar or bipolar depression face different sets of challenges in updating episodic memories – the memories that store information about our experiences throughout our lives.

‘Our memories are not static, they are dynamic,’ says Dongaonkar. We continuously integrate relevant new experiences with pre-existing memories, thus ​‘updating’ them. The authors were interested in seeing how depression would affect this process of storing and recalling memories in unipolar and bipolar patients.

Unipolar and bipolar depression are distinct disorders that derive their names from their characteristic symptoms. Unipolar patients show persistent depressive symptoms, whereas bipolar patients go through distinct phases of manically elevated and depressed mood. Nearly 200 unipolar or bipolar patients from the National Institute of Mental Health and Neuro-Sciences (NIMHANS) participated in this study. 

Over the course of the five-day study paradigm, the three groups — unipolar, bipolar and controls — were given 20 everyday objects to memorize (List 1). Two days later, participants from each of the three groups were randomly divided into two categories. One category was reminded of their experience of learning List 1 while the other wasn’t. Following this, all the participants were given a second set of 20 objects to memorize (List 2). Two days after this second session, all the participants were tested on their memory of items from either List 1 or List 2.

The researchers found that both unipolar and bipolar patients remembered fewer overall objects than controls on the final day. In addition to taking longer to memorize the lists of objects during both sessions, bipolar patients couldn’t quite remember whether a certain item belonged to list 1 or 2. They could remember the object, but often mixed up its context, a phenomenon known as source confusion. 

Unipolar patients, on the other hand, performed at par with controls on the same test and did not show random confusion during recall. Unlike bipolar patients, both controls and unipolar patients systematically ​‘updated’ List 1 memory with objects from List 2 but not the other way round.

“Attempts have been made to distinguish unipolar and bipolar depression clinically and this paper is a good contribution to possible differentiation between the two types of depression with potential treatment implications,” says K. Muralidharan from NIMHANS, who was not associated with this study.

The finding turned out to be a pleasant surprise for the researchers who are now planning to take this study forward. Dongaonkar is interested in trying out different experimental paradigms to validate their findings and ​“to see if that’s how their brains are wired fundamentally, or if it was just our testing paradigm.’’ 

Chattarji, on the other hand, is curious to see if brain imaging of bipolar patients would reveal damage to the hippocampus (a structure in the brain, that among other things, is important for creating memories from events happening in our day-to-day lives) and whether that would correlate with the severity of their disorder.

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