The Four Neural Sub-Types of Depression
Regular readers of The Mouse Trap will be familiar with my obsession with knowing how nature is carved at its joints or in other words what are the natural categories or basic kinds.
This translates into thinking a lot about what are the fundamental drives, basic emotions and personalty traits and what taxonomic system of mental illness is most reflective of underlying fundamental nosological differences.
While synthesizing the work of others, has great value, and one derives many valuable theoretical insights based on such musings; there is nothing better than finding empirical studies that shed some light on such matters.
For example, I have argued that one set of disorders that arise form emotional polarity of fear/interest is Anxiety disorders and Obsession disorders. When fear is disproportional/ inappropriate to circumstances, it leads to anxiety; when interest is disproportional/ inappropriate it leads to Obsession. Fear and interest though opposed are two separate constructs as per the first tenet of positive psychology that good is not the absence of bad.
Similarly, the set of disorders that arise form sadness/ Joy polarity is depressive disorders and manic disorders. I am deliberately using plural form while defining depressive/ manic disorders as they contain sub-types – as we will soon see in the case of depression.
Now while depression is characterized by excessive low affect (sadness), one way to conceive Mania is as having excessive energy; the opposite of manic symptoms thus might be conceived of as fatigue or anergia. Anxiety is of course marked by excess anxiety while Obsession is too much interest; a possible opposite of obsession may be anhedonia– a sort of disinterest or apathy.
Now, its common to find depression and anxiety disorders comorbid with each other and just like treating bipolar as well as schizophrenia under one umbrella of psychosis, one may conceivably treat depression/ anxiety / anergia and anhedonia under a common nomenclature- in this case that of depression.
But we are perhaps getting ahead of ourselves. Lets backtrack a bit and go straight to this new study that found four neural subtypes of depression.
Basically, Liston and colleagues, used resting state fMRI to look at the functional connectivity of depressed patients and developed an algorithm to predict who has depression and who does not have in a sample consisting of both depressed patients and healthy controls. They found abnormal functional connectivity in frontostraital and limbic systems in teh depressive patients.
They also used clustering techniques to find that their depressive subset of patients clustered around two dimensions- one of which they called anxiety dimension and the other anhedonia. When one takes into account that there could be 2×2 = 4 combinations of anxiety and anhedoinia they found that their patients neatly clustered in those four quadrants.
If you note in the figure 1f accompanying the article,
- cluster 1 subjects have low scores on anhedonia and high scores on anxiety
- cluster 2 subjects have neither anxiety nor anhedonia
- cluster 3 have high anhedonia but low anxiety
- cluster 4 have both high anxiety and high anhedonia
The authors note that all subjects had low mood (sadness, hoplessness, helplessness) and that is why they were classified as depressed patients in the first place. The core depressive signature was also associetd with anergia and anhedonia with majority of patients showing these symptoms across all subtypes.
They also found slightly different neural signatures for all the four subtypes. For eg. cluster 1 & 4 characterized by high anxiety had reduced frontoamygdalar connectivity, linking it with fear circuit. Cluster 3 & 4 were associated with hyper connectivity between thalamic and frontostriatal networks and had high anhedonia and psychomotor retardation associated with them. And cluster 1 & 2 had reduced connectivity between anterior cingulate and orbitofrontal coretx involved in reward and incentive salience and guess what they showed anergia or fatigue.
To me it seems apparent that what we are seeing here are different effects of low mood, anxiety, anhedonia and anergia playing out as four clusters.
Cluster 3 I would classify as primarily anehdoina related; cluster 4 as primarily anxiety related; cluster 1 may be thought of as anergia related and clusetr 2 as pure depression or low mood related.
If my hunches are true we should find similar subdivisions in diagnosed anxiety disorders, obsessive disorders , manic disorders too. Of course that is an empirical fact to be proved.
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