Attentional biases are typically measured by reaction time to or time spent looking at an emotional stimulus. Stroop task studies revealed that attentional deficits in depression extend beyond emotional biases. Eye-tracking studies show that depressed individuals spend more time attending to negative stimuli than healthy controls. Conflicting evidence has been found for if attentional tasks are independent of mood state. Despite this promising support, evidence for state-independence is mixed, heritability is mostly unknown, and cosegregation was not supported in relevant studies.

Attention Biases

Biased recall is a hallmark found in individuals with depression. Regarding endophenotypes, self-referential and autobiographical memory appear the most relevant. Multiple tasks are implemented to measure these biases. These recall biases are moderately state-dependent, but lack studies on heritability.

Memory Biases 

These endophenotypes are less studied and have a more limited evidence base. Both attention and memory have a prominent role in cognitive models of the etiology of depression. These endophenotypes also have important links to depression-related neural dysfunctions.

Appalachian State University

Endophenotypes describe the testable components or symptomology related to an overarching phenotype (in this case, depression). There are a large number of endophenotypes related to depression, a few are:

- Cortisol (CAR)
- Frontal asymmetry
- Reward learning (Anhedonia)
- Cognitive biases (attentional and behavioral)
- Neuroticism

Endophenotypes of Depression

https://doi.org/10.1016/j.cpr.2014.06.003

Goldstein, B. L., & Klein, D. N. (2014). A Review of Selected Candidate Endophenotypes for Depression. Clinical Psychology Review, 34(5), 417. https://doi.org/10.1016/j.cpr.2014.06.003

Neuroticism, defined as a dispositional tendency to experience negative emotions, is often elevated in individuals with depression. This personality trait is typically assessed through self-report measures and has been shown to increase during depressive episodes. Importantly, neuroticism is also moderately heritable. Twin studies further suggest that neuroticism and depression share common genetic influences, indicating overlapping biological foundations between the trait and the disorder.

Neuroticism as an Endophenotype

Cortisol, a marker of HPA axis functioning, has been widely studied in depression due to its links with stress and role in the development of the disorder. Depressed individuals often show elevated morning cortisol and an increased cortisol awakening response (CAR), although findings are somewhat inconsistent. CAR has stronger evidence of state-independence, as elevations are observed even in remission. Additionally, elevated cortisol levels have been found in healthy offspring of depressed parents and may predict the onset of symptoms. Overall, there is moderate support for CAR and morning cortisol as an endophenotype for depression.

Cortisol (CAR) as an endophenotype of depression

Asymmetry in electrical activity in frontal regions is measured via EEG. Generally, activity in the right frontal region is thought to reflect withdrawal behavior and negative affect. The left frontal region reflects approach behavior and positive affect. Depression has been associated with decreased relative left FA. These symptoms have been studied, and it has been found that this trait is only modestly heritable. However, this particular endophenotype is still being researched continually to discover its exact relation to depression.

Frontal Asymmetry

There is new research on deficits in reward processing as a possible endophenotype for depression. A probabilistic reward learning task (PRT) has the most relevant data. Using signal-detection procedures, participants are shown one of two stimuli and instructed to hit a corresponding key. The two stimuli represent different values (one with more value than the other to produce a response bias). Compared with control groups, depressed individuals tend to fail to create a response bias, therefore valuing both stimuli as the same. There is also preliminary evidence that response bias on the PRT shares genes with depression.

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