Many people who suffer from a mental illness have more than one diagnosis. This is also referred to as comorbidity. Several explanations have been given for the common existence of comorbidity, for example similar risk factors or an inconclusive diagnostic system. The DSM is widely used as a diagnostic system and holds the view that mental illnesses can be separated. It is possible that these separations do not occur in the human brain, causing comorbidity according to the DSM. More recently, the liability-spectrum model was introduced, stating that comorbidity reflects the existence of a smaller number of liability constructs that underlie multiple disorders. This article disrupted emotional processing as a liability-spectrum model.

Processing emotions

Our emotions impact a wide range of mental processes and manifests behaviours. Being able to recognize and regulate our emotions is important. There are different strategies in emotion regulation. Emotions are usually approached in two ways. The first, the direct emotion approach, states that there are basic emotions that can be empirically distinguished. Some brain models in line with this approach claim that all emotions can be specifically localized to either a distinct brain region. The second, the dimensional approach, suggests that emotions can be defined according to one or more dimensions. The underlying brain mechanisms of emotions and which or how many path ways the brain uses to process emotions are currently unknown. However, emotions play an important role in mental illnesses.

When we observe another person’s emotion we tend to look at facial expressions and their posture. There are a few brain areas involved in processing emotional faces. The amygdala for example seems to have an important role in regulating the intensity of the experienced emotion. With psychiatric illnesses, most people have difficulties in identifying and describing emotions. This condition is also known as alexithymia and causes interpersonal problems and problems in social interaction. Also, it is thought that mental illnesses emerge through wrong or maladjusted appraisals of a situation, which elicits exaggerated emotions. This is referred to as disrupted emotion regulation.

Emotions and mental illnesses

Anxiety disorders are characterized by feelings of fear (reaction to current events, including bodily reactions) and feelings of anxiety (worrying about future events). These disorders are partly genetic and often occur with other mental illnesses, such as mood disorders and eating disorders. Fear is an emotion that had an adaptive function in the past. Still it helps us survive, by warning us for danger. However, when a person often perceives a stimulus as threatening when this is not the case, normal fear reactions can turn into anxiety. There are several hypothesis trying to explain this. The vigilance hypothesis states that people suffering from anxiety disorders detect threat more rapidly and more often. The maintenance hypotheses states that state anxiety is increased by attentional biases, causing cognitive resources on threat to be maintained. fMRI studies showed consistent amygdala hyperactivity in anxious people, along with reduced activity in the ventromedial prefrontal cortex and thalamus. This seems to be especially the case in people with social anxiety.

Mood disorders are characterized by a loss of interest or pleasure in almost all aspects of everyday life and a regular depressed mood. Mood disorders are separated in bipolar mood disorders, where depressions and manic episodes are both present, and depression, where a person doesn’t experience manic episodes. Mood disorders often occur with anxiety disorders, impulse control disorder and substance abuse. People suffering from a mood disorder have difficulties in recovering from a negative event. They keep focussing on the negative and do not have effective coping strategies. It is thought that people who are vulnerable for a depression exhibit negative attentional biases towards emotional information, perceiving emotional information more negatively in comparison to individuals without a depression. fMRI studies showed over-activation of the amygdala, the parahippocampal gyrus and the striatum when processing negative stimuli. When processing positive stimuli, under-activation of these brain areas is commonly seen. Studies have found support for theories suggesting that over-activity in the limbic areas is not adequately controlled by prefrontal areas.

Schizophrenia is characterized by difficulties with the perception or expression of reality. Contributors to schizophrenia are genes, early environment, physiological and social processes. Schizophrenia is a rare mental disorder, and often occurs with mood and anxiety disorders. Studies have shown that the expression of emotions is impaired in people suffering from schizophrenia. One model to explain this called ‘aberant salience’ , stating that a feeling of increased awareness and emotionality caused by an increased release of dopamine starts a psychosis. This is followed by the assignment of salience to stimuli that others perceive as normal. fMRI studies have shown less involvement of the fusiform gyrus, amygdala and basal ganglia during emotion processing, compared to healthy individuals. More recently, studies have found that people suffering from schizophrenia show reduced effective connectivity from the amygdala to the theory-of-mind network, which makes it harder to accurately describe the emotions and intentions of others.

Autism spectrum disorders are neurodevelopmental disorders. People with autism experience difficulties in social interaction and communication, and often show repetitive and stereotyped behaviour. Autism spectrum disorders often co-occur with ADHD and anxiety. Regulating and recognizing emotions is difficult for people with autism. They give less attention to facial expressions or focus on irrelevant features. fMRI studies have shown reduced neural sensitivity to emotional cues (such as facial expression). Difficulties with the theory-of-mind and accompanying neural networks are thought to be the core deficit in social cognition in people with autism.

Borderline personality disorder is the most common personality disorder and is characterized by marked impulsivity and an instability of affect, interpersonal relationships and self-image. Borderline often occurs with substance use and mood and anxiety disorders. People with borderline feel emotions more easily, intensively and longer, making it more difficult to return to an emotional baseline after a triggering event. This is associated with self-injurious and suicidal behaviour and is related to social-cognitive perceptual biases. It has been suggested that emotional hyper-reactivity interferes with emotion perception and contributes to a specific pattern of altered emotion recognition in people with a borderline personality disorder. fMRI studies associated these deficits with a heightened amygdala activity.

Eating disorders are characterized by abnormal eating habits that may include eating as little as possible (anorexia) or binge-eating and compensating (bulimia). It is thought that cultural idealization of a certain body image contributes to the development of an eating disorder. Eating disorders occur mostly with mood and anxiety disorders and impulse control disorder. Anorexia has been associated with using maladaptive strategies to regulate emotions. These strategies cause a temporally positive feeling by reducing fear, but causes more problems on the long-term. People with anorexia have difficulties inferring emotional states in others. Because of the many forms of eating disorders and the physical effects of an eating disorder, it is hard to state general conclusions on which brain regions are involved.

fMRI studies have shown deficits in emotion processing in many mental disorders. This attributes to attentional biases and impaired recognition of emotion expression, leading to problems in emotion regulation, general adjusting, emotional and social functioning and well-being. Disrupted emotion processing is an important liability spectrum that underlies different mental disorders. It is also a possible explanation for the high comorbidity amongst several mental illnesses.