Anhedonia

Anhedonia is a diverse array of deficits in hedonic function, including reduced motivation or ability to experience pleasure. While earlier definitions of anhedonia emphasized the inability to experience pleasure, anhedonia is used by researchers to refer to reduced motivation, reduced anticipatory pleasure (wanting), reduced consummatory pleasure (liking), and deficits in reinforcement learning. In the DSM-5, anhedonia is a component of depressive disorders, substance related disorders, psychotic disorders, and personality disorders, where it is defined by either a reduced ability to experience pleasure, or a diminished interest in engaging in pleasurable activities. While the ICD-10 does not explicitly mention anhedonia, the depressive symptom analogous to anhedonia as described in the DSM-V is a loss of interest or pleasure. Anhedonia is a diverse array of deficits in hedonic function, including reduced motivation or ability to experience pleasure. While earlier definitions of anhedonia emphasized the inability to experience pleasure, anhedonia is used by researchers to refer to reduced motivation, reduced anticipatory pleasure (wanting), reduced consummatory pleasure (liking), and deficits in reinforcement learning. In the DSM-5, anhedonia is a component of depressive disorders, substance related disorders, psychotic disorders, and personality disorders, where it is defined by either a reduced ability to experience pleasure, or a diminished interest in engaging in pleasurable activities. While the ICD-10 does not explicitly mention anhedonia, the depressive symptom analogous to anhedonia as described in the DSM-V is a loss of interest or pleasure. While anhedonia was originally defined in 1896 by Théodule-Armand Ribot as the reduced ability to experience pleasure, it has been used to refer to deficits in multiple facets of reward. Re-conceptualizations of anhedonia highlight the independence of 'wanting' and 'liking'. 'Wanting' is a component of anticipatory positive affect, mediating both the motivation (i.e. incentive salience) to engage with reward, as well as the positive emotions associated with anticipating a reward. 'Liking', on the other hand, is associated with the pleasure derived from consuming a reward. The consciousness of reward-related processes has also been used to categorize reward in the context of anhedonia, as studies comparing implicit behavior versus explicit self-reports demonstrate a dissociation of the two. Learning has also been proposed as an independent facet of reward that may be impaired in conditions associated with anhedonia, but empirical evidence dissociating learning from either 'liking' or 'wanting' is lacking. Anhedonia has also been used to refer to 'affective blunting', 'restricted range of affect', 'emotional numbing', and 'flat affect', particularly in the context of post-traumatic stress disorders. In PTSD patients, scales measuring these symptoms correlate strongly with scales that measure more traditional aspects of anhedonia, supporting this association. Studies in clinical populations, healthy populations, and animal models have implicated a number of neurobiological substrates in anhedonia. Regions implicated in anhedonia include the prefrontal cortex as a whole, particularly the orbitofrontal cortex (OFC), the striatum, amygdala, anterior cingulate cortex (ACC), hypothalamus, and ventral tegmental area (VTA). Neuroimaging studies in humans have reported that deficits in consummatory aspects of reward are associated with abnormalities in the ventral striatum and medial prefrontal cortex, while deficits in anticipatory aspects of reward are related to abnormalities in hippocampal, dorsal ACC and prefrontal regions. These abnormalities are generally consistent with animal models, except for inconsistent findings with regard to the OFC. This inconsistency may be related to the difficulty in imaging the OFC due to its anatomical location, or the small number of studies performed on anhedonia; a number of studies have reported reduced activity in the OFC in schizophrenia and major depression, as well as a direct relationship between reduced activity and anhedonia.Researchers theorize that anhedonia may result from the breakdown in the brain's reward system, involving the neurotransmitter dopamine. Anhedonia can be characterised as 'impaired ability to pursue, experience and/or learn about pleasure, which is often, but not always accessible to conscious awareness'. The conditions of akinetic mutism and negative symptoms are closely related. In akinetic mutism, a stroke or other lesion to the anterior cingulate cortex causes reduction in movement (akinetic) and speech (mutism). Anhedonia occurs in roughly 70% of people with a major depressive disorder. Anhedonia is a core symptom of major depressive disorder; therefore, individuals experiencing this symptom can be diagnosed with depression, even in the absence of low/depressed mood. The Diagnostic and Statistical Manual of Mental Disorders (DSM) describes a 'lack of interest or pleasure', but these can be difficult to discern given that people tend to become less interested in things which do not give them pleasure. The DSM criterion of weight loss is probably related, and many individuals with this symptom describe a lack of enjoyment of food. They can portray any of the non-psychotic symptoms and signs of depression. Anhedonia is commonly listed as one component of negative symptoms in schizophrenia. Although five domains are usually used to classify negative symptoms, factor analysis of questionnaires yield two factors, with one including deficits in pleasure and motivation. People with schizophrenia retrospectively report experiencing fewer positive emotions than healthy individuals. However, 'liking' or consummatory pleasure, is intact in schizophrenics, as they report experiencing the same degree of positive affect when presented with rewarding stimuli. Neuroimaging studies support this behavioral observation, as most studies report intact responses in the reward system (i.e. ventral striatum, VTA) to simple rewards. However, studies on monetary rewards sometimes report reduced responsiveness. More consistent reductions are observed with regard to emotional response during reward anticipation, which is reflected in a reduced responsiveness of both cortical and subcortical components of the reward system. Schizophrenia is associated with reduced positive prediction errors (a normal pattern of response to an unexpected reward), which a few studies have demonstrated to be correlated with negative symptoms. Schizophrenics demonstrate impairment in reinforcement learnings tasks only when the task requires explicit learning, or is sufficiently complex. Implicit reinforcement learning, on the other hand, is relatively intact. These deficits may be related to dysfunction in the ACC, OFC and dlPFC leading to abnormal representation of reward and goals. Anhedonia is common in people who are dependent upon a wide variety of drugs, including alcohol, opioids, and nicotine. Although anhedonia becomes less severe over time, it is a significant predictor of relapse. While PTSD is associated with reduced motivation, part of the anticipatory 'wanting', it is also associated with elevated sensation seeking, and no deficits in physiological arousal, or self reported pleasure to positive stimuli. PTSD is also associated with blunted affect, which may be due to the high comorbidity with depression.