Costello (1993) has drawn attention to both the heterogeneity problem in depression, and the reliability of current classifications, and recommends a more focused approach on specific symptoms. Hence, rather than focus on syndromes, another approach has been to focus on affect systems and in particular positive affect (PA) and negative affect (NA) (Davidson, 2000; Watson et al., 1995a;b), which are orthogonal (that is, one can feel both excited and frightened) (Clark, 2000). This approach makes sense of comorbidity data, for while depression is primarily a disorder of (low) PA, or anhedonia, it can be associated with a variety of NAs (such as anxiety, sadness, and anger) and coping efforts (such as alcohol use).
There are, however, slightly different conceptualisations of anhedonia. One focuses on anhedonia as a broad category, encompassing general apathy with a marked lack of motivation to engage in almost all activities (Klinger, 1993) and diminished interest or pleasure in activities, reflecting a generalised lack of PA (Watson & Clark, 1988; Watson et al., 1995a;b). This definition views depression as the result of reductions in a range of PAs such as joy, energy, enthusiasm, alertness, self-confidence, and interest. Others (e.g., Willner, 1993) have suggested that anhedonic patients can remain 'interested' in hedonic experiences (they would like to engage in certain activities), but anticipate that they will be unable to enjoy them if they do engage in them. Loss of interest or lack of motivation may develop from repeated experiences of wanting but failing to feel pleasure or reward from various activities (Snaith et al., 1995).
Davidson (2000) offers another angle on PA systems by suggesting that there are different regulators for the anticipation of rewards that motivate engagement and effort, compared to the enjoyment of rewards if one's efforts are successful. This distinction may be important for a number or reasons. First, cognitive therapists (Beck et al., 1979) advocate the use of pleasure and mastery recording to help patients plan activities and to discover that they may enjoy them (and achieve) more than they thought they would. Clearly, if patients continued to enjoy them less than they predicted, this may be a less helpful intervention. If this is a successful approach, it suggests that it is the anticipation of rewards and enjoyment that is compromised in depression (that is, Willner's  view). Second, if this is the affect system that is compromised, it suggests that one of the functions of anhedonia in depression may be to inhibit engagement with the environment. As we shall see, this may have important implications for an evolutionary model to be presented shortly.
There have been a few studies that have explored the regulators of PA and NA separately. Clark's (2000) review indicates thatNA is associated with threats and irritations, whereas PA seems particularly associated with social engagement and activity. Interestingly, MacDonald (1988) argued that PA systems have undergone extensive evolutionary modification in the last few millions years. The 'drive' for this has been the role of social relationships (especially affiliative ones) in our evolution, making us into the highly social and interactive species we are. PA capacity and control have evolved along with the complexity of, and multiple functions of, social relationships. Interestingly, there have been suggestions in the literature that some people may suffer from social anhedonia as a trait (that is, they have less interest in, and enjoyment of, social relationships). Such a trait may be related to vulnerabilities to schizophrenia (e.g., Blanchard et al., 2000). However, anhedonia in depression is often a change in state and is not specific to social interactions (Snaith et al., 1995). This again suggests that in the context of depressive disorders anhedonia serves to inhibit general engagement in activities and functions as a variable strategy.
Whereas NA provides an individual with information about dangers and threats, and the type of emotion (anxiety and anger) indicates the defensive response (such as fight-flight), PAs are part of a safeness (Gilbert, 1993), behavioural approach (Davidson, 2000; Gray, 1987) and/or resource-seeking system (Panksepp, 1998) that controls levels of engagement in the environment and energy expenditure. Affect and energy expenditure are also separable systems, however, as when there is high PA but low activity (relaxing on a warm beach on holiday), the result of goals having been meet. In agitated depressions, there is low PA
with high-energy expenditure. Many depressions are marked by high fatigue and loss of energy, which some patients feel preceded their anhedonia and other depression symptoms clinically, although there is little research in this area.
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