A neuroeconomic investigation into major depression and anhedonia

Anhedonia is a key symptom of major depression, but also presents as a trait within the general population. The aim of this thesis was to examine how anhedonia affects simple decision-making and reward learning. A review of animal and human (both healthy and clinical) studies on the role of dopamine in anhedonia concludes that the dopaminergic system appears to mediate the anticipation, rather than the experience, of pleasure. A review of anhedonia measures highlights their predictive validity in identifying individuals at risk of, or diagnosed with, major depression or schizophrenia. While anhedonia is defined as a core feature of major depression, the DSM-V definition has failed to keep abreast of recent developments in the anhedonia literature. Theoretical perspectives make a case for anhedonia and depression affecting economic decision-making, via increased rumination, risk aversion, or negative cognitive bias, a position supported by a review of the current neuroeconomic literature on decision-making in mental illness. Three empirical studies were undertaken. The first, a factor analysis, investigated whether common measures of anhedonia clustered on a single, general anhedonia factor. 444 undergraduate psychology students at two Australian universities completed a range of personality questionnaires. While the exploratory factor analysis suggested either a two or three factor structure, neither factor structure provided an adequate fit when tested via confirmatory factor analysis. Two samples were recruited for the second and third studies: 30 first-year undergraduate students, scoring high and low on a measure of general anhedonia; and 13 depressed participants matched to 13 control participants. Participants completed a gambling task, placing high and low bets to win varying amounts of money, and a signal detection task (SDT), in which correct responses to one cartoon face briefly presented on-screen was rewarded more frequently than the other cartoon face. While the gambling task manipulation was successful, with participants typically choosing to place a high bet when the potential winnings were greater, neither general anhedonia nor depression status affected this behaviour. Similar findings were observed in the SDT, with neither general anhedonia, nor depression status affecting the tendency of participants to develop a bias to the more frequently rewarded response. While the ERP analysis indicated that P300 and FRN amplitudes varied in response to the presentation of trial feedback in the SDT, neither anhedonia or depression status moderated this effect. More promising results were observed using more specific measures of anhedonia. Anticipatory anhedonia appeared to moderate the relationship between bet choice and reward level in the gambling task; while in the SDT, high anticipatory anhedonia was consistently associated with a higher response bias. These results do not support the existence of general anhedonia factor, but support the view that anhedonia can be conceptualised as two distinct subtypes, motivational and consummatory, with separate neurobiological pathways. While further work is needed to more closely align these measures with their underlying trait or symptomology, these findings highlight the role neuroeconomics can play in furthering our understanding of mental illnesses, and inform novel treatment options.