Why don't you try harder? An investigation of effort production in major depression

Abstract

Depression is mainly characterized as an emotional disorder, associated with reduced approach behavior. It remains unclear whether the difficulty in energising behavior relates to abnormal emotional states or to a flattened response to potential rewards, as suggested by several neuroimaging studies. Here, we aimed to demonstrate a specific incentive motivation deficit in major depression, independent of patients' emotional state. We employed a behavioral paradigm designed to measure physical effort in response to both emotional modulation and incentive motivation. Patients did exert more effort following emotionally arousing pictures (whether positive or negative) but not for higher monetary incentives, contrary to healthy controls. These results show that emotional and motivational sources of effort production are dissociable in pathological conditions. In addition, patients' ratings of perceived effort increased for high incentives, whereas controls' ratings were decreased. Thus, depressed patients objectively behave as if they do not want to gain larger rewards, but subjectively feel that they try harder. We suggest that incentive motivation impairment is a core deficit of major depression, which may render everyday tasks abnormally effortful for patients.

Figure 1. Behavioral task.

Successive screenshots displayed in one trial are shown from left to right, with durations in ms. Neutral or arousing pictures (with positive or negative valence) were shown prior to physical effort exertion. Effort was cued by simultaneously showing the amount of money at stake, materialized as coin images (1 cent, 10 cents or 1 euro), and a graduated scale in which a cursor represented the force exerted on the handgrip. Subjects knew that the top of the scale corresponded to the monetary incentive, such that the more they squeezed the handgrip, the more money they would win. After force production, subjects rated the extent of their effort by positioning a cursor on an analog scale. The final screen informed subjects about the cumulative total of monetary earnings.

Figure 2. Group-level results.

Histograms show the effects of the main independent factors (incentive and arousal levels) on the main dependent variables (grip force, effort rating). Grip force is expressed as a percentage of the highest measure. Effort ratings were divided by the actual force produced on a trial-by-trial basis. Error bars are ± inter-subjects standard errors of the mean. * Significant difference (paired t-test, P<0.05), between negative and neutral picture and between 0.01€ and 1€ in the task. NS = non significant.

Figure 3. Individual results.

Each point is a healthy subject (empty circle) or a depressed patient (filled circle). Graphs show difference scores for incentive (1€ - 0.01€) versus arousal (emotional - neutral) effects on force production and effort rating. Grip force and skin conductance is expressed as a percentage of the highest measure. Effort ratings were divided by the actual force produced on a trial-by-trial basis.