Negative symptoms of schizophrenia are associated with abnormal effort-cost computations

Abstract

Background: Decision-making studies show that response selection is influenced by the "effort cost" associated with response alternatives. These effort-cost calculations seem to be mediated by a distributed neural circuit including the anterior cingulate cortex and subcortical targets of dopamine neurons. On the basis of evidence of dysfunction in these systems in schizophrenia (SZ), we examined whether effort-cost computations were impaired in SZ patients and whether these deficits were associated with negative symptoms.

Methods: Effort-cost decision-making performance was evaluated in 44 patients with SZ and 36 demographically matched control subjects. Subjects performed a computerized task where they were presented with a series of 30 trials in which they could choose between making 20 button presses for $1 or 100 button presses for higher amounts (varying from $3 to $7 across trials). Probability of reward receipt was also manipulated to determine whether certain (100%) or uncertain (50%) reward affected effort-based decision-making.

Results: Patients were less likely than control subjects to select the high-effort response alternative during the 100% probability condition, particularly when the value payoff was highest (i.e., $6 and $7). Patients were also less likely to select the high-effort option on trials after reward in the 50% probability condition. Furthermore, these impairments in effort-cost computations were greatest among patients with elevated negative symptoms. There was no association with haloperidol equivalent dosage.

Conclusions: The motivational impairments of SZ might be associated with abnormalities in estimating the "cost" of effortful behavior. This increased effort cost might undermine volition.

Figure 1

Effort-cost task trial sequence. Participants initially select which balloon they want to pop. The easy selection always offers $1 for 20 button presses. The hard selection offers $3–$7 for 100 presses. After each press the size of the balloon increases in proportion to how many presses are needed to reach the pin at the top of screen.

Figure 2

Proportion of responses allocated to the difficult option as a function of reward value. (A, B) Probability of selecting the harder response alternative in the 100% and 50% probability payoff conditions, respectively, in the overall patient group (SZ) and in healthy control subjects (CN). (C, D) The same data are displayed with the overall SZ group split into high (HI-NEG) and low negative (LOW-NEG) symptom groups.

Figure 3

Probability of selecting the difficult effort alternative after prior reward or nonreward in the 50% probability condition. (A) Both SZ and CN are more likely to select the more difficult response alternative after receiving a reward on the prior trial. (B) The patient data are split into HI-NEG and LOW-NEG symptom groups. Abbreviations as in Figure 2.