Probability and magnitude evaluation in schizophrenia

Abstract

Alterations in reinforcement learning and decision making in schizophrenia have been linked with orbitofrontal cortex (OFC) dysfunction, a region critical for weighing reward magnitude in the calculation of expected value (EV). However, much of this work has used complex tasks that require combined learning and EV calculation. Here we used a simple "Roulette" task that examined the calculation of EV directly through a combination of text and/or pictorial representation of reward probability and magnitude. Forty-four people with schizophrenia and 30 controls were recruited. Patients were less sensitive to adjustments in a parameter combining probability and magnitude into one EV construct. Breaking down the construct into independent contributions of probability and magnitude, we found that negative symptoms were associated with magnitude sensitivity. This is consistent with the hypothesized role of OFC in actively representing magnitude and the notion that negative symptoms may involve a failure to appropriately estimate and use future reward magnitude to guide decision making.

Fig. 1

Curve fits and group parameter estimates (+95% HDI) for the Sharp et al. model M2 (Left) and model M6 from the alternative logistic regression models (Right). Solid lines represent the median of the posterior estimates, while the faint lines represent a single curve fit to one posterior sample from the Bayesian model. Crosses represent the group median probability of selecting prospect 1 across each EV-ratio. The adjusted EV-ratio for the Sharp model reflects the EV-ratio after accounting for the constant Prospect and Prelec functions. N = 30 controls, 44 people with schizophrenia.

Fig. 2

Model averaged magnitude sensitivity (averaged over M3-M6) as a function of total negative symptom scores: BPRS negative, SANS total and BNSS total. BPRS Negative symptoms was significantly associated with magnitude sensitivity, and while SANS total and BNSS total were not significant, the effect size was similar to BPRS Negative and they contained several subscales that were significant (see Table 2).