Delay discounting and task performance consistency in patients with schizophrenia

Abstract

To study impaired goal-oriented behavior in schizophrenia (SZ), we used a delay discounting task, which consists of a series of choices between receiving a small immediate or larger delayed reward. Few studies of delay discounting have evaluated response consistency (R(2)), which is especially relevant in SZ because of documented inconsistency in task performance. We calculated the rate of discounting (k) and R(2) in SZ (n=35) and healthy controls (HC; n=21). Using a criterion value of R(2)>0.60 to define consistent performance allowed us to compare discounting in consistent SZ and HC, as well as in inconsistent SZ. Groups did not differ significantly in smoking. Compared to HC, consistent SZ showed greater delay discounting. Both groups exhibited similar patterns of decreasing immediate choices across trial categories, although the decrease was less for SZ. Separate analyses on smokers and non-smokers showed that this group difference was carried by the non-smokers. Inconsistent SZ discounted more than HC and consistent SZ, but their aberrant pattern of choices casts doubt on the validity of their calculated k values.

Keywords: Decision making; Executive function; Intertemporal.

Fig. 1

Delay discounting task. (A) Delay discounting task trial; (B) Sensorimotor control trial. All trials were 11 sec in duration, with the initial fixation cross presented for two, four, or six sec, followed by two grey boxes paired with (A) the choice of an immediate or a delayed hypothetical monetary reward ($28 now or $34 in five days; a trial k of 0.041) or (B) the no choice option. Participants had the remainder of the 11-sec trial (nine, seven, or five sec) to indicate their preference by pressing a button on the side corresponding to their choice. The box under the choice turned green, indicating the response selection. A return of the fixation cross indicated the start of the next trial.

Fig. 2

Model fit (R²) during estimation of k values for healthy controls (HC; all consistent) and consistent patients and inconsistent patients with schizophrenia (SZ). The reference line at 0.60 indicates the minimum R² value that was used to define consistent performance.

Fig. 3

Mean (± standard error) log(k) for the healthy controls (HC), consistent patients with schizophrenia (Con SZ), and inconsistent patients with schizophrenia (Incon SZ). Smaller negative values indicate greater delay discounting. The values at top above each group are the k values corresponding to the log(k) values on the y-axis and in Table 1. Letters (a, b, c) indicate that all group differences were significant (p < 0.05).

Fig. 4

Mean (± standard error) percentage of Now (%Now) choices as a function of the eight trial k s for the healthy controls (HC), consistent patients with schizophrenia (Con SZ), and inconsistent patients with schizophrenia (Incon SZ).