FMRI investigation of intertemporal discounting in schizophrenia

Abstract

Schizophrenia is most recognizable by positive symptoms of hallucinations and delusions, but the cognitive deficits and negative symptoms contribute more to functional deficits. The delay discounting (DD) task, with choices between a small immediate reward and a larger delayed reward, tapping into both executive function and reward processing, may prove useful in identifying cognitive and reward processing abnormalities relevant to schizophrenia. In the present study, we used the discounting parameter, k, to assess whether patients with schizophrenia preferred more immediate rewards than healthy controls. We used a model fit statistic, R², as a measure of choice consistency, quantified using a non-linear regression of participants' responses. Using functional magnetic resonance imaging (fMRI), we investigated group differences between patients with schizophrenia and controls in blood oxygen level-dependent (BOLD) responses to DD decisions in general. In addition, we investigated neural responses to delay discounting decisions varying in difficulty. Compared to controls, patients were more inconsistent in their pattern of responses and exhibited greater DD. However, the difference in DD disappeared when analysis was limited to patients who were consistent in their task performance. Controls, matched on performance and demographics to the consistent patients, displayed greater activation in executive function and reward areas in response to task trials compared to control trials. Compared to controls, consistent patients displayed greater activation to the task relative to the control trials in left insular and temporal cortices and in the precuneus. In response to hard DD trials, controls, when compared to patients, showed more activation in areas associated with executive function, such as the inferior frontal gyrus and the dorsal anterior cingulate cortex. In response to both hard and easy trials, controls showed more activation than consistent patients in the inferior parietal lobule and the ventral striatum. Patients unable to perform the task consistently, when compared to controls, showed greater activation to the DD task in the precuneus and posterior cingulate cortex. In general, patients with schizophrenia appear to have regions of hypoactivation contributing to executive function and reward processing deficits, in addition to hyperactivation in areas associated with resting state and conflict monitoring.