Intra-dimensional/extra-dimensional set-shifting performance in schizophrenia: impact of distractors

We sought to determine if a representative group of young chronic patients with schizophrenia would demonstrate selective impairments in set shifting processes of the CANTAB Intra-dimensional/extra-dimensional (IDED) task. We predicted that patients would have prominent difficulties with Compound Discrimination (C_D) (stage of the task in which irrelevant stimuli are introduced) and Extra-Dimensional Shifting (EDS) (stage of the task in which a new stimulus dimension must be attended) on the basis of the results of cortical hypodopaminergic states in subhuman primates (for C_D) and effects of dorsolateral prefrontal cortical lesions on set shifting and prior results in schizophrenia (for EDS).

We administered the IDED to 36 patients and 26 healthy controls. Additionally, we administered the Wisconsin Card Sorting Test (WCST), another test of set shifting, and a Continuous Performance Test (CPT) type task of attention to patients with schizophrenia in order to investigate which cognitive components accounted for performance difficulties at different stages of the IDED task.

Patients had selective difficulties on C_D and EDS stages of the task. In schizophrenic patients early stages of the task involving the introduction and establishment of attentional set were correlated to CPT performance, while later set shifting stages were correlated with WCST categories attained.

We found evidence that patients with schizophrenia were susceptible to introduction of unreinforced irrelevant stimuli at the C_D stage, such that the previously rewarded target stimuli no longer held hegemony as a representation. This type of processing failure may reflect difficulties in stabilizing a representation and is consistent with effects of prefrontal hypodopaminergia in primates. Secondly, "survivors" of this stage experienced marked difficulties on EDS-stage, suggestive of classic prefrontal failures.