Performance on a probabilistic inference task in healthy subjects receiving ketamine compared with patients with schizophrenia

Abstract

Evidence suggests that some aspects of schizophrenia can be induced in healthy volunteers through acute administration of the non-competitive NMDA-receptor antagonist, ketamine. In probabilistic inference tasks, patients with schizophrenia have been shown to 'jump to conclusions' (JTC) when asked to make a decision. We aimed to test whether healthy participants receiving ketamine would adopt a JTC response pattern resembling that of patients. The paradigmatic task used to investigate JTC has been the 'urn' task, where participants are shown a sequence of beads drawn from one of two 'urns', each containing coloured beads in different proportions. Participants make a decision when they think they know the urn from which beads are being drawn. We compared performance on the urn task between controls receiving acute ketamine or placebo with that of patients with schizophrenia and another group of controls matched to the patient group. Patients were shown to exhibit a JTC response pattern relative to their matched controls, whereas JTC was not evident in controls receiving ketamine relative to placebo. Ketamine does not appear to promote JTC in healthy controls, suggesting that ketamine does not affect probabilistic inferences.

Figure 1.

Flow chart depicting test procedure.

Figure 2.

Mean ketamine levels in blood plasma according to target dose.

Figure 3.

Probability distributions of ‘draws to decision’ under ketamine. Pooled data across all participants, according to dose.

Figure 4.

Probability distributions of ‘draws to decision’ for patients, matched controls, and controls receiving ketamine at the higher dose.