[Learned equivalence in schizophrenia: novel method for the measurement of memory functions of the hippocampus and basal ganglia].

The purpose of the present study was to investigate basal ganglia (BG) and medial temporal lobe (MTL) dependent learning in patients with schizophrenia. Acquired equivalence is a phenomenon in which prior training to treat two stimuli as equivalent (if two stimuli are associated with the same response) increases generalization between them. The learning of stimulus-response pairs is related to the BG, whereas the MTL system participates in stimulus generalization. Forty-three patients with DSM-IV schizophrenia and 28 matched healthy controls participated in the study. Volunteers received the acquired equivalence associative learning task (FISHES) during which associations between faces and pet fishes are learned. Results revealed that patients with schizophrenia showed a selective deficit on stimulus generalization, whereas stimulus-response learning was unaffected. The number of errors during stimulus-response learning correlated with the daily chlorpromazine-equivalent dose of antipsychotics. In conclusion, patients with schizophrenia showed deficits during MTL-dependent learning, but not during BG-dependent learning. High-dose first generation antipsychotics may disrupt BG-dependent learning by blocking dopaminergic neurotransmission in the nigro-striatal system.