Effect of transcranial direct current stimulation on semantic memory depends on anodal stimulation sites in patients with schizophrenia

Background: Impairments in language function, often associated with semantic memory disorganization, represent a core symptom of schizophrenia. Semantic memory disorganization is linked to poorer social functioning, while potential treatment methods remain uncertain. We previously reported on the ability of transcranial direct current stimulation (tDCS) when delivered to the left prefrontal (PFC) cortex, to improve semantic memory in patients with schizophrenia. The current study investigated whether anodal tDCS over the left temporal region would also improve semantic memory in patients with schizophrenia. Methods: Data were used from fifteen outpatients with schizophrenia, while previously published data from healthy control (HC) volunteers were used as a reference. Cognitive assessments, including the category fluency task (CFT), were conducted at baseline and one month after completion of tDCS (2 mA for 20 min, twice daily for five consecutive days), following the protocol of our previous study. Verbal responses from the CFT were analyzed using singular value decomposition. To evaluate the effect of tDCS, differences between HC and patients (baseline and follow-up) were examined. Results: Hierarchical clustering showed patients with schizophrenia at follow-up produced a better coherent category cluster than at baseline. However, the overall improvement in the organization of semantic memory did not reach statistical significance when using Meng's test. Conclusions: The effects of anodal stimulation of the left temporal region on semantic memory organization in outpatients with schizophrenia were limited. This indicates that when attempting to improve semantic memory, the PFC may serve as a better stimulation site.