Abstract
Background
Transcranial direct current stimulation (tDCS) is a neurophysiological therapy which modulates neural activities in the brain with weak electrical current. The beneficial effects of tDCS are relevant to positive and negative symptoms, as well as cognition and functional capacity in patients with schizophrenia (Narita et al, 2017). However, whether tDCS would improve higher-order cognition, e.g. semantic memory organization, has remained unclear. Recently, text-mining analyses have been proven to be effective to reveal semantic memory in patients with schizophrenia (Sung et al., 2012; Sumiyoshi et al., 2018). The purpose of the study was to investigate effects of tDCS to improve semantic memory, as evaluated by text-mining analyses of category fluency data, in patients with schizophrenia.
Methods
Twenty-eight patients meeting DSM-5 criteria for schizophrenia entered the study. All subjects gave written informed consent, and the study was approved by the Ethical Committee of National Center of Neurology and Psychiatry and the Ethical Committee of Osaka university. Data from healthy control subjects (N=335) were also used. The tDCS was conducted according to the previous study (Narita et al., 2017). tDCS was administered twice a day in 5 consecutive days. The Brief Assessment of Cognition in Schizophrenia, (BACS) was administered at baseline and one month after the last tDCS session. Verbal outputs of the category fluency task in the BACS were submitted to the singular valued decomposition (SVD) analysis. Semantic memory structures were estimated by calculating cosine values (i.e. similarities) among frequent items in the six-dimensional solution. These values were compared between healthy controls vs. patients at baseline and follow-up using Pearson’s correlational coefficients.
Results
The correlational coefficient for the frequent items (DOG, CAT, ELEPAHANT, GIRAFFE, LION, TIGER) between patients and healthy controls was considerably higher at the follow-up (r=0.58) compared to that at base line (r=0.12), reaching a marginal significance (p=0.058). A visual inspection for the cosine profiles of frequent items indicated that distinct clusters of Pet (DOG, CAT), Wild-Herbivorous (ELEPAHANT, GIRAFFE), and Wild-Carnivorous (LION, TIGER) in patients at follow-up, as in the case for healthy controls.
Discussion
To our knowledge, this is the first study reporting a facilitative effect of tDCS on higher-order of cognition in patients with schizophrenia. Semantic memory in the patients became structurally similar to that for healthy controls after administration of tDCS. Furthermore, animal names frequently produced were meaningfully clustered in patients at follow-up, similar to the pattern of healthy controls. These results may explain the efficacy of tDCS for improving other domains of cognitive function in patients with schizophrenia. We also could demonstrate that text-mining analysis is a powerful tool to elucidate higher-order cognition in people with mental disorders.
References
Anodal Transcranial Direct Current Stimulation over the Cerebellum Enhances Sadness Recognition in Parkinson’s Disease Patients: a Pilot Study