Transcranial Direct Current Stimulation Influences on Cognitive Inhibition in Schizophrenia

A deficit in cognitive suppression is a trait of patients with schizophrenia. Cognitive suppression is the ability to control or suppress irrelevant information and to adopt relevant responses instead. This ability is a basic skill which is essential to maintain a normal well-adjusted life. We need to suppress distracting information in order to focus attention, to suppress irrelevant cues in order to retrieve particular memories, and to suppress habitual responses in order to make adaptive choices (Dillon \& Pizzagalli, 2007). The purpose of this study is to investigate the effects of transcranial direct current stimulation (tDCS) on information suppression in schizophrenic patients. This is a noninvasive technique of brain stimulation that induces prolonged functional changes in the cerebral cortex through the application of a weak direct current to the scalp (Nitsche \& Paulus, 2001; Nitsche et al., 2003, 2004; Antal et al., 2004). The aim of this study is to test whether anodal and cathodal tDCS differentially modify performance on several cognitive tasks when applied to the left/right dorsolateral prefrontal cortex (DLPFC).

Sixty patients will be divided into four groups: 1) 15 schizophrenic subjects in a tDCS treatment group; 2) 15 schizophrenic subjects in a sham tDCS group; 3) 15 healthy subjects in a tDCS treatment group; 4) 15 healthy subjects in a sham tDCS group. All participants will complete baseline tests including an n-back test, the Hayling test and metaphoric comprehension test. The schizophrenic subjects only, will complete a bank of cognitive tests as a baseline (MATRICS Consensus Cognitive Battery (MCCB). The first meeting for the schizophrenic subjects only will be used to determine the best stimulation area (right/left DLPFC) and type (anodal/ cathodal). Schizophrenic subjects in both the treatment and the control groups will undergo six treatment sessions in total. The cognitive tests will be given to all participants once again at the end of the study. In order to evaluate improvement between the pre-test and the post-test, differences in reaction times and error rate will be calculated for each participant.

This study is important for few reasons. First, this study focus on the difficulty of suppressing irrelevant information, executive function ability essential to leading a normal life. Special populations, and specifically people with schizophrenia, have difficulty suppressing irrelevant information, and a correlation has been noted between this characteristic and weak executive functioning capabilities. In this study we evaluate the potential of minimizing these difficulties through use of tDCS. This particular tool was chosen for the study because it is easy to use, non-invasive, and painless. Research has shown that tDCS stimulation of the DLPFC can improve performance in cognitive tasks, a result that has not been mirrored through use of medications. Second, in recognition of the heterogeneity of schizophrenic patients (substantiated by the fact that the success of other treatments varies from patient to patient), our study will perform tests to identify the preferred stimulation region, DLPFC left/right. The results will help determine more specifically if there is a degree of heterogeneity in terms of the region responsible for suppressing irrelevant information . Related to this, we will be able to assess the impact of stimulation, including whether or not stimulation benefits all schizophrenic patients or, alternatively, if there are unknown variables involved which determine the relative success of this treatment amongst individuals. In addition, we will be able to explore differences or correlations between healthy subjects' verbal abilities in relation to the various tests conducted, and assess the impact of these tests on healthy subjects' cognitive inhibition and working memory. Finally, this study is critical because if the treatment method proves to be successful, we would be able to extend this study to other populations that have difficulties with executive functioning, and in addition, to examine the effect of electrical stimulation on other cognitive abilities.