Abnormal bihemispheric responses in schizophrenia patients following cathodal transcranial direct stimulation

Post-mortem and in vivo studies provide evidence for a link between reduced plasticity and dysconnectivity in schizophrenia patients. It has been suggested that the association between plasticity and connectivity contributes to the pathophysiology and symptomatology of schizophrenia. However, little is known about the impact of glutamate-dependent long-term depression (LTD)-like cortical plasticity on inter-hemispheric connectivity in schizophrenia patients. The aim of the present study was to investigate LTD-like cortical plasticity following excitability-diminishing cathodal transcranial direct current stimulation (tDCS) of the left primary motor cortex (M1) and its effects on the non-stimulated right M1. Eighteen schizophrenia patients and 18 matched (age, gender, handedness, and smoking status) control subjects were investigated in this study. Corticospinal excitability changes following tDCS and intra-cortical inhibitory circuits were monitored with transcranial magnetic stimulation. On the stimulated hemisphere, cathodal tDCS increased resting motor thresholds (RMT) in both groups and decreased motor-evoked potential (MEP) sizes in healthy controls to a greater extent compared to schizophrenia patients. On the non-stimulated hemisphere, RMTs were increased and MEPs were decreased only in the healthy control group. Our results confirm previous findings of reduced LTD-like plasticity in schizophrenia patients and offer hypothetical and indirect in vivo evidence for an association between LTD-like cortical plasticity and inter-hemispheric connectivity in schizophrenia patients. Moreover, our findings highlight the impact of plasticity on connectivity. Dysfunctional N-methyl D-aspartate receptors or modulation of dopaminergic transmission can explain these findings. Nevertheless, the effects of antipsychotic medication still need to be considered.