Modulating Semantic Integration in the Right Hemisphere: A Transcranial Direct Current Stimulation Study

Prior research has shown that the right hemisphere is essential for integrating information across a passage to form a coherent thematic meaning. The aim of this dissertation is to evaluate right hemisphere semantic integration processes by directly manipulating the neural excitability of the left or right hemisphere using transcranial direct current stimulation (tDCS) while participants read passages and complete a lexical decision task. For the lexical decision task, participants were presented with either a target word related to the passage’s local (word-level) context, a target word related to the passage’s global (thematic) context, or a non-word. Because processing global information requires semantic integration processes, right hemisphere anodal (i.e., active) stimulation was predicted to increase the speed and accuracy of lexical decision responses to global targets compared to sham (i.e., inactive) and left hemisphere anodal stimulation. Contrary to predictions, results indicated no effect of tDCS condition, such that participants in each stimulation condition revealed similar patterns of response time and accuracy. However, participants responded faster to local targets compared to global targets, regardless of stimulation condition. This finding provides additional evidence for fast activation of local contextual information and relatively slower activation of global contextual information during discourse comprehension. Future directions are discussed regarding the absence of a tDCS effect on lexical decision response time and accuracy.