Previous studies suggest that the left inferior frontal cortex is involved in the resolution of lexical ambiguities for language comprehension. In this study, we hypothesized that processing of lexical ambiguities is improved when the excitability of the left inferior frontal cortex is enhanced. To test the hypothesis, we conducted an experiment with transcranial direct current stimulation (tDCS). We investigated the effect of anodal tDCS over the left inferior frontal cortex on behavioral indexes for semantic judgment on lexically ambiguous and unambiguous words within a context. Supporting the hypothesis, the RT was shorter in the anodal tDCS session than in the sham session for ambiguous words. The results suggest that controlled semantic retrieval and contextual selection were facilitated by anodal tDCS over the left inferior frontal cortex.

One of the most fundamental and universal properties of human language is a phenomenon called displacement. In the present study, we used multichannel event-related potentials (ERPs) to identify the nature of this phenomenon with Japanese, a subject-object-verb (SOV) language of relatively free word order. The ERPs of sentences of canonical word order (CC) were compared with those of non-canonical word order in two types of Japanese complex sentences; namely, in those which can be described as being in a middle-scrambled condition (MSC) and in those in a long-scrambled condition (LSC). The sustained anterior negativity (SAN) and the P600 in the pregap position were observed in the LSC, compared to the CC, and they are consistent with previous findings. The SAN, exhibiting a tripartite nature in morphology and scalp distribution, mainly reflected a storage cost of scrambled elements in sentence comprehension. The subsequent P600 had a left fronto-temporal maximum, distinguished from a posterior P600, taken as a reflector of the thematic role assignment in previous related studies. It is argued that the P600 in the present study reflects a cost of structural integration intensively depending on the case marker information. A compositional interpretation of sentence meanings was also observed, reflected in an anterior negativity at the postgap verbal position, which cannot be differentiated at the pregap verbal position in the languages of subject-verb-object (SVO) word order.