On the basis of the notion of the argument/adjunct asymmetry in syntactic derivation, it will be argued that Japanese right dislocation constructions should be divided into two types, based on the argument/adjunct distinction of the dislocated element. This argument is also examined through an event-related brain potential (ERP) study. We will report that dislocated arguments elicit the left dominant positivity. On the basis of previous ERP studies of language processing, it will be argued that this left dominant positivity effect reflects a syntactic integration, and that this result supports our asymmetrical analysis of Japanese right dislocation constructions.