INTRODUCTION In recent years, the concept of zone of proximal development (ZPD) has come to be known as one of the cardinal features in learning from a sociocultural perspective. Thus, it has increasingly been applied to second language education. ZPD was first introduced by the Russian psychologist Lev Vygotsky in Thought and Language (1986). He believed that learners should be assessed not through their actual knowledge but through their potential knowledge. In his view, social context is the original source of knowledge. Lantolf (2007) purports that Vygotsky has proposed a wide view of cognition by considering the environment itself as the source of development. To Vygotsky, ZPD indicates "the discrepancy between a child's actual mental age and the level he reaches in solving problems with assistance" (1986: 187). In fact, any assessment regardless of social interaction cannot be the mirror of reality. This notion has a central role in sociocultural theory applied to second language education. It should be mentioned that in this study, the enlarged notion of ZPD is meant. The "enlarged" notion of ZPD refers to "a site of potential learning that is created when participants of all ages and levels--and not just children and adults or novices and experts collaborate in understanding a concept or solving a problem" (Wells, 1999, cited in Villamil & de Guerrero, 2005: 79-80). Furthermore, it "implies that mediation is not limited to assistance by other human beings but may come in the form of socially constructed semiotic artifacts, such as books, maps, and diagrams" (Wells, 1999, cited in Villamil & de Guerrero, 2005: 80).
