Four types of morpheme: evidence from aphasia, code switching, and second-language acquisition

This paper presents empirical evidence supporting a new model of morpheme classification called the 4-M model. This model emphasizes the notion that lemmas underlying different types of morphemes become salient at different levels of production. This explains their different distributions. While the 4-M model classifies morphemes, it is primarily a model of how morphemes are accessed. The argument is that particular instantiations of morphemes are classified as a consequence of the mechanisms that activate them. The evidence considered comes from studies of code switching, Broca's aphasia, and second-language acquisition. One finding that the 4-M model captures is that not all functional elements pattern alike. Some are conceptually activated at the level of the mental lexicon along with their content-morpheme heads. Two other types of functional element are structurally assigned and do not become salient until later in the production process. These differences explain their different distributions in the data considered