Word classes in the brain: implications of linguistic typology for cognitive neuroscience

Although recent research on the neural substrates of word classes has generated some valuable findings, significant progress has been hindered by insufficient attention to theoretical issues involving the nature of the lexical phenomena under investigation. This paper shows how insights from linguistic typology can provide cognitive neuroscientists with well-motivated guidelines for interpreting the extant data and charting a future course. At the outset, a fundamental distinction is made between universal and language-particular aspects of word classes. Regarding universals, prototypical nouns involve reference to objects, and their meanings rely primarily on the ventral temporal lobes, which represent the shape features of entities; in contrast, prototypical verbs involve predication of actions, and their meanings rely primarily on posterior middle temporal regions and frontoparietal regions, which represent the visual motion features and somatomotor features of events. Some researchers maintain that focusing on object nouns and action verbs is inappropriate because it conflates the semantic and grammatical properties of each word class. However, this criticism not only ignores the importance of the universal prototypes, but also mistakenly assumes that there are straightforward morphological and/or syntactic criteria for identifying nouns and verbs in particular languages. In fact, at the level of individual languages, the classic method of distributional analysis leads to a proliferation of constructionally based entity-denoting and event-denoting word classes with mismatching memberships, and all of this variation must be taken seriously, not only by linguists, but also by cognitive neuroscientists. Many of these word classes involve remarkably close correspondences between grammar and meaning and hence are highly relevant to the neurobiology of conceptual knowledge, but so far hardly any of them have been investigated from a neurolinguistic perspective.