He has did research in North American Indian languages specifically in those geographically centered around California. His contributions have been wide ranging including those to historical linguistics, computational linguistics, and the theory of linguistic structure. His work led to innovative designs of associative memory hardware for microcomputers. He is best known as the father of the relational network theory of language, which is also known as as 'stratificational theory'. Near the turn of the millennium, he has been developing the theory further and exploring its possible relationships to neurological structures and to thinking processes. In 1999, he published a book, Pathways of the Brain: The Neurocognitive Basis of Language expressing some these ideas.