Keith Johnson is a professor of Linguistics. The main ideas that he pursues in his research center around the nature of phonetic knowledge. He is a proponent of the idea that phonetic knowledge is best thought of as inclusive of all of one's experiences as a speaker or hearer. He has implemented this conception of phonetic knowledge in exemplar-based models and find that this style of modeling - in which, no processes of abstraction remove gradient information from memory - provide a better fit to the behavior of listeners.
He has also made some basic phonetic descriptive contributions, in providing descriptions of Xhosa clicks, Cherokee tones, Tamil liquids, Korean stops, creek vowels, and the speech of gay men in Columbus, Ohio.
Ultimately, linguistic phonetics is concerned with how phonetic factors influence the sound patterns of language. To him this means that explaining sound change is the ultimate goal of linguistic phonetics. so, he's done some theoretical and modeling work on sound change.
Recently he has been helping a team of researchers at UC San Francisco conduct research in phonetic neuroscience. This work involves imaging the brain during speech production and perception with the aim of providing basic facts about the neural coding of phonetic knowledge.