
As a boy, he played the autistic character Tommy Westfall on the TV series "St. Elsewhere" (1982). At first a minor character, Tommy took on great significance in the final episode when the entire series was revealed to have taken place inside his head. This has given rise to a fan theory known as the "Tommy Westphall Universe Hypothesis," which states that numerous shows that are connected to St. Elsewhere through character crossovers or dialog references have also taken place in Tommy's mind. For instance, the show had a crossover with "Cheers" (1982), and several characters from "St. Elsewhere" were featured in "Homicide: Life on the Street" (1993). From those links, the program can be linked to at least 280 other non-animated shows, ranging from "I Love Lucy" (1951) to "CSI: NY" (2004) (as of 2007).
(imdb.com)