http://io9.com/5847567/why-would-you-want-someone-to-drill-a-hole-in-your-skull
The listener often perceives a similarity between humorous comments by schizophrenic patients and linguistic word plays, especially puns. A structural analysis of specific examples illustrates the basic difference. In 'good' as well as 'bad' word plays the humorous effect stems from the surprising similarity of word pairs (homonyms and homophones), while the seemingly humorous comments of the schizophrenic patient are rooted in an alteration of the semantic field of the word(s) employed. In spite of certain similarities in superficial structure there is a basic and specific difference in the semantic structure of these two forms of word play.