Levelt, W. J. M. (1980). On-line processing constraints on the properties of signed and spoken language. In U. Bellugi, & M. Studdert-Kennedy (Eds.), Signed and spoken language: Biological constraints on linguistic form (pp. 141-160). Weinheim: Verlag Chemie.
It is argued that the dominantly successive nature of language is largely mode-independent and holds equally for sign and for spoken language. A preliminary distinction is made between what is simultaneous or successive in the signal, and what is in the process; these need not coincide, and it is the successiveness of the process that is at stake. It is then discussed extensively for the word/sign level, and in a more preliminary fashion for the clause and discourse level that online processes are parallel in that they can simultaneously draw on various sources of knowledge (syntactic, semantic, pragmatic), but successive in that they can work at the interpretation of only one unit at a time. This seems to hold for both sign and spoken language. In the final section, conjectures are made about possible evolutionary explanations for these properties of language processing.