All major writing systems mandate the use of commas to separate clauses and list items. However, casual writers often omit mandatory commas. Little empirical or theoretical research has been done on the effect that omitting mandatory commas has on eye movement control during reading. We present an eye-tracking experiment in Spanish, a language with a clear standard as to mandatory comma use. Sentences were presented with or without mandatory commas while readers' eye movements were recorded. There was a local increase in the go-past time for the pre-comma region when commas were presented, which was balanced out by shorter first-pass and second-pass times on the subsequent regions. In global sentence reading time, there was no evidence for an advantage of presenting commas. These findings suggest that, even when commas are mandatory, their effect is primarily to shift when processing takes place rather than to facilitate processing overall.
Keywords: Reading; eye movements; punctuation; sentence processing.