Predictability and decomposability separately contribute to compositional processing of idiomatic language.

When reading, comprehenders construct a message-level representation and integrate new information as it becomes available. Such compositional processing may differ for idioms, where the meanings of the individual words do not always relate to the figurative meaning. Here, we examined how predictability and idiom decomposability contribute to compositional processing. Participants' EEG was recorded while they read sentences containing idioms that varied in decomposability and phrase-final word cloze probability, or their literal match (break the ice/slip on the ice) with little context prior to the phrase, along with adjective insertion conditions (break the freezing ice/slip on the freezing ice). Cloze probability modulated N400 amplitudes to critical words for both idiomatic and literal phrases, whereas P600 amplitudes only elicited by idiomatic phrases were also modulated. Phrases with adjective insertions reduced acceptability judgments, particularly for idioms, and led to N400 amplitude differences compared to critical words, but only for idioms. N400 differences were also found between idiomatic and literal contexts at the point of adjective insertion. Additionally, both cloze probability and decomposability modulated gamma band activity, with greater gamma activity for more predictable and less decomposable idioms, but at different times and with different scalp topographies, suggesting dissociable components of processing. These results support a hybrid model of processing in which multiple linguistic factors determine the type of processing engaged by the brain to comprehend non-literal language. When prior context is minimal, compositional processing may still be engaged when reading idioms.