An important goal of research in language comprehension is to specify the nature of listeners' and readers' mental representations and the processes involved in constructing them. Psycholinguists study comprehension at three levels. At the word level, processes are necessary to encode the spoken or printed word and access its meaning in memory. At the sentence level, processes are devoted to the formation of structures that specify the syntactic and conceptual relations among words in a sentence. These processes help to encode propositions, abstract units that represent meaning. At the discourse level, processes form connections among successive propositions in discourse. These processes are involved in establishing referential connections, the knowledge that two text elements refer to the same entity, and coherence relations, the knowledge that ideas are related in terms of the situation or context to which a text refers.