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Dual coding is a theory of cognition introduced by Allan Pavio in late 1960s. This theory suggests two distinct subsystems contributing to cognition: one is specialized for language and verbal information, and the other for images and non-verbal information.
These two kinds of processing systems result in two types of stored representations:
Links between verbal and non-verbal representations are called referential connections and connections within each of those two systems are called associative connections. Referential connections enable performing operations like imaging to words and namings to pictures or images to words. For example, associations of an image of a school building or an unpleasant feeling (both non-verbal entities) elicited by the word school (a verbal entity). Associative connections on the other hand enable forming verbal-verbal or non-verbal-non-verbal associations. For example, the word school can elicit verbal entities blackboard, or boredom.
Both types of connections help forming the complex networks of human memory.
Paivio, Allan. Imagery and verbal processes. Holt, Rinehart and Winston. New York, 1971.