Dual-coding theory, a theory of cognition, was hypothesized by Allan Paivio of the University of Western Ontario in 1971. In developing this theory, Paivio used the idea that the formation of mental images aids learning. According to Paivio, there are two ways a person could expand on learned material: verbal associations and imagery. Dual-coding theory postulates that both sensory imagery and verbal information is used to represent information. Imagery and verbal information are processed differently and along distinct channels in the human mind, creating separate representations for information processed in each channel. The mental codes corresponding to these representations are used to organize incoming information that can be acted upon, stored, and retrieved for subsequent use. Both imagery and verbal codes can be used when recalling information. For example, say a person has stored the stimulus concept "dog" as both the word 'dog' and as the image (appearance, sound, smell, and other sensory information) of a dog. When asked to recall the stimulus, the person can retrieve either the word or the image individually, or both simultaneously. If the word is recalled, the image of the dog is not lost and can still be retrieved at a later point in time. The ability to code a stimulus two different ways increases the chance of remembering that item compared to if the stimulus was only coded one way. There has been controversy to the limitations of the dual-coding theory. Dual-coding theory does not take into account the possibility of cognition being mediated by something other than words and images. Not enough research has been done to determine if words and images are the only way we remember items, and the theory would not hold true if another form of codes were discovered. Another limitation of the dual-coding theory is that it is only valid for tests on which people are asked to focus on identifying how concepts are related. If associations between a word and an image cannot be formed, it is much harder to remember and recall the word at a later point in time. While this limits the effectiveness of the dual-coding theory, it is still valid over a wide range of circumstances and can be used to improve memory. Types of codes Analogue codes are used to mentally represent images. Analogue codes retain the main perceptual features of whatever is being represented, so the images we form in