there are several theories as to how mental images are formed in the mind. these include the dual-code theory, the propositional theory, and the functional-equivalency hypothesis. the dual-code theory, created by allan paivio in 1971, is the theory that we use two separate codes to represent information in our brains: image codes and verbal codes. image codes are things like thinking of a picture of a dog when you are thinking of a dog, whereas a verbal code would be to think of the word "dog". another example is the difference between thinking of abstract words such as "justice" or "love" and thinking of concrete words like "elephant" or "chair." when abstract words are thought of, it is easier to think of them in terms of verbal codes—finding words that define them or describe them. with concrete words, it is often easier to use image codes and bring up a picture of a "human" or "chair" in your mind rather than words associated or descriptive of them.