Dual coding and the classroom
Despite its long history, visual imagery has at times been neglected in the study of memory and learning. Behaviourist psychologist J. B. Watson considered it to be unimportant, with mental images seen as mere shadows or 'ghosts' of the verbal behaviour involved in language (Paivio, 1969), and visual processing was absent from the classic ‘modal’ model of short-term and long-term memory. However, from the late 1960s onwards, researchers such as Gordon Bower and Allan Paivio began to promote its use as a study strategy, and explore the psychology behind it.
Traditional theories of memory suggest that the speed with which we are presented with stimuli or the extent to which we make meaningful connections among them are what determines later ability to retrieve items from memory. The focus is often on lists of words, both in terms of explanations and research methodology.
Dual coding theory (DCT) sees this view as over-simplistic, instead arguing that the modality via which information is processed is of lasting significance to whether it is remembered or not. It suggests that when objects and events are encoded to memory, the mind processes two main types of stimuli - verbal and non-verbal (the latter including both images and other non-verbal stimuli such as sounds and movements).
These are then separately represented in the mind as two main types of mental structure - verbal items which Paivio and colleagues label 'logogens', and visual-based items which they label 'imagens' (Paivio, 1986). There are also mental links or 'referential connections' between these.
A number of studies have suggested that images can be better remembered than words, a phenomenon known as the picture superiority effect. Bower (1972) was one of the first to show that a dynamic visual image including two images can be better remembered than either item independently. A range of studies have found that items presented as pictures tend to be better remembered than the same items presented as words, while Paivio and Csapo (1973) found that the picture superiority effect is due in part to the superiority of the mental image over the word in terms of its rich meaningful content.
However, DCT does more than just encourage the use of pictures. It suggests that both pictures and words, when encoded together, will create a stronger memory trace, including two distinct types of representation. When viewing an image, people spontaneously think of the word, thus gaining the benefit of dual coding in most picture superiority experiments. In addition, it will be easier to retrieve the item from memory, as either the logogen or the imagen could provide sufficient information to trigger retrieval of an episodic memory.
There may be more aspects to a memory trace than the two described by Paivio and colleagues. A study by Wammes et al. (2016) looked specifically at the role of drawing as a mnemonic strategy. Their primary methodology was to read out list of words and ask participants to either write or draw the item named. In a series of experiments they found that drawing (rather than written note taking) had a powerful mnemonic effect, and one that cannot be explained purely by picture superiority or by other obvious factors such as more meaningful processing. They suggested that drawing promotes the integration of the verbal meaning of a memory trace with its visual and motor representations. This has become known as the 'drawing effect'.
Links with other theories
Although it is more concerned with long-term memory than with working memory, DCT seems to fit well with the work of Baddeley and colleagues, who suggest that in short term processing, too, we have distinct storage and processing systems for visual and verbal material. More recently, a form of episodic working memory system was later added to their theory. These are thought to link to the corresponding long-term stores of visual and verbal semantic long-term and to episodic long-term memory, respectively (e.g. Baddeley, 2000).
Biologically, the hippocampus appears to play an important role in long-term memory for visualisation and route planning, and may therefore be involved in the way the brain represents imagens. Maguire et al. (2006) found that this brain area was larger in volume among London taxi drivers than in a comparable group of bus drivers, and the taxi drivers also did better on a visual test of local landmarks. However, it was not all good news for the taxi-drivers - they performed worse on a test involving visualisation of novel items. This suggests that while our visual abilities may be able to adapt to circumstances due to neuroplasticity, the resulting specialisation may have a cost in terms of the ability to perform new tasks in the same domain.
As a technique for boosting memory, DCT has broad potential applications in a range of areas, most obviously education. It suggests that learning new information via both words and images (which could include experimental apparatus or short videos, as well as pictures) will be more successful than via words alone. Diagrams in notes or on whiteboards and worksheets are also important. Given the heavily verbal nature of school and university learning, this seems to imply that more use should be made of images. Clark and Paivio (1991) note that images contain important properties and that complexities that can 'fade' when represented verbally, including movement and the interrelation of parts.
Both DCT and the drawing effect suggest that making use of multiple means of encoding and representing information leads to a better-integrated and more robust mental representation, and this may also lead to better transfer in future. This is in contrast to the widely held - but scientifically unsupported - view that learners should identify a preferred learning style (see the work of Daniel Willingham for more about why that idea has been discredited).
Baddeley, A. (2000). The episodic buffer: a new component of working memory?. Trends in Cognitive Sciences, 4(11), 417-423.
Bower, G.H. (1972). Mental imagery and associative learning. In L. Gregg (Ed.), Cognition in learning and memory, pp. 51-88. New York: Wiley.
Clark, J. M., and Paivio, A. (1991). Dual coding theory and education. Educational Psychology Review, 3(3), 149-210.
Maguire, E. A., Woollett, K., and Spiers, H. J. (2006). London taxi drivers and bus drivers: a structural MRI and neuropsychological analysis. Hippocampus, 16(12), 1091-1101.
Paivio, A. (1969). Mental imagery in associative learning and memory. Psychological Review, 76(3), 241.
Paivio, A. (1986). Mental representations: A dual-coding approach. Oxford: Oxford University Press.
Paivio, A., & Csapo, K. (1973). Picture superiority in free recall: Imagery or dual coding?. Cognitive Psychology, 5(2), 176–206.
Wammes, J. D., Meade, M. E., and Fernandes, M. A. (2016). The drawing effect: Evidence for reliable and robust memory benefits in free recall. Quarterly Journal of Experimental Psychology, 69(9), 1752-1776.