Visual Working Memory

To understand visual working memory, it is helpful to consider human memory more generally. Human memory is not a single mental faculty or cognitive system; rather, the cognitive processes that we [Page 766]collectively call memory are composed of a number of independent and specialized cognitive systems that encode and store information in different formats. The various taxonomies of human memory agree on two main distinctions: First, memory may be explicit, or declarative, in the sense that personal knowledge and previous experiences are consciously recollected or recognized, or memory may be implicit, or nondeclarative, in the sense of being expressed indirectly in behavior without accompanying conscious recollections of previous learning episodes. Second, a distinction is drawn between short-term memory, called immediate memory by early memory researchers, which has been proposed as the seat of consciousness and active processing and which is able to store limited quantities of information for limited periods of time—in the range of seconds rather than minutes—and long-term memory, which stores unlimited amounts of information for unlimited periods of time. These distinctions are not completely orthogonal, since short-term memory is coupled to explicit memory.

The theory of working memory was proposed 35 years ago by Alan Baddeley and Graham Hitch and later developed in a very influential book by Alan Baddeley, Working Memory, published in 1986. According to this theory, short-term memory is not a simple mechanism for passive storage of information; rather, it is a coordinated set of mechanisms that combines incoming information from sensory systems with information retrieved from long-term memory and consists of a central executive operating with the assistance of domain-specific verbal, and visual support systems. Broadly speaking, the concept of visual working memory refers to the short-term memory system, which stores information that enters the brain through the eye and is maintained and manipulated by the support system referred to as the visual-spatial sketchpad. The definition may be too broad, however, because visual information, such as written words or pictures of naturalistic scenes, is easily recoded and stored in verbal-memory systems. A more restricted definition of visual working memory is the maintenance and manipulation of information represented as visual codes. Since performance with complex, meaningful visual stimuli may be supported by both visual and verbal working memory systems operating in concert, much of the research in visual working memory has aimed at isolating the visual component of memory. This entry considers the scientific evidence for visual memory codes and the organization and capacity limitation of visual working memory.