The working memory model of Baddeley and Hitch^ subdivides memory into three main types depending both on time-based and conceptual differences. The first system, sensory memory, is a brief and rather literal trace that results from a visual, auditory, or other sensory event, probably lasting no longer than a quarter of a second. This is the system we use to make sense of moving pictures (visual sensory memory) or language (auditory sensory memory). Most people with damage to this system would present with perceptual or language disorders and we would not normally think of them as having memory problems.
The second system, working memory, is considered to have two main components or functions. The first of these is short-term or immediate memory, which lasts for several seconds. This period of time can be extended to several minutes if the person is rehearsing or concentrating on the particular information. Unlike sensory memory, information in working memory has already undergone substantial cognitive analysis, so it is typically represented in meaningful chunks such as words or numbers. We use this system when looking up a new telephone number and holding on to it long enough to dial.
The second component of working memory is a central executive that can be conceived of as an organizer, controller, or allocator of resources. This component enables us to both drive a car and talk to our passenger at the same time. Sufficient resources are allocated to each of these tasks, and if a demanding or unusual situation occurs on the road we stop talking while all our resources are required to deal with the unexpected situation.
The third system in the Baddeley and Hitch model(1) is long-term memory, which encodes information in a reasonably robust form and can last for decades. Although there are differences in memory for things that happened 10 minutes ago and things that happened 10 years ago, the differences are less clear-cut than those between sensory (quarter of a second) and immediate (a few seconds) memory systems. Nevertheless, because long-term memory means different things to different people, the following terms can be used to reduce ambiguity:
1. Delayed memory refers to memory for information presented in the last few minutes.
2. Recent memory refers to knowledge accumulated in the last few days or weeks.
3. Remote memory refers to knowledge accumulated over several years.
All the systems described so far are connected with retrospective memory, that is remembering information or events that have already occurred. Frequently, however, we want to remember to do something in the future, such as take our medicine, water the plants, or make a telephone call. The system activated for remembering to do something is known as prospective memory. It is significant that many of the complaints of memory-impaired people refer to failures in prospective memory.
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