Short-term memory is the capacity for holding a small amount of information in an active, readily available state for a brief period of time. It is separate from our long-term memory, where lots of information is stored for us to recall at a later time. Unlike sensory memory, it is capable of temporary storage. How long this storage lasts depends on conscious effort from the individual; without rehearsal or active maintenance, the duration of short-term memory is believed to be on the order of seconds.
Capacity of Short-Term Memory
Short-term memory acts as a scratchpad for temporary recall of information. For instance, in order to understand this sentence you need to hold in your mind the beginning of the sentence as you read the rest. Short-term memory decays rapidly and has a limited capacity.
The psychologist George Miller suggested that human short-term memory has a forward memory span of approximately seven items plus or minus two. More recent research has shown that this number is roughly accurate for college students recalling lists of digits, but memory span varies widely with populations tested and with material used.
For example, the ability to recall words in order depends on a number of characteristics of these words: fewer words can be recalled when the words have longer spoken duration (this is known as the word-length effect) or when their speech sounds are similar to each other (this is called the phonological similarity effect). More words can be recalled when the words are highly familiar or occur frequently in the language. Chunking of information can also lead to an increase in short-term memory capacity. For example, it is easier to remember a hyphenated phone number than a single long number because it is broken into three chunks instead of existing as ten digits.
Rehearsal is the process in which information is kept in short-term memory by mentally repeating it. When the information is repeated each time, that information is re-entered into the short-term memory, thus keeping that information for another 10 to 20 seconds, the average storage time for short-term memory. Distractions from rehearsal often cause disturbances in short-term memory retention. This accounts for the desire to complete a task held in short-term memory as soon as possible.
Working Memory
Though the term "working memory" is often used synonymously with "short-term memory," working memory is related to but actually distinct from short-term memory. It holds temporary data in the mind where it can be manipulated. Baddeley and Hitch's 1974 model of working memory is the most commonly accepted theory of working memory today. According to Baddeley, working memory has a phonological loop to preserve verbal data, a visuospatial scratchpad to control visual data, and a central executive to disperse attention between them.
Phonological Loop
The phonological loop is responsible for dealing with auditory and verbal information, such as phone numbers, people's names, or general understanding of what other people are talking about. We could roughly say that it is a system specialized for language. It consists of two parts: a short-term phonological store with auditory memory traces that are subject to rapid decay, and an articulatory loop that can revive these memory traces. The phonological store can only store sounds for about two seconds without rehearsal, but the auditory loop can "replay them" internally to keep them in working memory. The repetition of information deepens the memory.
Visuospatial Sketchpad
Visual and spatial information is handled in the visuospatial sketchpad. This means that information about the position and properties of objects can be stored. The phonological loop and visuospatial sketchpad are semi-independent systems; because of this, you can increase the amount you can remember by engaging both systems at once. For instance, you might be better able to remember an entire phone number if you visualize part of it (using the visuospatial sketchpad) and then say the rest of it out loud (using the phonological loop).
Central Executive
The central executive connects the phonological loop and the visuospatial sketchpad and coordinates their activities. It also links the working memory to the long-term memory, controls the storage of long-term memory, and manages memory retrieval from storage. The process of storage is influenced by the duration in which information is held in working memory and the amount that the information is manipulated. Information is stored for a longer time if it is semantically interpreted and viewed with relation to other information already stored in long-term memory.
Transport to Long-Term Memory
The process of transferring information from short-term to long-term memory involves encoding and consolidation of information. This is a function of time; that is, the longer the memory stays in the short-term memory the more likely it is to be placed in the long-term memory. In this process, the meaningfulness or emotional content of an item may play a greater role in its retention in the long-term memory.
This greater retention is owed to an enhanced synaptic response within the hippocampus, which is essential for memory storage. The limbic system of the brain (including the hippocampus and amygdala) is not necessarily directly involved in long-term memory, but it selects particular information from short-term memory and consolidates these memories by playing them like a continuous tape.