Sensory memory

The sense organs have a limited ability to store information about the world in a fairly unprocessed way for less than a second. The visual system possesses iconic memory for visual stimuli such as shape, size, colour and location (but not meaning), whereas the hearing system has echoic memory for auditory stimuli. Coltheart et al. (1974) have argued that the momentary freezing of visual input allows us to select which aspects of the input should go on for further memory processing. In this visual modality, information that enters the sensory store will be eliminated from the store within several hundred milliseconds (Baddeley, 1966). The existence of sensory memory has been experimentally demonstrated by Sperling (1960) using a tachistoscope.
Short-term memory

Information is retained acoustically and visually long enough to use it, e.g. looking up a telephone number and remembering it long enough to dial it. Peterson and Peterson (1959) have demonstrated that STM last approximately between 15 and 30 seconds, unless people rehearse the material, while Miller (1956) has found that STM has a limited capacity of around 7+ or −2 ‘chunks’ of information. STM also appears to mostly encode memory acoustically (in terms of sound) as Baddeley (1966) has demonstrated, but can also retain visuospatial images. However in many cases STM can be at a semantic level. In addition to using the short-term store (STS) as the primary memory device when desired for certain tasks, such as remembering a telephone number after looking it up, the STS fulfills various other functions. Instead of the memory system having to pay moment-to-moment attention to the environment to account for all environmental changes, the (STS) serves as a buffer and separates the environment from the memory system. It also functions as a working memory in which alterations of information can occur. However, these manipulations are only temporary (Baddeley, 1966).

Long-term memory

LTM provides the lasting retention of information, from minutes to a lifetime. Long term memory appears to have an almost limitless capacity to retain information, but it could never be measured as it would take too long. LT information seems to be encoded mainly in terms of meaning (semantic memory) as Baddeley has shown, but also retains procedural skills and imagery.

Memory may also be transported directly from sensory memory to LTM if it receives instant attention, e.g. witnessing a fire in your house. This is known as a "Flashbulb Memory". Another example of this is the fact that most people living in the United States at the time can recall what they were doing on the day of September 11, 2001, as it was the day of an extreme event. Recent research has shown, however, that "flashbulb memory" is not as reliable as it was once thought to be.[citation needed]

Also if information in the LTM is not rehearsed it can be forgotten through trace decay