memory, short-term memory (STM), and long-term memory (LTM) (see Figure 11.1).
External events from our senses are held in our sensory memoryjust long enough to be
perceived. In sensory memory, visual or iconic memorythat completely represents a visual
stimulus lasts for less than a second, just long enough to ensure that we don’t see gaps between
frames in a motion picture. Auditory or echoic memorylasts for about 4 seconds, just long
enough for us to hear a flow of information. Most information in sensory memory is lost.
Our selective attention,focusing of awareness on a specific stimulus in sensory memory,
determines which very small fraction of information perceived in sensory memory is encoded
into short-term memory. Encoding can be processed automatically or require our effort.
Automatic processingis unconscious encoding of information about space, time, and
frequency that occurs without interfering with our thinking about other things. This is
an example of parallel processing,a natural mode of information processing that involves
several information streams simultaneously. Effortful processingis encoding that requires
our focused attention and conscious effort.
Short-Term Memory
Short-term memory(STM) can hold a limited amount of information for about 30 sec-
onds unless it is processed further. Experiments by George Miller demonstrated that the
capacity of STM is approximately seven (plus or minus two) unrelated bits of information
at one time. STM lasts just long enough for us to input a seven-digit phone number after
looking it up in a telephone directory. Then the number disappears from our memory. How
can we get around these limitations of STM? We can hold our memory longer in STM if
we rehearsethe new information, consciously repeat it. The more time we spend learning
new information, the more we retain of it. Even after we’ve learned information, more
rehearsal increases our retention. The additional rehearsal is called overlearning. While
rehearsal is usually verbal, it can be visual or spatial. People with a photographic or eidetic
memory can “see” an image of something they are no longer looking at. We can increase
the capacity of STM by chunking,grouping information into meaningful units. A chunk
can be a word rather than individual letters, or a date rather than individual numbers,
for example.
Although working memory is often used as a synonym for STM, Alan Baddeley’s work-
ing memory model involves much more than chunking, rehearsal, and passive storage of
Cognition 129Figure 11.1 Atkinson–Shiffrin three-stage model of memory.