Cognition: Memory, Attention, and Learning - Foundations for Designing User-Centered Systems - page 121

an image left when you close your eyes or after an image you have been looking at
has disappeared. This is called iconic memory.
Visual iconic memory holds only a few items. Some suggest a limit of two or
three items (Zhang and Simon 1985 ); others suggest a few more. These items also
decay (disappear) at a fairly fast rate. Sperling ( 1961 ) had subjects view a screen of
numbers and then asked them to retrieve items after the items were no longer
displayed. He determined that items exist in a temporary memory store for about
500 ms, with an exponential decay rate. If items are not processed, about half of
the records of these items disappear in each half second.
Items can be put into short-term or long-term memory by processing them.
However, as this takes time, the other items can decay and be lost from iconic
memory. Thus items that appear on an interface for a short time have to be noticed
and processed to be remembered.
5.2.1.2 Short-Term Memory
Short-term memory (STM) is a temporary memory store. Work by Atkinson and
Shiffrin ( 1968 ) helped establish STM as a common concept in memory research. It
can be considered analogous to the registers in a computer. Cognition writes
information into short-term memory but, unlike the computer, the contents decay
with time. You might start out across the room knowing the phone number you
want to dial, or start to bring up a browser to type in a URL, but by the time you
get there, physically or metaphorically, you may have forgotten the number or
URL. This type of loss is one of the first ways that people get introduced to short-
term memory.
George Miller, in a famous study (Miller 1956 ) found that, for unrelated
objects, users could remember around seven meaningful items (plus or minus two).
The estimate of the rate of loss of these items varies somewhat based on who is
studied and what they are trying to remember. Some authors have found that half
the information disappears in about 5 s if it is not rehearsed (practiced).
Short-term memory is often used to store lists or sets of items to work with.
There are several interesting and immediate effects of memory of lists that are
worth knowing about. The first effect, called primacy, is that items that appear at
the start of a list are more easily retrieved from memory.
The second is that distinctive items in a list are better retrieved (the Von
Restorff effect). For example, if they are printed in red ink or a bell goes off when
you read them, or they are in some other way distinct or important, they will be
better retrieved. The improvement in memorability requires distinctiveness—
highlighting a whole text and putting a box around it does not help because
nothing stands out. Similarly, writing everything in red does not help, but writing
only one word in ten in red will help. This effect is also illustrated later in this
chapter by discussing Nepal and encoding.
The third is that items in a list that make more sense, IBM, PDQ and XYZ, are
better retrieved than items that do not have associations for everybody, such as