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96 Chapter 3 Understanding users

3.4.2 information processing
Another approach to conceptualizing how the mind works has been to use
metaphors and analogies (see also Chapter 2). A number of comparisons have
been made, including conceptualizing the mind as a reservoir, a telephone net-
work, and a digital computer. One prevalent metaphor from cognitive psychology
is the idea that the mind is an information processor. Information is thought to
enter and exit the mind through a series of ordered processing stages (see Figure
3.11). Within these stages, various processes are assumed to act upon mental rep-
resentations. Processes include comparing and matching. Mental representations
are assumed to comprise images, mental models, rules, and other forms of knowl-
edge.
The information processing model provides a basis from which to make predic-
tions about human performance. Hypotheses can be made about how long some-
one will take to perceive and respond to a stimulus (also known as reaction time)
and what bottlenecks occur if a person is overloaded with too much information.
The best known approach is the human processor model, which models the cogni-
tive processes of a user interacting with a computer (Card et al., 1983). Based on
the information processing model, cognition is conceptualized as a series of pro-
cessing stages, where perceptual, cognitive, and motor processors are organized in
relation to one another (see Figure 3.12). The model predicts which cognitive
processes are involved when a user interacts with a computer, enabling calculations
to be made of how long a user will take to carry out various tasks. This can be very
useful when comparing different interfaces. For example, it has been used to com-
pare how well different word processors support a range of editing tasks.
The information processing approach is based on modeling mental activities
that happen exclusively inside the head. However, most cognitive activities involve
people interacting with external kinds of representations, like books, documents,
and computers-not to mention one another. For example, when we go home from
wherever we have been we do not need to remember the details of the route be-
cause we rely on cues in the environment (e.g., we know to turn left at the red
house, right when the road comes to a T-junction, and so on). Similarly, when we
are at home we do not have to remember where everything is because information
is "out there." We decide what to eat and drink by scanning the items in the fridge,
find out whether any messages have been left by glancing at the answering machine
to see if there is a flashing light, and so on. To what extent, therefore, can we say
that information processing models are truly representative of everyday cognitive
activities? Do they adequately account for cognition as it happens in the real world
and, specifically, how people interact with computers and other interactive devices?

Input output
or or
stimuli response

Figure 3.1 1 Human information processing model.