This chapter is the first of four in which we introduce some of the ‘foundations’ of
HCI. We start with the human, the central character in any discussion of interactive
systems. The human, the user, is, after all, the one whom computer systems are designed
to assist. The requirements of the user should therefore be our first priority.
In this chapter we will look at areas of human psychology coming under the general
banner of cognitive psychology. This may seem a far cry from designing and building
interactive computer systems, but it is not. In order to design something for someone,
we need to understand their capabilities and limitations. We need to know if
there are things that they will find difficult or, even, impossible. It will also help us to
know what people find easy and how we can help them by encouraging these things.
We will look at aspects of cognitive psychology which have a bearing on the use of computer
systems: how humans perceive the world around them, how they store and
process information and solve problems, and how they physically manipulate objects.
We have already said that we will restrict our study to those things that are relevant
to HCI. One way to structure this discussion is to think of the user in a way that
highlights these aspects. In other words, to think of a simplified model of what is
actually going on. Many models have been proposed and it useful to consider one of
the most influential in passing, to understand the context of the discussion that is to
follow. In 1983, Card, Moran and Newell [56] described the Model Human Processor,
which is a simplified view of the human processing involved in interacting with
computer systems. The model comprises three subsystems: the perceptual system,
handling sensory stimulus from the outside world, the motor system, which controls
actions, and the cognitive system, which provides the processing needed to connect
the two. Each of these subsystems has its own processor and memory, although
obviously the complexity of these varies depending on the complexity of the tasks
the subsystem has to perform. The model also includes a number of principles of
operation which dictate the behavior of the systems under certain conditions.
We will use the analogy of the user as an information processing system, but in
our model make the analogy closer to that of a conventional computer system.
Information comes in, is stored and processed, and information is passed out. We
will therefore discuss three components of this system: input–output, memory and
processing. In the human, we are dealing with an intelligent information-processing
system, and processing therefore includes problem solving, learning, and, consequently,
making mistakes. This model is obviously a simplification of the real
situation, since memory and processing are required at all levels, as we have seen in
the Model Human Processor. However, it is convenient as a way of grasping how
information is handled by the human system. The human, unlike the computer, is
also influenced by external factors such as the social and organizational environment,
and we need to be aware of these influences as well. We will ignore such
factors for now and concentrate on the human’s information processing capabilities
only. We will return to social and organizational influences in Chapter 3 and, in
more detail, in Chapter 13.