which many functionally differentiated structures participate, rather than a
function controlled uniquely by a single centre.’’
Divided Attention
What happens when people try to do two things at once? The answer obviously
depends on the nature of the two ‘‘things.’’ Sometimes the attempt is successful,
as when an experienced motorist drives a car and holds a conversation at the
same time, or a tennis player notes the position of his or her opponent while
running at speed and preparing to make a stroke. At other times, as when
someone tries to rub their stomach with one hand while patting their head with
the other, there can be a complete disruption of performance. In this section of
the chapter, we will be concerned with some of the factors determining how
well two tasks can be performed concurrently (i.e. at the same time).
Hampson (1989) made the important point that focused and divided atten-
tionaremoresimilarinsomewaysthanonemighthaveimagined.Factors
such as use of different modalities which facilitate focused or selective atten-
tion generally also make divided attention easier. According to Hampson (1989,
p. 267), the reason for this is that ‘‘anything which minimises interference be-
tween processes, or keeps them ‘further apart’ will allow them to be dealt with
more readily either selectively or together.’’
At a more theoretical level, the breakdowns of performance often found
when two tasks are combined shed light on the limitations of the human infor-
mation-processing system. It has been assumed by many theorists that such
breakdowns reflect the limited capacity of a single multi-purpose central pro-
cessor or executive that is sometimes simply referred to as ‘‘attention.’’ Other
theorists are more impressed by our apparent ability to perform two relatively
complex tasks at the same time without disruption or interference. Such theo-
rists tend to favour the notion of several specific processing resources, arguing
that there will be no interference between two tasks provided that they make
use of different processing resources.
More progress has been made at the empirical level than at the theoretical
level. It is possible to predict reasonably accurately whether or not two tasks
can be combined successfully, but the accounts offered by different theorists are
very diverse. Accordingly, we will make a start by discussing some of the fac-
tual evidence before moving on to the murkier issue of how the data are to be
explained.
Factors Determining Dual-Task Performance
Task Similarity When we think of pairs of activities that are performed well
together in everyday life, the examples that come to mind usually involve two
rather dissimilar activities (e.g. driving and talking; reading and listening to
music). There is much evidence that the degree of similarity between two tasks
is of great importance. As we saw earlier in the chapter, when people attempt
to shadow or repeat back prose passages while learning auditorily presented
words, their subsequent recognition-memory performance for the words is at
chance level (Allport et al., 1972). However, the same authors found that mem-
ory was excellent when the to-be-remembered material consisted of pictures.
378 Michael W. Eysenck and Mark T. Keane