Earlier in the chapter we considered various studies (e.g. Hirst et al., 1980;
Spelke et al., 1976) in which two complex tasks were performed remarkably
well together. Such findings make it difficult to argue for the existence of a
bottleneck in processing. However, as Pashler (1993) pointed out, studies of the
psychological refractory period effect have the considerable advantage that
there is very precise assessment of the time taken to respond to any given stim-
ulus. The coarse-grained measures obtained in studies such as those of Spelke
et al. (1976) and Hirst et al. (1980) may simply be too insensitive to permit de-
tection of bottlenecks.
Even if there is a bottleneck that disrupts dual-task performance, it is clearly
not the only relevant factor. Accordingly, we now turn to theoretical accounts
that consider other factors such as the effects of practice and similarity.
Central Capacity Theories An apparently straightforward way of accounting
for many of the dual-task findings is to assume there is some central capacity
which can be used flexibly across a wide range of activities (e.g. Johnston &
Heinz, 1978). This central processor possesses strictly limited resources, and is
sometimes known as attention or effort. The extent to which two tasks can be
performed together depends on the demands that each task makes on those
resources. If the combined demands of the two tasks do not exceed the total
resources of the central capacity, then the two tasks will not interfere with each
other. However, if the resources are insufficient to meet the demands placed on
them by the two tasks, then performance disruption is inevitable.
According to central capacity theories, the crucial determinant of dual-task
performance is the difficulty level of the two tasks, with difficulty being defined
intermsofthedemandsplacedontheresourcesofthecentralcapacity.How-
ever, the effects of task difficulty are often swamped by those of similarity
between the tasks. For example, Segal and Fusella (1970) combined image con-
struction (visual or auditory) with signal detection (visual or auditory). As can
be seen in figure 15.8, the auditory image task impaired detection of auditory
signals more than the visual task did, suggesting to central capacity theorists
that the auditory image task is more demanding than the visual image task.
However, the auditory image task was less disruptive than the visual image
Figure 15.7
Response times to the first and second stimuli as a function of time between the onset of the stimuli
(stimulus–onset asynchrony) and whether or not the order of the stimuli was known beforehand.
Adapted from Pashler (1990).
382 Michael W. Eysenck and Mark T. Keane