interpreting their findings. Perhaps the dictation task was performed rather
automatically, and so placed few demands on cognitive capacity, or there might
have been a rapid alternation of attention between reading and writing. Hirst
et al. (1980) claimed that writing to dictation was not done automatically be-
cause the subjects understood what they were writing. They also claimed that
reading and dictation could only be performed together with success by the
strategy of alternation of attention if the reading material were simple and
highly redundant. However, they discovered that most subjects were still able
to read and take dictation effectively when less redundant reading matter was
used.
It is sometimes claimed that the studies by Spelke et al. (1976) and by Hirst
et al. (1980) demonstrate that two complex tasks can be performed together
without disruption, but this is not so. One of the subjects used by Hirst et al.
was tested at dictation without reading, and made fewer than half the number
of errors that occurred when reading at the same time. Furthermore, the read-
ing task gave the subjects much flexibility in terms of when they attended to
the reading matter, and such flexibility means that there may well have some
alternation of attention between tasks.
There are other cases of apparently successful performance of two complex
tasks, but the requisite skills were always highly practised. Expert pianists can
play from seen music while repeating back or shadowing heard speech (Allport
et al., 1972), and an expert typist can type and shadow at the same time
(Shaffer, 1975). These studies are often regarded as providing evidence of com-
pletely successful task combination, but there are signs of interference when the
data are inspected closely (Broadbent, 1982).
There are several reasons why practice might facilitate dual-task performance.
First, subjects may develop new strategies for performing each of the tasks so
as to minimise task interference. Second, the demands that a task makes on
attentional or other central resources may be reduced as a function of practice.
Third, although a task initially requires the use of several specific processing
resources, practice may permit a more economical mode of functioning relying
on fewer resources. These possibilities are considered in more detail a little
later in the chapter.
Task Difficulty The ability to perform two tasks together undoubtedly depends
on their difficulty, but there are several ways in which one task can be more
difficult than another one. However, there are several studies showing the
expected pattern of results. For example, Sullivan (1976) gave her subjects the
two tasks of shadowing an auditory message and detecting target words on
a non-shadowed message. When the shadowing task was made more difficult
by using a less redundant message, fewer targets were detected on the non-
shadowed message.
It has sometimes been assumed that the demands for resources of two tasks
when performed together equal the sum of the demands of the two tasks when
performed separately. However, the necessity to perform two tasks together
often introduces fresh demands of co-ordination and avoidance of interference.
Duncan (1979) asked his subjects to respond to closely successive stimuli, one
requiring a left-hand response and the other a right-hand response. The rela-
tionship between each stimulus and response was either corresponding (i.e.
380 Michael W. Eysenck and Mark T. Keane