task when each task was combined with a task requiring detection of visual
signals, which suggests exactly the opposite conclusion. In this study, task
similarity was clearly a much more important factor than task difficulty.
Many theorists have become so disenchanted with the notion of a central ca-
pacity or attentional system that they deny the existence of any such capacity
or system. For example, Allport (1989, p. 647) argued that the findings ‘‘point
to a multiplicity of attentional functions, dependent on a multiplicity of speci-
alised subsystems. No one of these subsystems appears uniquely ‘central.’’’
According to Allport, it is possible to ‘‘explain’’ dual-task interference by
assuming that the resources of some central capacity have been exceeded, and
to account for a lack of interference by assuming that the two tasks did not ex-
ceed those resources. However, in the absence of any independent assessment
of central processing capacity, this is more like a re-description of the findings
rather than a proper explanation.
Modular Theories The views of central capacity theorists can be compared with
those of cognitive neuropsychologists. Cognitive neuropsychologists assume
that the processing system is modular (i.e. it consists of numerous relatively
independent processors or modules). Some of the most convincing evidence for
modularity comes from the study of language in brain-damaged patients. This
has revealed, for example, that reading is a complex skill involving several
rather separate processing mechanisms. If the processing system consists of a
large number of specific processing mechanisms, then it is clear why the degree
of similarity between two tasks is so important: similar tasks compete for the
same specific processing mechanisms or modules, and thus produce interfer-
ence, whereas dissimilar tasks involve different modules, and so do not inter-
fere with each other.
Allport (1989) and others have argued that dual-task performance can be
accounted for in terms of modules or specific processing resources, but there
are significant problems with this theoretical approach. First, there is no con-
Figure 15.8
Sensitivity (d^0 ) to auditory and visual signals as a function of concurrent imager modality (auditory
vs. visual). Adapted from Segal and Fusella (1970).
Attention and Performance Limitations 383