Ts the right way up. The time taken to detect the target was scarcely affected
by the number of non-targets, presumably because they were all the same.
According to feature integration theory, the fact that the target was defined by
a combination or conjunction of features (i.e. a vertical line and a horizontal
line) means that visual search should have been slow and much affected by the
number of non-targets.
At a more explanatory level, the following assumptions are incorporated into
the attentional engagement theory:
.There is an initial parallel stage of perceptual segmentation and analysis
involving all of the visual items together..There is a subsequent stage of processing in which selected information
is entered into visual short-term memory; this corresponds to selective
attention..The speed of visual search depends on how easily the target item enters
visual short-term memory..Visual items that are well matched to the description of the target item
are most likely to be selected for visual short-term memory; thus, non-
targets that are similar to the target slow the search process..Visual items that are perceptually grouped (e.g. because they are very
similar) will tend to be selected (or rejected) together for visual short-term
memory; thus, dissimilar non-targets cannot be rejected together and this
slows the search process.
Some of the differences between this theory and Treisman’s feature integra-
tion theory can be seen if we reconsider the study by Treisman and Gelade
(1980). It will be remembered that there were long search times to detect a
green letter T in a display containing an approximately equal number of brown
Ts and green Xs (see figure 15.6), and Treisman and Gelade (1980) argued that
this occurred because of the need for focal attention to produce the necessary
conjunction of features. In contrast, Duncan and Humphreys (1989, 1992)
claimed that the slow performance resulted from the high similarity between
the target and non-target stimuli (all of the latter possessed one of the features
of the target stimulus) and the dissimilarity among the non-target stimuli (the
two different non-targets did not share any features).
Section Summary
The speed of visual search appears to depend on a number of factors. It is likely
that the similarity between target and non-targets (accepted by Duncan and
Humphreys, and by Treisman), the degree of similarity among non-targets
(emphasised by Duncan and Humphreys), and conjunction of features (em-
phasised by Treisman) all affect visual search. There are indications that the
differences between feature integration theory and attentional engagement
theory are becoming less as the theories are modified. As Treisman (1992,
p. 589) concluded: ‘‘There is substantial convergence between the respective
theories, but it still appears that conjoining features poses a special problem
that cannot be explained solely by the grouping and matching mechanisms of
Duncan and Humphreys.’’
Attention and Performance Limitations 375