the target and the distractors is a factor influencing visual search time. They
claimed (with supporting evidence) that visual search for an object target
defined by more than one feature is typically limited to those distractors pos-
sessing at least one of the features of the target. For example, if you were look-
ing for a blue circle in a display containing blue triangles, red circles, and red
triangles, then you would ignore red triangles. This contrasts with the views of
Treisman and Gelade (1980), who argued that none of the stimuli would be
ignoredinsuchcircumstances.
Attentional Engagement Theory Duncan and Humphreys (1989, 1992) have
proposed an attentional engagement theory of visual attention. They assumed
that the time taken to detect a target in a visual display depends on two major
factors:
- Search times will be slower when the similarity between the target and
the non-targets is increased. - Search times will be slower when there is reduced similarity among
non-targets. Thus, the slowest search times are obtained when non-targets
are dissimilar to each other, but similar to the target.
Some evidence that visual search can be very rapid when the non-targets are
all the same was obtained by Humphreys, Riddoch, and Quinlan (1985). Sub-
jects were asked to detect a target of an inverted T against a background of
Figure 15.6
Performance speed on a detection task as a function of target definition (conjunctive vs. single fea-
ture) and display size. Adapted from Treisman and Gelade (1980).
374 Michael W. Eysenck and Mark T. Keane