Covert Attention: Directing Attention Without Eye Movements • 103(Marino & Scholl, 2005). Although experiments
have shown that the spotlight idea is a useful way
to think about the way attention enhances process-
ing, directing covert attention is more complicated
than this. One complication becomes apparent
when considering what happens when attention is
directed to specifi c objects.OBJECT-BASED ATTENTION
Studies of attention using the precueing proce-
dure consider how people move their attention
from one location to another. But other experi-
ments have used precueing to show that atten-
tion can also be associated with specifi c objects.
Experiments studying object-based attention have
shown that when attention is directed to one place
on an object, the enhancing effect of this attention
spreads throughout the object.
Consider, for example, the experiment dia-
grammed in ● Figure 4.29 (Egly et al., 1994). As
participants were instructed to keep their eyes on the +, one
end of the rectangle was briefl y highlighted (Figure 4.29a).
This was the cue signal that indicated where a target, a
dark square (Figure 4.29b), would probably appear. In this
example, the cue indicates that the target is likely to appear
in the upper part of the right rectangle. The participants’
task was to press a button when the target appeared any-
where in the display (Figure 4.29b). Reaction times were
fastest when the target appeared where the cue signal pre-
dicted it would appear (at A in this example; note that
the letters were not present in the actual experiment) and
slower at other locations. However, the most important
result of this experiment is that participants responded
faster when the target appeared within the same rectan-
gular object at location B than when it appeared in the
other object at location C. Note that B and C are the same
distance from A. However, participants respond more
rapidly when the target was presented at B, which is in
the same object as A. Apparently, the enhancing effect of
attention had spread within the rectangle on the right, so
even though the cue was at A, some enhancement occurred
at B as well. This result is called same-object advantage
(Marino & Scholl, 2005).
We have seen that attention can be based both on
where a person is looking in the environment (location-
based attention) and on where a person is looking on a
specifi c object (object-based attention). We can think of
these two modes of visual attention as involving two dif-
ferent mechanisms that operate under different conditions.
For static scenes or scenes that contain few objects, loca-
tion-based visual attention can be likened to a spotlight
that scans different locations (● Figure 4.30a). In dynamic
environments, object-based visual attention can involve
a mechanism that locks onto objects and follows them
as they move (Figure 4.30b; Behrmann & Tipper, 1999;
Luck & Vecera, 2002). Recent physiological evidence has● FIGURE 4.29 Stimuli for Egly et al.’s (1994) object-based attention
experiment. (a) The cue signal, darkened lines, appears at the top or bottom
of one of the rectangles to indicate where the target will probably appear.
The letters were not present in the display viewed by participants. (b) The
target, a darkened square, appears at one end of one of the rectangles.
Numbers indicate how long it took, in milliseconds, to respond to targets
presented at positions A, B, and C when the cue had appeared at position A.
CueCA 374B324358Present cue............. Cue off............ Present target++(b)● FIGURE 4.30 (a) Location-based attention can be compared
to a spotlight that scans a scene. (b) Object-based attention
involves focusing attention on specifi c objects. These objects can
be stationary or moving.
Spotlight(a) Location-based
AttentionAttention(b) Object-based
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