Feature Integration Theory • 105analyzed the book into individual features such as lines with different
orientations.
To provide some perceptual evidence that objects are, in fact, ana-
lyzed into features, Anne Treisman and H. Schmidt (1982) did an inge-
nious experiment to show that early in the perceptual process, features
may exist independently of one another. Treisman and Schmidt’s display
consisted of four objects fl anked by two black numbers (● Figure 4.33).
They fl ashed this display onto a screen for one-fi fth of a second, followed
by a random-dot masking fi eld designed to eliminate any residual per-
ception that might remain after the stimuli were turned off. Participants
were told to report the black numbers fi rst and then to report what they
saw at each of the four locations where the shapes had been.
In 18 percent of the trials, participants reported seeing objects that
were made up of a combination of features from two different stimuli. For
example, after being presented with the display in Figure 4.33, in which
the small triangle was red and the small circle was green, they might
report seeing a small red circle and a small green triangle. These combi-
nations of features from different stimuli are called illusory conjunctions.
Illusory conjunctions can occur even if the stimuli differ greatly in shape and size. For
example, a small blue circle and a large green square might be seen as a large blue square
and a small green circle.
Although illusory conjunctions are usually demonstrated in laboratory experi-
ments, they can occur in other situations as well. Recently I ran a class demonstra-
tion to illustrate that observers sometimes make errors in eyewitness testimony. In the
demonstration, a male wearing a green shirt burst into the class, grabbed a yellow
purse that was sitting on a desk (the owner of the purse was in on the demonstration!)
and left the room. This event happened very rapidly and was a surprise to students in
the class. Their task was to describe what had happened as eyewitnesses to a “crime.”
Interestingly enough, one of the students reported that a male wearing a yellow shirt
grabbed a green purse from the desk! Interchanging the colors of these objects is an
example of illusory conjunctions (Treisman, 2005).
According to Treisman, illusory conjunctions occur because at the beginning of
the perceptual process each feature exists independently of the others. That is, features
such as “redness,” “curvature,” or “tilted line” are, at this early stage of processing,
not associated with a specifi c object. They are, in Treisman’s (1986) words,
“free fl oating,” as shown in ● Figure 4.34, and can therefore be incorrectly
combined if there is more than one object, especially in laboratory situations
when briefl y fl ashed stimuli are followed by a masking fi eld.
You can think of these features as components of a visual “alpha-
bet.” At the very beginning of the process, perceptions of each of these
components exist independently of one another, just as the letter tiles in a
game of Scrabble exist as individual units when the tiles are scattered at
the beginning of the game. However, just as the individual Scrabble tiles
are combined to form words, the individual features combine to form
perceptions of whole objects. According to Treisman’s model, these fea-
tures are combined in the second stage, called the focused attention stage
(Figure 4.32). Once the features have been combined in this stage, we
perceive the object.
During the focused attention stage, the observer’s attention plays an
important role in combining the features to create the perception of whole
objects. To illustrate the importance of attention for combining the features,
Treisman repeated the illusory conjunction experiment using the stimuli
in Figure 4.33, but this time she instructed her participants to ignore the
black numbers and to focus all of their attention on the four target items.
This focusing of attention eliminated illusory conjunctions so that all of the
shapes were paired with their correct colors.● FIGURE 4.33 Stimuli for illusory conjunction
experiment. (Source: Reprinted from A. Treisman & H. Schmidt,
“Illusory Conjunctions in the Perception of Objects,” Cognitive
Psychology,14, 107–141, 1982, with permission from Elsevier.)
18
FIGURE 4.34 Illustration of the idea that in
the preattentive stage an object’s features
are “free fl oating.” Because they are not
attached to a particular object, they can
potentially become associated with any object
in a display. When this happens, an illusory
conjunction is created. (Source: Reprinted from
A. Treisman & H. Schmidt, “Illusory Conjunctions in the
Perception of Objects,” Cognitive Psychology,14, 107–141,
1982, with permission from Elsevier.)
Small Triangle Red
“Free-Floating” FeaturesCircle Large YellowCopyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.