known task. According to the Stroop effect, people take a long time to name the ink color when that color is used in printing an incongruent word; in contrast, they can quickly name that same ink color when it appears as a solid patch of color.
In a typical study on the Stroop effect, people may require about 100 seconds to name the ink color of 100 words that are incongruent color names (for example, blue ink used in printing the word YELLOW). In contrast, they require only about 60 sec- onds to name the ink colors for 100 colored patches (C. M. MacLeod, 2005). Notice why the Stroop effect demonstrates selective attention: People take longer to name a color when they are distracted by another feature of the stimulus, namely, the mean- ing of the words themselves (Styles, 2006).
Since the original research, hundreds of additional studies have examined varia- tions of the Stroop effect. For example, Elliot and Cowan (2001) demonstrated that practice can improve people’s selective attention, as measured by the Stroop task. Fur- thermore, clinical psychologists have created a related technique called the emotional Stroop task (C. MacLeod, 2005; MacLeod & MacLeod, 2005; C. M. MacLeod, 2005). On the emotional Stroop task, people are instructed to name the ink color of words that are related to a possible psychological disorder.
For example, suppose that someone appears to have a phobic disorder, which is an excessive fear of a specific object. A person with a fear of spiders would be instructed to name the ink colors of printed words such as hairy and crawl. People with phobias are significantly slower on these anxiety-related words than on control words. In con- trast, people without phobias show no difference between the two kinds of words
Demonstration 3.1 The Stroop Effect
For this demonstration, you will need a watch with a second hand. Turn to Color Figure 3 inside the back cover. First, measure how long it takes to name the colors in Part A. Your task is to say out loud the names of the ink colors, ignoring the meaning of the words. Measure the amount of time it takes to go through this list five times. (Keep a tally of the number of repetitions.) Record that time.
Now you will try a second color-naming task. Measure how long it takes to name the colors in the rectangular patches in Part B. Measure the amount of time it takes to go through this list five times. (Again, keep a tally of the number of repetitions.) Record the time.
Does the Stroop effect operate for you? Are your response times similar to those obtained in Stroop’s original study?
(Williams et al., 1996). These results suggest that people who have a phobic disorder are hyper-alert to words related to their phobia, and they show an attentional bias to the meaning of these stimuli. As a result, they pay relatively little attention to the ink color of the words.
Other research shows that people who are depressed take a long time to report the color of words related to sadness and despair. In addition, people with eating disorders take a long time to report the ink color of words related to food or body weight (C. MacLeod, 2005). Furthermore, the Stroop test can be used to assess addiction to alcohol and cigarettes (Cox et al., 2006).
Researchers have examined a variety of explanations for the Stroop effect. Some have suggested that it can be explained by the parallel distributed processing (PDP) approach (e.g., Cohen et al., 1998; C. M. MacLeod, 2005). According to this explana- tion, the Stroop task activates two pathways at the same time. One pathway is activated by the task of naming the ink color, and the other pathway is activated by the task of reading the word. Interference occurs when two competing pathways are active at the same time. As a result, task performance suffers.
Furthermore, adults have had much more practice in reading words than in nam- ing colors (T. L. Brown et al., 2002; Cox et al., 2006; Luck & Vecera, 2002). The more automatic process (reading the word) interferes with the less automatic process (nam- ing the color of the ink). As a result, we automatically—and involuntarily—read the words that are printed in Part A of Color Figure 3. In fact, it’s difficult to prevent our- selves from reading those words—even if we want to! For instance, right now, stop reading this paragraph! Were you successful?
Let’s review what we have discussed so far in this section on selective attention. According to the research on dichotic listening, people usually have trouble picking up much information about the auditory message that they were instructed to ignore. The research on the Stroop effect shows that people have trouble naming the color of a stimulus when the letters of the stimulus are used to spell the name of a different color. Because we read words quite automatically, it’s difficult to pay attention to the less auto- matic part of the message on the Stroop task. Let’s now consider visual search, a third kind of selective-attention task.
Visual Search. You’ve probably conducted several visual searches within the last hour, perhaps a notebook, a sweater, or a yellow marking pen. In some cases, our lives may depend on accurate visual searches. For instance, airport security officers search travelers’ luggage for possible weapons, and radiologists search a mammogram to detect a tumor that could indicate breast cancer (Wolfe et al., 2005).
Researchers have identified an impressive number of variables that influence visual searches. For example, Jeremy Wolfe and his colleagues (2005) found that people are much more accurate in identifying a target if it appears frequently. If the target appears— in a visually complex background—on 50% of the trials, participants missed the target 7% of the time. When the same target appeared in this same complex background on only 1% of the trials, participants missed the target 30% of the time.
Let’s examine two stimulus variables in more detail: (1) whether we are searching for a single, isolated feature or a combined set of features; and (2) whether we are
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searching for a target for which a particular feature is present or a target for which this feature is absent. As you’ll see, two researchers—Anne Treisman and Jeremy Wolfe— have been especially active in studying visual search. Before you read further, however, try Demonstration 3.2.
1. The isolated-feature/combined-feature effect. Demonstration 3.2 is based on classic research by Treisman and Gelade (1980). According to their research, if the target dif- fered from the irrelevant items in the display with respect to a simple feature such as color, observers could quickly detect the target. In fact, they could detect this target just as fast when it was presented in an array of 24 items as when it was presented in an array of only 3 items (Geng & Behrmann, 2003; Styles, 2006; Treisman, 1993; Treisman & Gelade, 1980). If you tried Part A of Demonstration 3.2, you probably found that the blue X seemed to “pop out,” whether the display contains 2 or 23 irrelevant items.
In contrast, Part B of Demonstration 3.2 required you to search for a target that is a combination (or conjunction) of two properties. When you searched for a blue X among red X’s, red O’s, and blue O’s, you probably found that you had to pay attention to one item at a time, using serial processing. You were distracted by stimuli that resem- bled the target because they had either a blue color or an X shape (Serences et al., 2005). This second task is more complex, and the time taken to find the target increases dramatically as the number of distracters increases (Wolfe, 2000, 2001). As a result, Figure B2 required a more time-consuming search than Figure B1 did. Now try Demon- stration 3.3 before you read further.
2. The feature-present/feature-absent effect. Theme 3 of this book states that our cog- nitive processes handle positive information better than negative information. Turn back to Demonstration 1.3 on page 24 to remind yourself about this theme. The research of Treisman and Souther (1985) provides additional support for that theme, as you can see from Demonstration 3.3.
Notice in Part A of this demonstration that the circle with the line seems to “pop out” from the display. The search is rapid when we are looking for a particular feature that is present. Treisman and Souther (1985) found that people performed rapid searches
Demonstration 3.2
The Isolated-Feature/Combined-Feature Effect
After reading this paragraph, turn to Color Figure 4 inside the back cover. First, look at the two figures marked Part A. In each case, search for a blue X. Notice whether you take about the same amount of time on these two tasks. After trying Part A, return to this page and read the additional instructions.
Additional instructions: For the second part of this demonstration, return to Part B inside the back cover. Look for the blue X in each of the two figures in Part B. Notice whether you take the same amount of time on these two tasks or whether one takes slightly longer.
for a feature that was present (like the circle with the line in Part A), whether the dis- play contained zero irrelevant items or numerous irrelevant items. When people are searching for a feature that is present, the target item in the display usually captures their attention automatically (Franconeri et al., 2005; Wolfe, 2000, 2001). In fact, this “pop-out” effect is automatic, and researchers emphasize that locating the target is strictly a bottom-up process (Boot et al., 2005).
In contrast, notice what happens when you are searching for a feature that is absent (like the circle without the line in Part B). Treisman and Souther (1985) found that the search time increased dramatically as the number of irrelevant items increased. People who are searching for a feature that is absent must use focused attention, emphasizing both bottom-up processing and top-down processing. This task is substantially more challenging, as Wolfe (1998, 2000, 2001) has also found in his extensive research on the feature-present/feature-absent effect.
Another example of the feature-present/feature-absent effect was discovered by Royden and her coauthors (2001). According to their research, people can quickly locate one moving target when it appears in a group of stationary distracters. In con- trast, they take much longer to locate one stationary target when it appears in a group of moving distracters. In other words, it’s easier to spot a movement-present object than a movement-absent object.
As we have seen in this discussion of visual search, we search more quickly for an isolated feature, as opposed to a combination of two features. Furthermore, we search more quickly for a feature that is present (as opposed to a feature that is absent).
Demonstration 3.3
Searching for Features That Are Present or Absent
In Part A, search for the circle with the line. Then, in Part B, search for the circle without the line.
Source: Based on Treisman & Souther, 1985.
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