by Joseph Nano
Airport-security screeners are important in keeping weapons off aircraft. On March 11, 2001, American Airlines Flight 11 was hijacked by 5 Al-Qaeda members, and the plane crashed into the North Tower of the World Trade Center in New York City. This plane crash was part of the September 11 attacks, which caused the deaths of approximately 3000 people. This incident raised questions regarding the effectiveness of airport security at the time, and airport checkpoint screenings have been tightened since this incident. For example, the Transportation Security Administration (TSA) improved security on aircraft, security screening, and increased identification checks. McCarley et al. (2004) investigated whether practice performing an x-ray screening task improves search and recognition. Also, the purpose of the study was to determine whether search and recognition that are learned with practice were specific to the target stimuli during training. The term “target stimulus” refers to the stimulus to which subjects in an experimental procedure should attend. In this study, 8 knives served as target stimuli. This journal article is of interest to non-experts because it focuses on human performance in the task of aviation security screening, which can help to optimize screener training and prevent future terrorist attacks. Simply put, the topic of security screening is important because it can prevent the next terrorist attack.
The topic of McCarley et al. (2004) focused on human performance in the task of aviation security screening. There are two general theoretical issues related to the topic. In medical image reading, learning can affect recognition and scanning. Experienced medical image readers are more likely to recognize abnormalities in an x-rayed image than others. This skill appears to reflect increased low-level visual sensitivity, and this raises a question about visual expertise. There are two theories that explain visual expertise. First, is it actual visual sensitivity? In other words, is this a skill that reflects increased low-visual sensitivity? Second, there is evidence, which indicates that the ability to perceptually identify shape representations from degraded imagery is based on observers’ ability to retrieve their stored mental representations of familiar objects. In other words, a second theory would question whether the performance of the expertise reflects not necessarily visual sensitivity but enhance ability to retrieve mental representations of particular familiar objects. To sum up, there are two general theories that can explain visual expertise: visual sensitivity and mental representations.
McCarley et al. (2004) hypothesized that there will be a significant improvement in both sensitivity for detecting target stimuli and reaction times (RTs) with practice. For the experimental design, 16 adults were recruited to the study. All participants had normal color vision. Eyelink eye tracker was used to record eye movements. Stimuli were presented on a 19-inch monitor. Participants viewed displays from a distance of 91 centimeter. The Federal Aviation Administration provided chromatic x-ray images of 89 bags that produced stimuli for this study. These images of bags included objects, such as clothes, pharmacy bottles, hair dryers, and target objects. The similarity rate for the items within each set was higher than the similarity rate of items between sets. A total of 8 knives (2 sets of 4 knives) served as target objects. These target objects were inserted digitally into the images at random locations and orientations. Furthermore, 1 target object was inserted into each image, and it was presented with its flat side.
For the experimental procedure, participants completed 5 experimental sessions across several days, and they were given written instructions explaining their task. First, participants were asked to gaze at a central fixation mark and press the spacebar on the computer’s keyboard. Second, they were asked to imagine they were employees at an airport and their job was to search for the presence of knives in x-ray images of luggage. If the bag contained a knife, then participants were instructed to press the “F” key on the keyboard. In contrast, they were instructed to press the “J” key in the absence of knives. After each response, participants were given a text feedback. Third, during Session 1 through 4, all images given to participants were drawn from one set of targets. However, Session 5 contained targets drawn from the alternative set. Participants were informed that they would encounter a new set of target knives that would look different from those they had seen previously.
Reaction times and a signal detection measure of sensitivity (Az) were used to measure participants’ task performance for accurate target-present and target-absent responses. In addition, the study measured target recognition by calculating hit rate and false alarm rate. The term “hit rate” refers to the number of times in which the participant correctly responds to the target stimulus. In contrast, the term “false rate” refers to the incorrect observation by participants when they state that a signal is present in a trial when in fact it is absent. Furthermore, the study examined changes between the first and fourth sessions and fourth and fifth sessions to provide a better understanding about the impact of practice on participants’ performances.
The results of the study showed a significant increase in Az and a significant decline in RTs for target-present and target-absent responses between Sessions 1 and 4. This indicates some improvement in detecting the target stimuli as a result of practice. Also, the data showed some improvements in sensitivity for detecting target stimuli, which is produced by changes in participants’ ability to recognize objects. When the study compared participants’ performances between Sessions 4 and 5, they noticed a decline in sensitivity and an increase in RTs for target-present and target-absent responses during Session 5. In addition, the study compared the performance of participants between Session 1 and Session 5 to investigate whether practice with one set of target has any benefits to the performance with the alternative set. The results showed an increase in sensitivity and a decline in RTs for target-present and target-absent responses between Sessions 1 and 5. To sum up, the results of the study showed a significant decline in RTs between Sessions 1 and 4, and Session 5 revealed a significant increase in RTs in comparison to Session 4.
The main conclusion that can be drawn from the experiment is that both sensitivity and RT improved with practice. In other words, participants were faster to recognize the target once they fixated on it. Although scanning became more efficient with practice, it did not become more effective. For example, the study argues that practice reduced search time for hits and correct target-absent responses. Also, practice had little effect on the ability to identify the targets through visual foraging, which refers to searching for multiple targets in the same trial. These results show that airport security officers should not rely on security screenings. In order to prevent the next aircraft hijacking, security officers should be more thoroughly trained to detect weapons or explosives. Also, airports should employ full-body scanning machines and check all baggage.
There are some limitations to this study. First, McCarley et al. (2004) presented targets that did not differ dramatically in appearance between trials. There are different kinds of knives and some types of knives may not have a sharp blade. In other words, a follow-up study should ensure skill generalization. Second, in the real case scenario, there are other elements that may affect officer’s recognition and decision-making, such as time pressure and noises. Airports are often crowded during the holidays, and customers are always anxious to board a plane. A follow up study should make participants work under conditions characterized by high levels of noise and time stress. Third, future studies should include more than one target. In real case scenario, the job of the screeners is to examine x-ray images to detect the presence of any suspicious or threatening objects. In addition, they look for any illegal objects, such as pressure containers and infectious materials. In this study, there was one type of target and a follow-up study should include more than one target.
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