The goal of the experiment was to determine the effect selective attention played in subjects’ abilities to detect change blindness. Change blindness occurs when large changes are missed under natural viewing conditions because they occur simultaneously with a brief visual disruption, perhaps caused by an eye movement, a flicker, a blink, or a camera cut in a film sequence (Regan, Rensink, and Clark, 1999). Subjects were tested for change detection using dynamic displays. Both the cut-contingent technique and the gradual change method were adopted, using a video camera, to make changes to several stimuli in the scene. During these changes, attention was focused for one group on a conversation, while for the other attention was focused on the actual scene itself. Four different types of stimuli were used to test the deletion of a stimulus from the scene, the change in orientation to a particular stimulus, change of color, and change of motion. Previous studies have shown that subjects find it easier to detect changes in motion as opposed to changes in existence or orientation (Rensink, 2002). The study not only shows the effect of attention on the ability to detect changes to the environment but also what stimuli are easier to detect.
Methods:
Subjects were 20 Alma College students. Two groups were shown a video. The first group (A) was instructed that this was an experiment on selective attention, and that they were to pay close attention to the conversation, as questions pertaining to it would be asked afterwards. Volume of the conversation was played at a relatively normal hearing level. The second group (B) was also told this was an experiment on selective attention and were instructed to pay close attention to the scene in particular, and to see if they specifically noted any changes to it. Volume of the video was turned completely down to mute.
A video recording was produced in a small laboratory room in the basement of the Psychology department, which included such objects as a desk, a computer, and a chair, with no objects on the walls. Four devices (stimulus) were added to the room in a random placement of the scene. Devices used were an empty pop can, a rotating wheel with a cardboard cutout of a black and white swirl attached to it, a light projector box including a selection of different colored slides, and an eye test poster. The video depicted two people discussing an issue.
The first fifteen seconds depicted the opening dialogue between the two people. At the fifteen-second mark, a change was made to the existence of an object using the cut-contingent method, which is a cut in the video to another shot from a different angle. The pop can was removed in between the cut, so the first fifteen seconds included dialogue with a pop can in the background, while the fifteen seconds following did not have a pop can in the scene. At the thirty-second mark, the tape was stopped temporarily to ask the subjects questions. Group A was asked a few questions about the content of the dialogue. At the end of the questioning, group A was asked if they noticed any thing else about the video. Group B was simply asked if they saw any change or changes in the scene, and if yes, what change or changes there were.
The video was then continued for about another thirty seconds (the pop can was never added back to the scene), as second change was made to the scene. The cut-contingent method was used to change the orientation of an object, the eye test poster, by rotating it 180 degrees from the original normal positioning. The video was once again stopped and the subjects were asked the same series of questions respectively (with the exception that Group A was asked questions pertaining to the conversation between the interval of thirty seconds and the one minute mark).
The video was continued for another thrity seconds (with no further change to the orientation of the poster) with a new change, again using the cut-contingent method. This time one of the color slides in the light projector box was changed. After watching the dialogue under this condition the subjects were asked their respective questions.
The video was once again continued, with the final change to the scene. A gradual change method was adopted to begin rotating the wheel with the black and white swirl. No cut was made in the video; instead the video was run continuously for thirty seconds from the same vantage point, and at the fifteen-second mark, one of the experimenters who was outside the frame of the video used a control switch attached to a power chord to begin rotating the wheel. Therefore, the subjects never saw anyone physically turn on the rotating wheel in the video. At the two minute mark the video was stopped, and once again, the questions were asked to the subjects.
Results:
Responses to the questions varied. Therefore, a graph was compiled in which any subject who detected any change with the stimulus that was changed that frame was counted as a subject detecting change. See Figure 1.0. This means that a subject who correctly stated that the light changed from white to red and a subject who claimed that there was some change with the light in the light box were both counted in the number of subjects who reported some change. This does not mean, however, that a subject who detected change in the poster orientation after the second set of questions was counted. Only subjects who reported change in the set of questions directly following the change in stimulus were counted.
It was found that by directing subjects’ attention to the conversation, the ability to detect change was lessened for two particular stimuli: the pop bottle, and the spin wheel. Both the eye poster and the light box showed no particular difference between the two groups. The change in orientation of the eye poster was the most easily detectable stimulus, with ninety percent of subjects detecting change without sound, and eighty percent detecting change with sound. The deletion of the pop bottle was the hardest stimulus to detect; not a single subject with sound detected any change in the pop bottle, and only thirty percent of the subjects without sound reported any change.
In addition, it was found that very few subjects were able to correctly detect the exact change that occurred. For example, many subjects described that the eye chart poster was upside down, but were not sure if it was like that previously. A correct response would be that the eye chart poster was right side up in the first frame, but the second frame it was turned upside down. Also, a few subjects had a hard time discriminating between the colors in the light box. Some said that the color changed from blue to red, some said it was now red but that they did not know what it was before, and some described that they noticed a change in the light but could not tell that it was the color of the light that changed. The spin wheel also brought a wide variety of responses. Subjects noticed that the wheel slowed down and the sped up, some noticed a directional change, and some noticed that the wheel was going counterclockwise, slowed to a stop, and then started moving clockwise. Almost every subject failed to notice that during the video, the spin wheel was moving clockwise, but at the start of the last frame, it was moving counterclockwise. All of the above responses were counted as subjects detecting some change.
However, the pop bottle is different. Most subjects noticed that one of the people in the video (Kym) took a drink of the pop bottle, but then failed to notice that several seconds later the pop bottle was gone. Only subjects that said the pop bottle was gone were counted. Discriminating between the existence of the pop bottle on the one hand, and the deletion of the pop bottle from the frame on the other, is an important distinction. For example, a subject would not be counted if they simply noticed that the eye chart was hanging on the wall, but would be if they noticed that the eye chart had changed from its previous position. It was not crucial that the subject detect in what way the chart had changed but simply the fact that it had changed. However, with the pop bottle, the only change that a subject could report would be that it was gone. Subjects did not have to discriminate between where it was on the desk, or where it went, but simply that it had been moved from its previous position.
Discussion:
Subjects whose attention was not focused on the conversation detected change with the pop bottle and the spin wheel better than subjects whose attention was focused on the conversation. This shows that selective attention plays some part in change detection. Attentional rather than sensory limits are a key factor because changes are often missed in less attended regions (Rensink et al., 1997). Perhaps a line exists somewhere that determines which stimuli are more easily detectable and which are not. However, this is probably true of only the extremes: that is the stimulus that is extremely obvious or the stimulus that is particularly hard to detect. Specifically, the eye chart was extremely obvious, while the change in the direction of the spin wheel from clockwise to counterclockwise between frames was very difficult to detect. Most subjects varied in which stimuli they were able to detect. That is, some stimuli may have been obvious for some subjects and not for others, and vice-versa.
In addition, short-term memory and perhaps long-term memory probably play a large part in change detection. Discriminating between the fact that a change has occurred with a particular stimulus and what that change actually is tells researchers a great deal about the capability of short-term memory. For a subject to notice what has changed about a particular stimulus, the subject must have a mental representation of the previous scene, and be able to compare that with the new scene. For example, a subject would have to know what color the light in the light box was originally in order to know specifically what change in color has occurred. Some subjects described that the color changed from blue to red. This is interesting because the light was never blue, but yet subjects still knew that the light was some other color originally other than red. It seemed easy for subjects to describe what color the light is since its change, but when trying to describe what color the light was previously, subjects couldn’t remember accurately enough. Therefore, subjects short-term memory was successful in recognizing that the color was different, but when trying to remember what color the light was previously, the short-term memory failed. Also, most subjects reported that they were not sure what the stimulus was like before they noticed the change, and some said that it could have been like that before and that they just noticed it now. Short-term memory helps subjects recognize that a change occurred, but it is perhaps the role of long-term memory to help subjects determine exactly what that change was (such as a change from white light to red light). Experiments on change detection seriously question the accuracy of short-term memory and the role of long-term memory.
Also, it was found that it took a long time before some changes were noticed, which support the finding (Rensink et al,. 1997) that it can take a remarkably long time before changes to the scene is noticed, even for substantial changes. For example, one subject said during the last set of questions that they noticed the eye chart was upside down. In actuality, the eye chart had been upside down almost the entire length of the film. The findings above show that most people noticed the change at the time the change occurred, which suggests that the change in orientation of the eye poster was a substantial change.
There are many reasons why some stimuli were detected more than others. As described in the introduction, change blindness can occur because of a brief disruption, such as an eye movement, a flicker, a blink, or a camera cut in a film sequence. This was taken into account when making the video. Perhaps the reason why the pop bottle was missed so frequently was because of a camera cut in a film sequence. The camera zoomed in on the lecturer (Jeff) after the listener (Kym) took a drink of pop and set it on the desk. After zooming in on Jeff and after Kym was out of the frame, the camera cut back to the shot that showed the whole scene, this time with the pop bottle gone. This camera cut could be why so many subjects missed the deletion of the pop bottle from the scene. In addition, the change in direction of the spin wheel during the last frame occurred at the exact moment when Kym coughed loudly. Eye movement could play a large role in why so many of the subjects with sound missed the change in direction. Their eye moved from Jeff, who was talking, to Kym, who coughed, and then back to Jeff, who began talking again. This eye movement could have deterred subjects from focusing on the spin wheel by making them focus on the coughing. This is supported by (Irwin, 1992), which showed that eye movement alone is sufficient to disrupt any complete representation of the scene.
Another reason why subjects without sound did better at detecting changing stimuli was that they were explicitly told to pay attention to the scene. Attention to the background appears necessary to consciously perceive changes there. If no explicit instructions are provided about attending the background, attention is apparently directed by default towards foreground items (Turatto, Angrilli, Mazza, Umilta, Driver, 2002).
Since one group was told to pay attention to background changes, they naturally would do better than subjects not looking for background changes. The difference between detected versus missed (background) changes depended here primarily on instructions/cues indicating that background changes were possible (Turatto et al., 2002).
Figure 1.0 This graph illistrates the number of respondents who reported noticing a change for each of the four change stimuli.
REFERENCES
Driver, Jon, Davis, Greg, Russell, Charlotte, Turatto, Massimo, Freeman, Elliot (2001). Segmentation, attention, and phenomenal visual objects. Cogniton, 80, 61-95.
Irwin, D.E. (1992). Perceiving an integrated visual world. In D.E Meyer and S. Kornblum, Attention and performance XIV: Synergies in experimental psychology, artificial intelligence, and cognitive neuroscience--A silver Jubilee (pp. 121-142) Cambridge, MA: MIT press.
O’Regan, J. Kevin, Rensink, Ronald A., Clark, James J., (1999). Change-blindness as a result of ‘mudsplashes’. Nature, 398, 34.
Rensink, Ronald A.(2002). Change detection. Annual Review of Psychology, 53, 245-272
Shore, David I, Klein, Raymond M (2000). The effects of scene inversion on change blindness. The Journal of General Psychology, 127, 27-43.
Simons, Daniel J., Levin, Daniel T., (1998). Failure to detect changes to people during a real-world interaction. Psychonomic Bulletin and Review, 5, 644-649.
Turatto, Massimo, Angrilli, Alessandro, Mazza, Veronica, Umilta, Carlo, Driver, Jon (2002). Looking without seeing the background change: electrophysiological correlates of change detection versus change blindness. Cognition, 84, B1-B10.
Contact Information - Email
Kym Kolodziejski - emeraldprincesss@hotmail.com
Jeff Natke - bluesnat1@aol.com
Jonathon Musser - string_thumper@yahoo.com