However, we use top-down processes to “fill in the gaps”. This creates the illusion we can see the entire visual scene clearly and in detail (Freeman &
Simoncelli, 2011; see Chapter 5 and Figure 5.10).
Other reasons for change blindness blindness were identified by Loussouarn et al. (2011). Change blindness blindness was most common when observers were led to believe the time they took to detect changes was short and their perceived success at detecting changes was high. Thus, change blindness blindness depends in part on overconfidence.
Change blindness vs. inattentional blindness
Change blindness and inattentional blindness are similar in that both involve a failure to detect some visual event that appears in plain sight. There are also some important similarities between the processes underlying change blindness and inattentional blindness. For example, failures of attention often (but not always) play an important role in causing both forms of blindness.
In spite of these similarities, there are major differences between the two phenomena (Jensen et al., 2011). First, consider the effects of instructing observers to look for unexpected objects or visual changes. Target detection in change blindness paradigms can still be hard even with such instructions. In contrast, target detection in inattentional blindness paradigms becomes trivially easy. Second, change blindness involves memory so that pre-change and post-change stimuli can be compared, whereas inattentional blindness does not. Third, inattentional blindness occurs when the observer’s attention is engaged in a demanding task (e.g., counting passes between players), which is not the case with change blindness.
In sum, more complex processing is typically required for successful performance in change blindness tasks than inattentional blindness ones. More specifically, observers must engage successfully in five separate processes for change detection to occur (Jensen et al., 2011):
1 Attention must be paid to the change location.
2 The pre-change visual stimulus at the change location must be encoded into memory.
3 The post-change visual stimulus at the change location must be encoded into memory.
4 The pre- and post-change representations must be compared.
5 The discrepancy between the pre- and post-change representations must be recognised at the conscious level.
In the real world, we are often aware of changes in the visual environment because we detect motion signals accompanying the change. Ways of preventing observers from detecting motion signals include making the change during a saccade (rapid movement of the eyes), making the change during a short temporal gap between the original and altered stimuli, or making the change during an eyeblink.
What causes change blindness?
There is no single (or simple) answer to the question, “What causes change blindness?” The main reason is that (as we have just seen) change detection requires the successful completion of five different processes.
Change blindness often depends on attentional processes. We typically attend to regions of a visual scene most likely to contain interesting or important information. Spot the difference between the pictures in Figure 4.12. Observers took an average of 10.4 seconds to do so with the first pair of pictures but only 2.6 with the second pair (Rensink et al., 1997). The height of the railing is of marginal interest whereas the position of the helicopter is of central interest.
Hollingworth and Henderson (2002) studied the role of attention in change blindness. They recorded eye movements while observers looked at a visual scene (e.g., kitchen, living room) for several seconds. It was assumed the object fixated at any given moment was being attended. Two kinds of changes could occur to each visual scene:
1 Type change, in which an object was replaced by an object from a different category (e.g., a plate was replaced by a bowl).
2 Token change, in which an object was replaced by an object from the same category (e.g., a plate was replaced by a different plate).
Figure 4.12
(a) the object that is changed (the railing) undergoes a shift in location comparable to that of the object that is changed (the helicopter) in (b). However, the change is much easier to see in (b) because the changed object is more important.
From Rensink et al. (1997). Copyright © 1997 by SAGE. Reprinted by permission of SAGE Publications.
What did Hollingworth and Henderson (2002) find? First, change detection was much greater when the changed object had been fixated prior to the change (see Figure 4.13a). There was very little evidence observers could accurately detect change in objects not fixated prior to change.
Second, there was change blindness for 60% of objects fixated before they were changed. Thus, attention to the to-be-changed object was necessary (but not sufficient) for change detection.
Third, Hollingworth and Henderson (2002) studied the fate of objects fixated some time prior to being changed. The number of fixations on other objects occurring after the last fixation on the to-be-changed object had no systematic effect on change detection (see Figure 4.13b). Thus, the visual representations of objects last for some time after receiving attention.
Fourth, change detection was much better when there was a change in the type of object rather than merely swapping one member of a category for another (token change) (see Figure 4.13b). This makes sense given that type changes are more dramatic and obvious.
Figure 4.13
(a) Percentage of correct change detection as a function of form of change (type vs. token) and time of fixation (before vs. after change); also false alarm rate when there was no change. (b) Mean percentage correct change detection as a function of the number of fixations between target fixation and change of target and form of change (type vs. token).
Both from Hollingworth and Henderson (2002). Copyright © 2002 American Psychological Association. Reproduced with permission.
Do observers showing change blindness remember the changed object? Busch (2013) gave a recognition-memory test to participants showing change blindness or change detection. Even those showing change blindness manifested some memory for the pre-change and post-change objects. That means the changed object was processed to some extent even in the absence of change detection.
Change blindness may occur because observers fail to compare the pre- and post-change representations of the visual display. Evidence that useful information about the pre-change representation can still be available even when there is change blindness was reported by Angelone et al. (2003). In one experiment, observers watched a video clip in which the identity of the central actor changed. This was followed by photographs of four individuals, one of whom was the pre-change actor.
Angelone et al. (2003) compared performance on the line-up task of observers who did or did not detect that the actor’s identity had changed. Those who showed change blindness performed as well as those who showed change detection (53% vs. 46%, respectively).
Varakin et al. (2007) extended the above research in a real-world study in which a coloured binder was switched for one of a different colour while participants’ eyes were closed. Some participants showing change blindness nevertheless remembered the colour of the pre- and post-change binders and so had failed to compare the two colours. Other participants showing change blindness had poor memory for the pre- and post-change colours and so failed to represent these two pieces of information in memory.
Landman et al. (2003) argued that we initially form detailed representations of visual scenes. However, these representations decay rapidly or are overwritten by subsequent stimuli. Observers were presented with an array of eight rectangles (some horizontal and some vertical) followed 1,600 ms later by a second array of eight rectangles. The task was to decide whether any of the rectangles had changed orientation.
There was very little change blindness provided observers’ attention was directed to the rectangle that might change within 900 ms of the offset of the first array. Landman et al. (2003) concluded we can have access to fairly detailed information about the current visual scene for almost 1 second. However, it is important that what we currently perceive is not disrupted by what we perceive next. Such disruption occurs when there is overwriting of the previous scene with the current one. A consequence of such overwriting is that we often exhibit change blindness.
Busch et al. (2009) argued that we should distinguish two types of change detection: (1) sensing there has been a change without conscious awareness of which object has changed; and (2) seeing the object that has changed. Busch et al. used event-related potentials (ERPs; see Glossary). ERP components associated with selective attention and conscious processing of visual changes were associated with seeing but not sensing. Much research is problematical due to a failure to distinguish clearly between sensing and seeing.
Howe and Webb (2014) argued that sensing often occurs when observers detect a global change in a visual scene. Observers were presented with an array of 30 discs (15 red, 15 green). On some trials, three discs all the same colour changed from red to green or vice versa. On 24% of trials, observers detected the array had changed without being able to identify any of the discs that had changed. Thus, there was frequently sensing of global change without seeing which objects had changed.
Finally, Fischer and Whitney (2014) proposed a new theoretical approach that may well enhance understanding of change blindness. They argued that perceptual accuracy is sacrificed to some extent so that we can have continuous, stable perception of our visual environment. Observers reported the perceived orientation of black-and-white gratings presented several seconds apart. The key finding was that the perceived orientation of a grating was biased in the direction of the previous grating even when it had been presented ten seconds earlier. The visual system’s emphasis on perceptual stability may inhibit our ability to detect changes within the visual scene.