Chapter 6 Explaining two-picture change blindness
For this unit, the term change blindness refers to the failure to notice changes in animations that alternate between two pictures of a scene. Later we will also talk about other situations in which people miss changes, but this chapter focuses on the two-picture alternation animations. In previous years, you probably already saw some of those amazing demonstrations. Here, however, we will learn somewhat different lessons than what you learned before.
6.1 Blindness for gradual changes
Let’s start with very gradual changes rather than alternation of two pictures of a scene. Please view this movie. One might expect that when continuously viewing a scene, you would notice any major changes that occur during that time.
6.1.1 The “grand illusion of visual experience”
Most ordinary people are surprised by change blindness. Many researchers were very surprised, too, and some arrived at the conclusion that there is a grand illusion of visual experience. This is the claim that while people think that they are simultaneously experiencing the whole visual field, they are wrong about that - it is an illusion. These researchers explain change blindness with the claim that at any one time, you are only experiencing a small portion of the visual field, parts that you are particularly attending to. In other words, these researchers claim that visual experience is subject to a strong bottleneck.
However, this conclusion that there is a bottleneck on visual experience may be premature. To understand why, we need to consider in more detail what the failure to notice changes might mean. We need to consider the processing that’s needed to detect a change.
6.1.2 What is needed to detect a change?
Let’s consider what it takes to detect the change of an object or part of a scene:
- An internal representation of that object that is different before and after the change.
- A process that compares what was identified earlier to what is being identified now.
- A process that calls attention to, or brings into conscious awareness, the instances of change.
Apparently, at least one of the above three processes is lacking. Let’s consider #1 first. It is the case that all incoming retinal signals across the scene get processed. Unfortunately, however, if the object is in the periphery, the retinal signals may not be high-resolution enough for the representation to be different before and after the change. This is because vision is low resolution in the periphery (4).
For many real-world scenes, then, #1 above is sufficient to explain why people don’t notice changes. However, #1 is not enough to explain all failures to notice changes. Even when researchers create displays in which all the objects are big enough and widely-spaced enough to see in the periphery, still people miss many changes. For example, the changes are large enough in some classic demonstrations like this boat scene.
So, #2 or #3 or both are lacking. This is likely due to a bottleneck. These processes are limited in capacity, so they cannot simultaneously process all objects in the visual scene. And what about the “grand illusion of visual experience?” Well, it seems quite possible that we may have experience of objects without having processes that correspond to #2 and #3. In other words, the conclusion that there is a grand illusion of visual experience may be a hasty one (Noë, Pessoa, and Thompson (2000)). When people are surprised by change blindness, their mistake may be failing to realise that there’s various processes required to notice a change, and visual experience may not always involve those.
Only a finite number of neurons can fit in our head, and evolution seems to not have prioritized processing of #2 and #3. The brain has not devoted neurons to constantly comparing what you’re seeing now to what you saw half a second ago. Comparing what was present at two different times requires the limited resources of attention to be at that location at the two different times. We don’t know why evolution did not prioritize these, but one possibility is that a full comparison process (#2) would require a lot of neurons, and animals like us have been able to get by with other, simpler processes, which we will discuss next.
6.3 Two-picture change blindness
Animations like that of the boat scene or this Paris scene sandwich a blank screen in between the two versions of the picture. It’s like a blank screen sandwich! (The two pictures of the scene are analogous to the two chocolate biscuits and the ice cream is analogous to the blank screen).
Here is a schematic of the timeline, wherein the arrow represents time.
That blank screen is critical - it creates flicker everywhere in between the two frames. That is, when the picture of the scene is replaced by the blank scene, it creates a flicker signal everywhere, and then more flicker everywhere when the second scene comes on.
Please view this animation of a change blindness video with the blank screen removed. In that animation, the only location that tickles your transient detectors is that of the change. As a result, your attention goes straight to the location of the change.
Without the blank screen, the only location of flicker was the location of the changing object. The flicker called your attention to that location. With the blank screen, there’s flicker everywhere, so there is no indication of which of the many locations contains the change.
6.4 Searching without a clue
When the blank screen is in the animation, the flicker/motion detectors provide no clue as to the location of the change. You might think that in this situation, people would search about randomly, or perhaps in a systematic fashion, something like searching from left to right and then top to bottom.
Consider the dinner date change blindness scene.
Researchers have recorded eye movements of people viewing these animations to see where they look - what they fixate on. These eye movements provide a pretty good indication of where people direct their attention. In a previous chapter (4), you learned that most movements of attention are overt - if people are interested in something, they look at it.
The above image shows some data from an eyetracking experiment. The long straight lines represent big jumps of the eyes from one place to another as the participants tried to determine what was changing. As you can see, the eyes dwell mostly on the couple’s faces, their hands, and some objects on the table. So, the locations that people looked were not random at all.
You already knew that unique colors and other features are salient and attract attention, but here you can see that other properties of a scene affect attention. Over two thousand years ago, Aristotle wrote that “Man is by nature a social animal.” People are very interested in people, and in working out what they’re thinking and feeling. Things like people, bodies, and objects like food and wine are sometimes referred to as a scene’s high-level properties. The word “high-level” is used in part to indicate that they take more processing to extract, and thus are represented at later (“higher level”) stages of the brain compared to, say, color.
To understand the meaning of this scene involves working out the facial expressions of the people and how they are interacting with each other, based on their postures and the objects in front of them. People are so captivated by this that these participants never looked at some of the other objects, like the railing behind the couple (which was what was actually changing in the change blindness animation).
The above image shows only a few participants’ data on one particular scene. Does the pattern of fixating the eyes more on face and bodies hold for other stimuli as well? Rigby, Stoesz, and Jakobson (2016) investigated this issue. Sixteen participants watched twelve four-second movie clips and twelve still-frame images from several episodes of a TV show (the Andy Griffith show) which had a lot of dialogue and characters. No soundtrack was presented.
These data provide further support for the hypothesis that attention is biased towards faces. Rösler, End, and Gamer (2017) flashed pictures of scenes for just a fifth of a second, so people had time for only one eye movement, and found that people disproportionately looked at parts of the scene with faces or bodies.
These results are used by webpage and advertisement designers who seek to control or guide your attention. You may have noticed that many ads often have a talking person in them, even when this is completely unnecessary and superfluous to the information provided. Advertisers strive to hack your attentional system to get you to read/watch their ads.
There is some evidence that the attention of many children with autism spectrum disorder is less biased towards faces than is that of typically-developing children. The study whose results are plotted above also included a group of sixteen adults with autism spectrum disorder, as shown below.
What would you expect, then, for the pattern of performance in change blindness in people with autism spectrum disorder? Kikuchi et al. (2009) conducted a change blindness experiment and varied whether what changed was the head of a person, another object, or a change to the color of the background.
The blank screen sandwich was looped until the participants pressed a key. The participants were then required to report what the change was, by pointing at it or with a verbal description. As one measure of performance, the researchers examined only those trials where the participants correctly detected the change and plotted the response time on those trials. Lower response times suggest that the person detected the change faster.
Answer these questions and relate them to the first four, and the last, learning outcome ( 2 ):
- Why can classic change blindness animations be described as a “blank screen sandwich”?
- Why are gradual changes hard to detect?
- What effect do mudsplashes and other irrelevant sudden changes in a scene have on our ability to detect important changes? How do they have that effect?
Kikuchi, Yukiko, Atsushi Senju, Yoshikuni Tojo, Hiroo Osanai, and Toshikazu Hasegawa. 2009. “Faces Do Not Capture Special Attention in Children with Autism Spectrum Disorder: A Change Blindness Study.” Child Development 80 (5): 1421–33.
Noë, Alva, Luiz Pessoa, and Evan Thompson. 2000. “Beyond the Grand Illusion: What Change Blindness Really Teaches Us About Vision.” Visual Cognition 7 (1-3): 93–106.
O’Regan, J. Kevin, Ronald A. Rensink, and James J. Clark. 1999. “Change-Blindness as a Result of ‘Mudsplashes’.” Nature 398 (6722): 34–34. https://doi.org/10.1038/17953.
Rigby, Sarah N., Brenda M. Stoesz, and Lorna S. Jakobson. 2016. “Gaze Patterns During Scene Processing in Typical Adults and Adults with Autism Spectrum Disorders.” Research in Autism Spectrum Disorders 25 (May): 24–36. https://doi.org/10.1016/j.rasd.2016.01.012.
Rösler, Lara, Albert End, and Matthias Gamer. 2017. “Orienting Towards Social Features in Naturalistic Scenes Is Reflexive.” PLOS ONE 12 (7): e0182037. https://doi.org/10.1371/journal.pone.0182037.