Last week, Matt Lieberman posted the video below on Twitter. (I believe it was originally posted on TikTok by the dancer featured in it.) The video illustrates change blindness. Large visual changes occur during the course of the video, which the viewer typically doesn’t notice. You may like to watch the video a few times and see if you notice the changes.
If you still haven’t noticed the changes, I’ll tell you. [Spoiler follows]
. . .
The colours of the dancers’ tops change markedly during the video. You can see the difference easily by jumping to different points in the video and noting the colours of the tops at each point.
You may be surprised that you missed such a large change, but it is not really surprising. Our visual systems are designed to keep track of things through changes in their appearance. Imagine you’re in a forest with a tiger stalking you. As the tiger moves in and out of the shade, the light reflected from its coat changes continually, but you need to ignore these changes and focus on what’s constant: the individual animal moving through the trees. And that is exactly what your brain does. In the case of the video, your brain accurately tracked the individual dancers, not confusing one with another, and it ignored changes in colour that were irrelevant to this task. Looked at in this way, change blindness is not a bug in our visual system but a feature, and we should expect it to occur all the time.
But this feature of vision presents a problem for a certain view of consciousness, which I’ll call the three-stage view. The view goes like this. Conscious perception proceeds in three stages. First, physical stimuli impact on our sense organs and produce neural activity in sensory processing regions of our brains. Second, this neural activity produces a visual sensation — a private mental quality which makes it like something to perceive the stimuli. Third, the sensation in turn produces reactions appropriate to its nature (unpleasant sensations cause us to shun the things generating them, pleasant sensations cause us to seek them out, etc). We react as we do because of what our experiences are like.
Now, when the colours changed without your noticing it, what happened at each of the three stages? There were certainly changes at the first stage. The light rays hitting your retina changed, and, presumably, at least some of the activity in your visual cortex did, too. There were no changes at the third stage, however — or at least no major ones. Maybe there were subtle changes in your behavioural dispositions that could be detected under experimental conditions. (I don’t know if this has been investigated; it would be an interesting project.) But your reactions did not change in a way that was noticeable either to you or to a casual observer.
What about the second stage? Did your visual sensations change? Was there a change in what the experience was like for you?
If there wasn’t, and your sensations didn’t change until you reacted, then the three-stage view is undermined. For the view says that our reactions are produced by our sensations; we react as if there is something green in front of us because we are having a sensation of green. But if we don’t have a green sensation until we react, then the sensation can’t be playing this role. Indeed, it is not clear why we need sensations at all, since the reactions occur independently of them.
So a defender of the three-stage view must say that our sensations change before we react. They can then continue to claim that when we do react we are reacting to a change in our sensations, albeit after a delay.
But this has a strange consequence. It means that we can be mistaken about our own current sensations. At the mid-point of the video, you thought you were having the same colour sensations as at the start, but you were in fact having different ones. And does that even make sense? Your sensations are supposed to define how things seem to you, and while you may be wrong about what colour something is, you can’t be wrong what colour it seems to be.
That’s not all. If you can fail to notice your sensations changing, then maybe you could fail to notice them fading out altogether. Maybe your sensations faded out an hour ago, and you haven’t noticed yet. You’ve continued to react as if they hadn’t faded out, of course, but during the video you continued to react as if your colour sensations hadn’t changed. Maybe you’ve never had colour sensations at all. Maybe you’re just reacting as if you have them. How do you know? All you can be sure of is what you think about your sensations. Maybe sensations are a sort of illusion.