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Change blindness is a neat and convincing visual illusion that provides insights as to how we perceive the world.

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(Video showing some visual illusions, including change blindness.)

We do not see the world around us, but perceive it. What we perceive may not be precisely what is there. These are notions I love to think about. How much of what we see is really what’s there?

As everyone knows, our brains make crazy errors when things are presented in particular ways. We usually call visual failings visual illusions, but perceptual failings would be more accurate. These perceptual failings of our brains are exploited by magicians and in advertising and modern high-tech movies. They also tell us something of how our brains work.

In the video, Richard Brown of The Exploratium presents a series of perceptual failings. I’m writing about the first he presents: change blindness. If you’re in a hurry, you can start at 24 seconds in and play until about 1:19.

Try it.

If you don’t quite believe your eyes–or more accurately your brain–either replay the illusion looking at only one part of it (try cupping your hands around one part of the image), or replay it frame by frame by “grabbing” the button on the play scrollbar and dragging it slowly. (There’s a catch, though: once you know of a change, you can’t help noticing it.)

Feel like wasting lots of time? Type “visual illusion” or “optical illusion” into the search bar at the of the YouTube website… and have fun!

Change blindness raises interesting questions about what we see and remember, and the differences of visual short-term memory to long-term memory. For a popular account, see Natalie Anger’s story in the New York Times. (Natalie is also the editor of Best American Science Writing that I previewed a few weeks ago.†)

Change blindness is generally believed to be associated with visual attention; in the absence of “dominant” changes, we remember things that we visually “attend” to.

When there is a brief flash of a blank image between changing images, we do not perceive dominant changes that we would passively respond to, forcing us to seek out the changes. Coupled to this is that we probably can only pay attention to a small number of things at once, the rest is mostly “filled in”.

We have pretty good long-term visual memories; we can remember what we’ve seen before surprisingly well. Recent research suggests that a lack of time to “attend” to images causes the details not to be memorised in long-term memory (as opposed to visual short-term memory); if sufficient time is given, people are able to memorise considerable detail and hence recognise differences. You can read their short paper, summarising their current thinking followed by an experiment test their ideas at Communicative and Integrative Biology (open access).

Our visual memories capture a lot, but it may be that our longer term memories are not acting like a camera faithfully snapping a series of images, but record the things that either demand our attention or startle us (passive), or that we pay particular attention to (active). As a result, if we neither see changes in a series of images passively, nor have sufficient time to “attend” to the images to see the changes actively, perhaps the changes are not stored in long-term memory for later retrieval and comparison, with the up-shot that we simply don’t notice anything has changed.

An interesting application of change blindness as an experimental method is comparing the attention to faces or facial features in autistic people compared to “normal” people. For example, a recent study (subscriber access only) reported that unlike normal children, autistic children were not better at detecting changes in faces compared to changes in non-face objects. While autistic children have greater attention to detail, these results would suggest that it is not prioritised to faces as it is in “normal” children.

If this sort of thing interests you, you’ll also like fellow scibling Fabiana’s recent post pointing out an excellent description of one variation of effects of the “perceived before consciously aware” type on Neurophilosophy, one of my favourite blogs (see my blogroll).


PS: Those who want further reading, these links might be useful:

Encyclopedia of Cognitive Science: Change Blindness (Now dated, but this gives an account readable to a newcomer.)

When Good Observers Go Bad: Change Blindness, Inattentional Blindness and Visual Experience (A more academic read by one of the “founding” researchers of this topic; click on ‘PDF’ for full acticle.)


PPS: My Amazon order arrived yesterday, so now I have my copy of Best American Science Writing and few other books. Yay!