Change Blindness

One might think that humans and other animals would be good at spotting small changes in images, and this is true but only when the image is stabilised upon the retina. Watch a bird of prey hovering on a windy day, and you will see that while the body of the bird weaves wildly, the eyeballs appear locked in 3 dimensional space. The reason? To stabilise the image of the scene upon the retina. As a consequence of the evolutionary route through which our eyes evolved, they lack the ability to see a stationary image. If our eyes were to stare unmoving at an image, it would fade away within about 15 Seconds. In our normal life our eyes make unconscious movements (saccades), flicking around the scene to overcome this limitation.

The bird of prey (and many other creatures that pursue moving prey) capitalises on this. The overall scene fades away leaving any moving element highlighted. (For the bird this is likely to be a living moving source of food.) This enhanced sensitivity to changes in a familiar image does not involve the consciousness, but is instead a simple signal processing in the retina itself. As I am interested in measuring the rate that information reaches our consciousness using images, I must find a way to avoid the above scenario.

Change Blindness examples

We can eliminate the sensitivity of the retina itself to changes very simply, by briefly introducing a third neutral grey image while switching between the other two. Explanation here

Then it becomes very difficult to notice the changes. Try it