'Silencing' is an illusion that happens when an object that is moving is also changing it's appearance. Normally we can immediately tell if an object becomes lighter or darker, if it changes colour, or if it changes shape. However a recent paper by Jordan Suchow and George Alvarez (Suchow and Alvarez, Motion Silences Awareness of Visual Change, Current Biology (2011), doi:10.1016/j.cub.2010.12.019) of Harvard University, published in January 2011, shows something quite extraordinary.

They created a number of videos in which an array of dots change their appearance. In one video, the dots keep changing their colour, in another, they keep changing their shape. Initially this is very easy to see. In fact, it is completely obvious what is happening. What happens next however is amazing.

The pattern of dots starts rotating. Play the video below, staring all the time at the little white dot in the middle. Initially all the dots are changing colour. What do you see happening to the colour of the dots when they start to rotate?

Motion silences awareness of color changes from Jordan Suchow on Vimeo.

For most people, when the dots start to rotate, they either completely stop changing colour, or they hardly change colour at all. However the reality is that the dots are still changing colour, just as they were before they started rotating. Even when you know that the dots are changing colour, it does not affect the way you see the illusion.

The authors explore various hypotheses as to why our visual system is fooled. It seems that the faster the movement, the greater the degree of silencing. It seems that movement of the image across the retina is the key factor - if instead of staring at the white dot in the centre of the screen, you watch a particular dot as it moves around the screen, you will then see that it is changing colour, or changing shape, or whatever the change is.

So when an image of an object moves across our retina, our ability to detect changes in that object is substantially diminished.

You can see a range of different videos created by Jordan Suchow and George Alvarez here.

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