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THE LAST WORD

See the lightI was on holiday in Jersey and visited Mont Orgueil Castle in Gorey. In one of the rooms there was a strange light display. The room was darkened and there were small spots of moving red light projected on the floor, walls and ceiling. There was also a vertical white light. It didn’t seem particularly impressive until, as you glanced away from the vertical white light, in the corner of your eye it expanded briefly into a picture of Queen Elizabeth II. When you looked back, it was a simple vertical white light again. Not everybody could see the picture, but probably around two-thirds did. What was happening, and how was the picture produced?

n The picture of the queen has been divided into vertical strips and these are projected in quick succession onto the same bit of wall. When you look directly at it, all you see is a bright vertical strip

because each strip is overlaid on all the others. However, when you glance away, your eye is also moving so each strip hits your retina adjacent to the previous strip and your brain then fuses them into a single image of the queen. If you tilted your head through a right angle, you would have to flick your eyes up and down (relative to your head) to

see the image and it would also be rotated through a right angle. The moving red lights wouldn’t appear to have much to do with the queen’s image, but are there perhaps to encourage you to move your eyes so that you see it.Terry CollinsHarrogate, North Yorkshire, UK

n The more interesting question is why some people saw the image and some did not. Your visual system has two different modes for moving your eyes around, each with its own control circuitry in the brain. The first is called “smooth pursuit”, a fairly self-explanatory name. The second is called a “saccade”, the motion by which your focus jumps from one place to another. You make these motions constantly – usually several times a second, as you move the high-resolution centre of your retina to point at different parts of a scene and build up a fully detailed picture of it. Normally you are unaware of these movements.

In fact, a great deal of neural processing power is dedicated to keeping you unaware of saccades so the brain can create a smooth picture of the world. Think of the lurching appearance of handheld video, and now imagine the camera is being deliberately jerked around to point at different targets, several times each second. One of the ways that your brain avoids the nauseating disorientation this would cause is to stop registering visual input during a saccade. This will usually

prevent you seeing the image while looking directly at it. But peripheral vision is less strictly filtered because it’s important to notice when something is approaching you, which is why you were able to see the image “in the corner of your eye”.

The most likely explanation for why some people could see the image and some could not is that it’s actually very difficult to make yourself look at something with your peripheral vision. When you direct your attention to something peripheral, your brain automatically plans and executes a saccade to it to get a clear, detailed view. Steve GisselbrechtBoston, Massachusetts, US

n The image is broken down into vertical stripes from left to right, which are shown on top of each other one at a time. You can see this if you can process information from the eye during a saccade. Many people’s visual cortices can be fooled into seeing this “mid-movement” information in dark surroundings.

I am especially interested in the phenomenon as I can do this easily in normal lighting but have yet to meet anyone else who can. It made the world an interesting place as a child: if a television programme was dull I could make

the presenter fat by scanning my eye upwards or tall and thin by going in the other direction. It must be noted, though, that this only works with a cathode ray tube television. Fluorescent strip lights go from black to yellow to white and back again following the mains 50 hertz frequency when laid out like a stripy scarf in front of me and the LED boards in train stations break up into strange bundles of light that give clues about the display algorithm used to drive them.Jeremy RichemontLondon, UK

This week’s questionsBuzz offI have the back door open and a bee has entered my kitchen and is buzzing around. Is the best course of action to leave the door open and hope it goes out, or is there a greater chance of more coming in?Simon O’CallaghanLondon, UK

Train Brain drainIf I don’t know the time of the next half-hourly train to depart from my local station, is there any point in running to the station? My wife tells me it won’t make any difference to whether I catch the next train or not, but I insist it will. I must admit, my journey time door-to-door does not seem to be affected by the speed of my approach to the station.Jeremy BrowneElm Park, Essex, UK

“if you turned your head through a right angle, you’d have to flick your eyes up and down to see the image”

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“it’s actually very difficult to make yourself look at something using your peripheral vision”

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