Choosing What We Perceive

Excerpt from A Natural History of Seeing  by Simon Ings:

In 1965 the Russian psychologist Alfred Yarbus reported the results of experiments that tracked eye movements. In some of them, he used Ilya Repin’s classic painting They Did Not Expect Him (aka An Unexpected Visitor, 1884).

“Hold your eyes still, and the image of a moving target is quite likely to flash by individual photoreceptors in milliseconds. If it moves past very quickly, your photoreceptors won’t have a chance to respond, and you won’t see the target at all. Even if it’s moving quite slowly, the target is likely to be blurred.

This is why humans fixate so strongly on objects moving across their field of view — if they didn’t, they wouldn’t be able to see them.

If you concentrate on a moving object, the eye will turn smoothly in an attempt to track it. The more you are interested in what you are looking at, the more it will try to hold on.

But there is a limit: move your pen too fast, and the image will blur. The moment that happens, the eye will give up its ‘tracking shot’ and revert to ‘angle shots’. It will saccade, snapping from location to location, capturing a selection of ‘stills’.

In French, le saccade, describes the way a sail snaps in the wind. There is nothing subtle about a saccade; nothing delicate. The movement is sudden, ballistic, and powerful enough that the eyeball commonly overshoots its target; a barrage of smaller correcting saccades follows any large eye movement.

The eye exists to detect movement. Any image, perfectly stabilized on the retina, vanishes. Our eyes cannot see stationary objects, and must tremble constantly to bring them into view.”