Not knowing where a person is looking makes us uneasy. For this reason, it can be awkward to converse with somebody with dark sunglasses. And it's why someone might wear dark sunglasses to look "mysterious." A recently identified visual illusion takes advantage of the unsettling effect of uncertainty in gaze direction. The "Ghostly Gaze" illusion, by Rob Jenkins (University of Glasgow, U.K.), was awarded the 2nd Prize in the last edition of the Best Visual Illusion of the Year Contest (held May 12, 2008, in Naples, Florida; http://illusioncontest.neuralcorrelate.com). In this illusion, two twin sisters appear to look at each other when seen from afar. But as you approach them, you realize that the sisters are looking directly at you!

The illusion works in this way: it is a "hybrid" that combines two overlapping pictures of the same woman. The two overlapping photos differ in two important ways: their spatial detail (fine or coarse) and their direction of gaze (sideways or straight ahead). The photos that look at each other contain only coarse features, and are thus best seen from afar. The photos that look straight ahead are made up of sharp details, which can be best seen at close range. When you approach the pictures, you are able to see all the fine detail, and so the sisters seem to look straight ahead. But when you move away, the gross detail dominates, and the sisters appear to look into each other's eyes.