The eyes of some pictures seem to follow you around the room, like those of the famous WWI recruitment poster which helped garner almost 3 million volunteers in two years:
Try it. Get up and look at your screen from the side. Is he still looking at you? He should be.
Recently published research in the journal Perception  discusses how this effect works. The story was covered in the press (e.g. here). Turned around into a ‘how to’ rather than a simple ‘explanation’ it’s perfect material for a hack. I saw it too late to include in the book so I’m putting it here.
So here we go: How can you design pictures of faces with eyes that will follow you round the room?
The answer is simple: photograph, or paint, the face looking straight out. If it’s a photograph they must look straight at the lens of the camera. In the words of James Todd of Ohio State University, one of authors of the study, ‘If a person in a painting is looking straight out, it will always appear that way, regardless of the angle at which it is viewed’
How does it work? First of all, this is only possible because pictures and paintings aren’t 3D. They are semblances of 3D on a flat surface. This stops our brains calculating depth by comparing the images in the two eyes (how our brain calculates depth in images is covered in the book). Instead, our brains rely on other cues to depth, such as shading (the use of shadows to imply depth) and movement (all this is also covered in the book).
The explanation lies in how we interpret three-dimensional objects portrayed on a flat surface. Real three-dimensional objects look different depending on the angle because of the changing way light falls across them. But on the flat canvas, shading and light are fixed and the image looks the same from every angle. If the face is looking straight out from one angle, it will appear to be looking straight out at whatever angle it is viewed at.
In fact the only clue that the object in a picture isn’t really looking straight out is that the near side of an object should get smaller if you look at it from one side. This doesn’t happen in a natural way with a painting. Theoretically your visual system could use this information to figure out that pictures of objects aren’t real and thus the eyes aren’t really following you around the room, but it appears that they don’t. The contradictory information is either overridden or disregarded.
If you want look at the original paper you see that the how-to-make-eyes-follow-you-around-the-room result is actually more of side-issue of the main thrust of the paper – which is a discussion of the visual mechanisms behind and correct interpretation of effect.
But the best thing, and the thing which I didn’t see picked up by any of the press, is that you can do the trick with any object which has shading, and that for their investigation the authors used a statue showing a woman’s bum.
Hilarious! Why did no one mention it in the press?
And it works, too, look:
They’ve made a picture of a bottom that follows you round the room. Ain’t science great. (I’m absolutely convinced that more psychology should involve the analysis of naked bottoms.)
1. <a href="http://www.phys.uu.nl/~wwwpm/HumPerc/koenderink.html
“>Koenderink J.J., Doorn A.J. van, Kappers A.M.L., Todd J.T. (2004). Pointing out of the Picture. Perception, 33, 513-530. Here for subscribers