Coutesy: New Scientist Magazine.
Psychologist Alan Stubbs has large prints on his office walls that often catch visitors off guard: by glancing at the images the whole area seems to light up. Now you can experience the same illusion by looking at the images below. As you stare at each image, move your head towards the screen and then back again. The bright spot should appear to spread quite dramatically.
This illusion, discovered by Alan Stubbs at the University of Maine and Simone Gori at the University of Padua, Italy, contradicts the way we typically perceive size and brightness. When we move closer to an object, it appears to be the same size even though the size of the image on our retina has changed. Similarly, the brightness of an object usually remains constant despite changes in illumination. Since we’re typically looking around at a 3D environment, our brain automatically makes these adjustments to help us make sense of our surroundings.
So why does this illusion break these rules? Stubbs and Gori noticed that the edges of the bright spot have to be blurry for the effect to occur. "One possible explanation could be that our visual system cannot bring the blurred boundaries into focus and for this reason the correction of the brain for size constancy does not occur," they write. Another possibility is that the brightness gradient suggests to our brain that we are looking at a 3D tunnel, with a light at the end. In this case, it’s simply an error of interpretation and our brain is behaving as if it was presented with the actual scenario.