Hermann Grid Illusion

In the Hermann Grid Illusion, the white dots at the centre of each square seem to shift from white to grey.

What Do You See?

The Hermann grid was first discovered by a physiologist named Ludimar Hermann in 1870. When the viewer looks at the grid, the white dots and the centre of each 'corridor' seem to shift between white and grey. When the viewer focused his or her attention on a specific dot, it is obvious that it is white. But as soon as attention is shifted away, the dot shifts to a grey colour.

How Does the Hermann Grid Illusion Work?

So why do people see grey where there should be white? Why do we see something so different from reality?

Researchers have traditionally used what is known as lateral inhibition to explain why people see these grey areas. This phenomena demonstrates a very important principle of perception: we don't always see what's really there. Our perceptions depend upon how our visual system responds to environmental stimuli and how our brain then interprets this information.1

However, there is evidence suggesting that this explanation is likely inaccurate. The fact that the illusion is not dependent upon size, can be seen with contrast reversal and can be negated by slightly distorting the lines have been cited as reasons why the classic theory is wrong. One possible explanation that has been proposed is known as the S1 simple-cell theory.2

source http://psychology.about.com
image of Hermann http://commons.wikimedia.org