Lightness is Relative

Which side of the grey strip is lighter? You may not believe this, but the stripe is uniform.

Our vision judges lightness of an area in a relative manner, by comparison with adjacent areas. Thus the stripe is perceived as darker when presented on a light background and vice versa.

The strip's lightness is actually uniform along its length. The graded change in the lightness of the background makes the strip appear to be graded in the opposite direction.

This seems to be an effect of lateral inhibition. This is the suppression of sensitivity to a stimulus by a visually adjacent stimulus. It probably occurs in the retina. The result of this mechanism is that the apparent lightness of an area is always reduced by stimulation of adjacent areas, but this reduction increases as the lightness of the adjacent area increases. This results in the illusion: When the background is dark, the apparent lightness of the stripe is only slightly reduced by lateral inhibition, while a bright background strongly reduces the apparent lightness of the stripe.

Actually, the lateral inhibition has a limited range, as we see in the Lift the Strip exhibit, but it is enough to cover the width of the stripe.