Lift the Strip!

We are very sensitive to small differences in lightness when they occur across a sharp boundary.

Click the image above. The strip in the exhibit obscures the boundary between two uniform areas of grey that seem to be very similar. Only when the sharp boundary is exposed we realize that their lightness is in fact quite different.

Near the boundaries one can observe an additional illusion: the darker surface seems darker near the edge (compared to the rest of the dark half), and the light surface seems lighter.

Illumination is seldom uniform, and is generally graded. This gradation gives little information about the object illuminated, so we have become insensitive to it. Instead, we are very sensitive to small changes in lightness if they occur at a sharp boundary, as this is unlikely to arise from illumination. Accordingly, when we see two areas that differ only a little in lightness and are separated in space, we judge them as being of similar lightness. When the sharp boundary between the two is exposed, we immediately see that we were wrong, and that there is indeed a significant difference in lightness between the two areas.

Interestingly, as soon as we see a difference across a boundary, we extrapolate this difference to the rest of the areas, since our default assumption is uniformity.

The mechanism for this 'salience enhancement' is lateral inhibition, a neural process that takes place primarily in the retina. A stimulus at any point in the visual field inhibits or reduces the responses to nearby points. The strength of the inhibition depends on the stimulus strength, so if two adjacent points have different lightness, the brighter one inhibits the darker one more than vice versa. The result is that the difference between the two is enhanced.

The effect has a finite range. As a result, the response to the band of points near the boundary, on the darker side, is more depressed than points further from the boundary, resulting in the appearance of a band darker than the rest of the area; a similar effect leads to a light band on the lighter side of the boundary.