Artists use the technique of “equiluminance” to blur outlines and suggest motion. We cannot perceive the edges of objects where object and background have the same luminance. If parts of a painting are equiluminant, their positions become ambiguous. They may seem to shift position or to float.

Equiluminant colors have special properties. They can make a painting appear unstable. Adjust the colors in the painting above. Somewhere in the middle (the exact point varies among computers), the shapes may appear jittery.

The red and blue seem to move around because they are equiluminant. The “What” system sees the shapes because of the strong color contrast, but the “Where” system can’t because the colors are equiluminant.

An object that can be seen by both subdivisions of the visual system will be perceived accurately. It will appear to move correctly or appear stable and appropriately three-dimensional. But if the two subdivisions are not balanced in their response to an object, it may look peculiar. For example, an object defined by equiluminant colors can be seen by the What system but is invisible (or poorly seen) by the Where system. It may seem flat, it may seem to shift position or it may seem to float ambiguously because there is too little luminance contrast to provide adequate information about its three-dimensional shape, its location in space or its motion (or lack of motion). Conversely, something defined by very low contrast contours is seen by the Where system but not the What system and may seem to have depth and spatial organization but no clear shape.

Equiluminant colors have long been recognized by artists as being special because they can generate a sense of vibration, motion or sometimes an eerie quality. This strange quality arises because the What system can see something that the Where system cannot; with only What system activation in isolation we can identify a particular object, but its position and motion (or lack of motion) are undetermined.

Use of equiluminance in painting can make sunsets twinkle and flowers shimmer. We will explore this effect in a series of paintings from Claude Monet, Piet Mondrian and others.