blelb The visual perception lab
 

Colour illusions with Spot 08

Experimental procedure
The square contains 64 repetitive modules with four square-shaped elements of different colours and similarly to Spot 04 shows what is known as colour harmony, which changes dramatically as soon as the elements are distorted either horizontally or vertically.

Constant juxtaposition of colours
A physical eye could only detect black, cyan, yellow and red in the entire picture. The human eye, however, sees violet-red instead of red if the boundary line to the cyan is lengthened and apparently sees orange if the boundary line to yellow is dominant. Colour juxtaposition remains constant in this experiment.

Constant element geometry
The following interactive image generator is a supplement to Spot 08 and deals with the second cause of Bezold's illusion: element geometry is kept constant and the colours of the neighbouring elements are permutated. Another example of a manipulation of this kind is the pair of pictures (19, 20) in the supplements to Spot 01.

Colour logic of element juxtaposition
If the colours black, cyan, yellow and red are chosen, it is easy to discern slightly different shades of black, cyan, yellow and red in the 6 fields that are generated with their 36 repeating modules. The colour illusions are no longer as dramatic as they were in Spot 08, but they are no less interesting. The black patch is placed in the top left position of the four elements in all the fields. The second cyan patch has just 3 different positions available to it, while the red patch has only 2 locations open to it once the second patch has been positioned. The fourth yellow patch is compelled to take the last position remaining in the module. This produces 6 different modules and thus 6 different fields are generated.

Colour logic

Colour illusions
It is evident that three of these square fields differ relatively strongly, while the other three look like twins. This peculiarity is easy to explain: the two dark elements black and red can be positioned in the module either diagonally, side by side or one under the other. The lightest element yellow can be positioned either diagonal to the darkest or not diagonal. This results in three very different possible patterns. For each of these three there is a twin, in which only the horizontal juxtaposition of elements is swapped with the vertical. We call this weak effect horizontal-vertical illusion, which has already been defined in Spot 01 in the discussion of the image pair (17, 18) and interpreted there: on the one hand, the brain has various rapid reading programs for horizontal and vertical structures*), and on the other, most eyes have a slight degree of astigmatism, which also produces distortion of the images.

The Bezold effect
The changes in colour cast in the three strongly differentiated fields is known as the Bezold effect in its pure form; this continues to be a neurological puzzle even today. One prerequisite for this effect is that the colour elements are big enough when projected onto the retina that the colour contamination at the boundaries of the elements as a result of the limited resolution cannot be the cause of the illusion. In this latter case, we speak of the assimilation effect, which we present in Spot 09.

*) Vertical structures are read stereographically and provide vital information on distance. Unstructured horizontal elements are not seen spatially due to the horizontal position of the two eyes. Perception of images presented horizontally is different from that of images presented vertically. To see this, compare two identical images when they are swivelled through 90 degrees (e.g. pictures 17 and 18 in the appendix to Spot 01).
 
07/11/2003