Here again, just as in the simple opposition of light and dark, the perception of coloured after-images is connected with a breaking-down process in the nerve region of the eye, and a corresponding building-up activity coming from the blood. Only in this case the eye is not affected by simple light, but by light of a definite colouring. The specific destructive process caused by this light is answered with a specific building-up process by the blood. Under certain conditions we can become dreamily aware of this process which normally does not enter our consciousness. In such a case we see the contrasting colour as coloured after-image.