In addition to this article, and due to the discussion that followed about the relation between phosphenes and HI, I would like to add here some excerpts of a text I just found.
It has been written by Yves Delage in 1903 (the translation -sometimes summarizing- is mine). Yves Delage is a french zoologist who is also known for his interest in dreaming and particularly for having described lucid dreaming in 1920.
By explaining the phosphene/HI relation in a different way, this text may add to my article.
In this first excerpt, Delage describes what is an hypnagogic image:
“An object appears suddenly, generally the figure of a man or an animal, with a perfect accuracy of contour. The image has no frame and it is not surrounded by other objects depicting a background: it is isolated on a uniform, usually black background.”
In this second excerpt, Delage explains what may be observed when the HI disappears and the relation between HI and phosphenes. Delage refers to phosphenes as “entoptic lights”:
“When the hypnagogic image vanishes, the entoptic lights appear; by their compliance of color, position and distribution, the colored spots fits the position and color of the parts of the vanished image. It follows that entoptic lights can be the basis of hypnagogic images and that it is not necessary that they have a close resemblance with them. Now, far from it that the entoptic element makes the entire image: it is supplemented by a mental image which has the largest part in the constitution of the total image. The first is a vague outline, without contours, without details, while the second adds what is missing to the first in order to form a perfect representation of the object.”
This understanding can be usefully compared with the two illustrations at the end of chapter II, as well as comments about phosphenes and HI being sorts of two different layers.
Delage Yves (1903). La nature des images hypnagogiques et le rôle des lueurs entoptiques dans le rêve. Bulletin de l’Institut général psychologique, août-septembre 1903, p. 235-247.[/size]