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Thank you for your opinion, Dr. Rick. It certainly makes more sense than any explanation I have got so far. However, something still puzzles me seriously and stops me from taking your answer as relevant to what I was trying to describe. This type of "consultation" has its obvious limits, but still, I think with good fait on both sides, we should be able to understand each other.
So, here's the problem: I think you are referring to the sort of random "ghost" images that one can see after closing the eyes in a dark environment. I'm very familiar with that phenomenon, but this is NOT what I'm talking about.
Many things with the "dark current" hypothesis do not fit to what I am experiencing. First, I told you that it is NOT a random phenomenon, as one would expect from that residual electrical activity in the brain visual centers: I see a clear-cut, well-shaped yellowish and rather bright half-circle with my left eye only. One would expect that "dark current" activity to be a LOT, a WHOLE LOT more random than this !!!
I don't want to extend this to an endless discussion that would end up being very costly. So, unless you can propose me an alternative hypothesis that takes into account the problems the first one has, I will simply accept your opinion as a fair attempt to help me. You are right, I should persist and ask for an exam.
Please forgive me, but my mother had a detachment of the retina in one eye that happened rather abruptly. She told us about some warning signs that she had experienced prior to the virtually complete loss of her vision in that eye. I may be too cautious, but my mother's sad experience has raised my awareness to that possibility.
Thanks again for your honest answer!