Thanks so much for the link.
I had long assumed that it was due to an occurrence at that point, but I have looked for, and never been able to detect, any synchronization with the heartbeat. Neither can I guess what causes it to be insignificant most of the time now I'm older.
Just lately I had wondered if it was some kind of "cross talk" from the blood vessels where they run--in what also might be compared with electronics–as a twisted pair with the nerves, on their route to the eye. This is why I wondered if excessive flexing was contributory.
I too found it very tedious while staring into a microscope. But only on some days...I wonder if it was due to the occasional use of a lower colour-temperature in the light source.
I can't explain my ‘negative' symptoms all those years ago, except by a the brain's temporary reluctance to mask the signal. I recall being very concerned about it. One of those things that just go away.