I had this experiance many years ago. It is apparently something called "Migraine Aura". People who get migraines will experiance this prior to suffering a migraine headache. In some cases, people only experiance the aura, and not the headache. Turned out both my uncle and father experianced these, which is how I found out what was going on. It was a relief to find out it wasn't something more serious. Google "migraine aura" and you'll find a lot of info about this and can determine if it's what you're experiancing.
This is interesting but it isn't exactly what I am experiencing since it isn't only present when I look to the left or the right. Sometimes, I will wake up with the flashing light in the corner of my vision or it will come and go during the day. It is more visible when I look down and to the right but it stays for maybe a few minutes to an hour and then goes away. When the flashing light goes away, I will occasionally experience three quick flashes of light when I look down to the right and rarely when I look up to the left. It only happens in one eye. Interestingly enough, when I was in Italy on my honeymoon the dang thing went away. When I returned home, it came back. It's not as prominent as it was when it first started but it comes and goes with some days being worse than others.
I'm on the computer all the time (my job requires me to look at a computer screen for 8+ hours a day) and I often come home from work and go back on the computer for research, game playing etc...
If it is, as you say, pressure phosphene, when does it go away? Is it something I will have to live with for the rest of my life or does it usually go away in a few months, a year...???
pressure phosphene. many threads here about this. the short version is that a flash that ONLY occurs when you look in a certain direction is usually caused by the presure that your extraocular musculature exerts on the outside of your eye. totally harmless. assuming, of course that retinal problems have been ruled out, as you say they have...
http://www.medhelp.org/forums/EyeCare/messages/1052.html
http://www.medhelp.org/forums/EyeCare/messages/897.html
http://en.wikipedia.org/wiki/Phosphene