Seeing black spot in vision. Advise.

For the last couple of months, I'm having some kind of visual disturbances in my eyes. I'm seeing a black spot in my vision which is coming and going lasting for 3 seconds, then a single flickering spot sometimes, then seeing a kind of white image with little brightness in different shapes, then a black shadow same like the white image I mentioned etc in either of my eyes. I have previous history of migraine as well as seeing floaters too. I've consulted an ophthalmologist and have undergone various tests like vision test, tonometry, slit lamp, fundus examination,HFA, ERG, Autofluorescence etc and couldn't find anything wrong. My doctor concluded that it's migraine aura. I'm taking medicine also for migraine like Tryptomer, Betacap tr 80 and Sibelium 5 mg. So now I have no headache but still frequently seeing those visual disturbances especially a black spot. These are more visible in dim light almost all the symptoms. Kindly advise.