There are 2 separate problems that can both be caused by bright sunlight shining through the disc. Quite a few of the posts confuse the two.
The first problem is induction of an epileptic electrical activity in the brain by a bright flashing light. Some individuals (the CAA quotes 0.5% of applicants) are susceptible. The usual frequency range that causes problems is 1 to 30 Hz and in the UK an EEG with photic stimulation in this range forms part of the initial Class 1 medical exam. You're either susceptible or you're not. If you do get epileptiform waveforms, you have a greater than 1% per annum chance of having a seizure and are therefore not considered medically fit.
The second problem is a separate one and is nothing but an optical illusion that can be very disorientating. As the sun shines through the disc, the eye sees a large shadow sweeping across the visual field rapidly and repetitively. The brain "believing" that the visual field is moving across the retina interprets this as meaning that the body is moving in the visual environment (rather than the visual environment is moving across a steady body). The conflict between this and other cues to spatial orientation causes a disorientation that is true flicker vertigo.
You can demonstrate the effect by getting someone to stand infront of a disc that is painted with a pattern. If they're close enough to the disc such that the pattern fills most of their visual field you can get them to fall over just by rotating the disc. Great fun and oft demonstrated on RAF medical officer courses.
We can all get flicker vertigo in the right circumstances and unless it occurs in on individual on an unusally frequent basis should not be cause for disqualification.
FOM