Some of our pax in the North Sea used to experience this on Chinook flights when sitting next to the window on the sunny side. It is related to epilepsy I believe; some people are more prone than others. The frequency of the flicker ing sun through the large, slow-moving rotors seems to upset the brain's natural frequencies - alpha waves I believe; the seizure that follows is extremely alarming to both the victim and to those sitting nearby. You could say that this might happen to anybody who sat in a Chinook for five hours but that would be unkind. I never saw it on the 61 or the 76; nor on any Bell I flew. Don't know if it happens on military Chinooks.
The pilot can remove the source of the problem simply by moving the pax to another seat or by changing course to take him out of the sun. Usually these guys don't get to fly again once their companies hear of an occurrence; sad for them as it affects their job.
For aircrew the matter would be serious though; I remember the Dutch RLD tests pilots for sensitivity to flicker effect as part of the initial aircrew medical. I don't recall the CAA doing this.
Any flying AME out there? Roger Green or anybody?