Thanks everyone, I would love to be quicker with my replies but my skills in typing on the phone are pathetic and free time at the computer is an endagered species in my life.
I have read before about those spots where people can get confused about whether they are going uphill or downhill, but I have never been at one so never experienced anything of the sort. Also, I have seen many fantastic optical illusions - printed or on the screen - and my brain has been fooled countless times (and I am not even including my son's shenanigans), but those somehow feel different because I know someone has made a concerted effort to construct something with a singular purpose, which is to fool human brain. Stumbling upon something like that out there, in nature, feels different, if for no other reason then because it is totally unexpected.
What interests me about those locations is if people's perception changes when they turn around and look in other directions. I didn't find that answer in their stories and couldn't deduce it. The way I understand it, it is the shapes and slopes of/in the surrounding terrain that fools your brain into thinking that the road is descending when it is ascending, so it is a general observation which does not depend (much) on your exact position. Not much should change if you look around a bit (but probably not just everywhere). In my case, there is a point in my surroundings where perception changes suddenly and very sharply, and that point is exactly where I am standing. And it moves as I move. And it is not alcohol, acid or mushroom induced (maybe this is a sign that I should start, I could have lots of fun). I would say it's about 10% difference in gradients that I see in front and in the back, and the change starts right at my feet (or bicycle) and extends to the front and back.