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A "gravity hill" is one where a downhill slope appears to be uphill, such that an object such as a car will appear to roll uphill when in fact it is moving downhill.

This is described in a wikipedia article as a feature of the lie of the surrounding land.

What relevant feature of the land would cause this misperception/illusion in human beings? I assume it’s related to some aspect of vision, but would have thought our proprioceptors would feed us opposing information, inducing discomfort of some kind.

What is the biological basis of this illusion?

Edited to add my research: the phenomenon is explained here as a gentle decline between two steeper declines. The illusion is explained as comparable to the Zöllner illusion in which lines intersected by oblique line segments appear to tilt (Kitaoka, 2007; Oyama, 1960; Robinson, 1972/1988; Zöllner, 1860).

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The American Psychological Association’s definition of the Zollinger illusion is

a visual illusion in which parallel lines appear to diverge when one of the lines is intersected by short diagonal lines slanting in one direction, and the other by lines slanting in the other direction.

The illusion in 2 dimensions is clear. But how does that translate into the 3D illusion? Are the “parallel lines” the edges of the road? Or is the road the line (visually implied) transacted by oblique angles of the landscape?

If That question is confusing, it’s because I am confused. Sorry.

This is not original to me but the original poster chose to leave in a huff. Now I really want to know, because when I was a girl, my mom told me great stories about the strange things she herself had experienced. One of the stories involved being pulled uphill in a car placed in neutral at the bottom of a hill. TIL… it was true, for her at least!

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    $\begingroup$ I don't have any sources, but usually these sorts of things in the outdoors are related to the layout of the landscape, perhaps with a horizon that appears horizontal, but isn't actually, or maybe with a horizon that is steeper than your brain interprets. I've been on one in a "puzzle world" with a chair that rolls "up" the slope, but it's actually that the wall decor and floor are sloped. $\endgroup$
    – bob1
    Commented Jun 29, 2022 at 20:09
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    $\begingroup$ I recall doing instrument flying (IFR) in pilot school and how difficult it is to maintain spatial orientation without a visual horizon. Under these conditions, it is actually impossible to maintain level flight without reference to the instruments, especially the attitude indicator/artificial horizon. I have heard of plane crashes caused by instrument failures during IFR flight: the pilots thought the plane was straight and level when in fact it was in a banking dive. I don’t know if this is related, but it seems like it could be. $\endgroup$
    – canadianer
    Commented Jun 30, 2022 at 5:17
  • $\begingroup$ @bob1 - I didn’t consider the horizon. I just assumed (a dumb thing I often do) it would be hilly. Good point. $\endgroup$
    – anongoodnurse
    Commented Jul 1, 2022 at 1:25
  • $\begingroup$ I've found some footage of the one I went to here (youtube; timestamped, but it's only 2min anyway) with a couple of similar illusions. Not easy to see what's going on though, other than a sloped floor and ceiling. $\endgroup$
    – bob1
    Commented Jul 6, 2022 at 10:05
  • $\begingroup$ @bob1 - write this as an answer! It may not say exactly what is going on. But your comments do help (as did canadianer’s) and the bounty should go to one of you! $\endgroup$
    – anongoodnurse
    Commented Jul 6, 2022 at 10:20

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I don't have any proof, but these illusions are typically by visual effects, much like was postulated in the question.

Basically it is a matter of subverting the expectations of the viewer. In most cases this seems to be through a horizon that the viewer interprets as horizontal, but isn't.

An couple of examples of this can be found at Puzzling World in Wanaka in New Zealand, where there are illusions of balls rolling up a pool-table, and a rail (with chair) that the chair rolls "up". Footage of these can be found at this timestamp on Youtube. In both these cases, the floor and/or ceiling are sloped, but the eye interprets them as horizontal (I think through expectations/familiarity).

As commented by @canadianer on the question, pilots doing instrument flying find it difficult to maintain spatial orientation and level flight if their instruments fail and tend to bank the plane, and in some cases dive/rise, when flying without artificial horizons.

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  • $\begingroup$ The YouTube video is impressive! The brain is a funny thing, and I think you’re right when you say familiarity is behind it. Much more simple example of familiarity is interpreting a broken circle as a circle still. No reason to think illusions can’t be very complex. $\endgroup$
    – anongoodnurse
    Commented Jul 6, 2022 at 16:51

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