To elaborate - people talk about “pathways” in the brain. This does not have a very precise meaning, but it generally refers to how different parts of the brain are activated when you’re performing different tasks, or evaluating different kinds of information. In the context of vision, there are all sorts of processing tasks your brain performs automatically without your conscious control, effort, or awareness. Your brain automatically fills in the image when you blink, for instance, automatically fills in the rather large blind spot caused by the optic nerve attachment to the retina, and automatically processes the geometry and extent of objects even when you look at unmoving, 2D scenes. Have you ever wondered why, even with one eye closed, you have an innate awareness of the size, shape, orientation, etc. of all the objects in a scene, as opposed to just seeing a mess of colors? That is this part of the brain at work. Optical illusions all work by fooling this part of the brain, and the best illusions fool parts of the brain you can’t consciously bypass. anyway…
The “uncanny valley” refers to the fact that people find it disturbing to look at replicas of human beings that are highly realistic but not quite perfect. Of course, people have no problem looking at highly stylized replicas of human beings, nor at perfect replicas, but as you go up in accuracy you pass through a close-but-not-quite-perfect region in which the replica looks indescribably revolting.
Wikipedia as always has a good description and examples, although some of their proposed explanations for the effect are a little wacko. Uncanny valley - Wikipedia
The theory proposed by certain experts on human vision, greatly simplified, is that, if a replica is sufficiently different from a human, your brain processes it using perceptual pathways that it uses for other inanimate objects. As the replica becomes more human-like, the perceptual pathways that are used to handle humans begin to become activated, and these unconsciously pick up on all the subtle ways in which the replica is “wrong” or inconsistent with past experience.
I suspect the “soap opera effect” has a similar basis. When you are a watching an ordinary 24 fps movie, the motion is sufficiently different from reality that some of the normal pathways your brain uses to process a moving visual scene are not activated. After all, your brain evolved for billions of years to process motion at the fastest rate the eye can see, and the subtle stutter-step of 24 fps movies has no connection to anything in nature. Various specialized, high-speed processes in your brain, that are normally inactive when staring at still images, remain inactive when staring at a quasi-rapid succession of still images.
A high-frame-rate movie is faster than your eye can detect (or very close, anyway), and this is enough to activate some additional parts of your brain that are normally suppressed when watching low-frame-rate movies. Many of these are probably related to the interaction between motion and movement, and motion and lighting, and are normally suppressed because 24 fps stutter-steps is not the input they evolved to handle over billions of years. 48 fps is fast enough to at least partially activate them, and so these other pathways activate, and you notice all the things that are “wrong” in various ways - armor that is supposed to be solid and heavy but actually flexes slightly when the actor moves, a piece of “wood” in the background that actually interacts with the light more like the piece of painted plastic it really is, etc.
