In other words, 99% of people don't know that.

Yeah, that's true.

I just would have thought that people like Ziz and their followers, who ostensibly think of themselves as "rational" and are interested in how the brain hemispheres really work, would have searched for even superficial-level information like "each eye feeds both hemispheres" that supports—or contradicts—their mental model about the brain.

Or not. Zizians, like all of us, are only human. They might be in a cult, but, while I here cast the first stone, I'm sure have I too have erroneous beliefs that could be corrected with just "superficial-level" expert information in other domains.

I just hope that I'm not overconfident about my own errors. An even bigger problem than known ignorance in a knowledge domain is overconfidence in that domain. If I forget how to calculate a cone, and someone explains to me that "the volume of a sphere is a cylinder's minus a cone's", I ought not to be that surprised, because I shouldn't have held a strong preexisting erroneous intuition about this thing in the first place. In contrast, a Zizian, I imagine, might well be surprised (or contrary) that "blocking your left eye's vision reduces input to both hemispheres evenly", because they overconfidently held a strong (and wrong) belief about basic brain neuroanatomy and invested a ton of time and pain into practices based on that belief. While figuring out their "unihemispheric sleep" practices, they should have actually done the research...or maintained a healthy amount of "I don't really know much about that field right now".

The relationship between the sphere, cylinder, and cone is still pretty cool, though.

I appreciate this article because it correctly characterizes how both hemispheres are involved in both eyes' processing, a fact which is not known enough and which is probably very important to stereopsis and binocular vision.

For those who like technical terminology and Greek, the medical term for when you lose the one side your left and right eyes' vision—is hemianopsia (hemi- "half" + an- "not" + opsia "seeing").

There are several types of hemianopsia depending on how far upstream or downstream the nerve damage occurred between your eyes and your cerebrum. When it happens because one of your cerebral hemispheres got damaged, then you get homonymous hemianopsia (same-sided hemianopsia):

• If your right hemisphere's visual cortex gets damaged, then the left sides (left hemifields) of both of your eyes will stop working.
• If your left hemisphere's visual cortex gets damaged, then the right hemifields of both of your eyes.

Here is a picture on Wikipedia of what your vision looks like with left homonymous hemianopsia (i.e., when your right hemisphere's visual processing is not working).

This weirdness happens because of the way the optic chiasm of your optic nerves works:

• The nerves from the inner, midline/nose-sided halves of your retinae (your eyes' "nasal" hemifields) cross sides in the chiasm:
  • Your left eye's nasal hemifield (the right half of your left eye's vision) crosses over to your brain's opposite, right hemisphere.
  • Your right eye's nasal hemifield (the left half of your right eye's vision) crosses over to your brain's opposite, left hemisphere.
• But the remaining, outside, temple-sided halves of your retinae (your eyes' "temporal" hemifields) do not cross sides in the chiasm.
  • Your left eye's temporal hemifield (the left side of your left eye's vision) travels to your brain's left hemisphere.
  • Your right eye's temporal hemifield (the right side of your right eye's vision) travels to your brain's right hemisphere.

Here is a picture on Wikipedia of how your optic nerves cross over (for your eyes' nasal hemifields) or don't cross over (for their temporal hemifields).

And here's a picture from Polaski and Tatro, 1996 of how damage at various points in the chain of optic processing causes different types of hemianopsia.

Using this, doctors can predict pretty well where in someone's brain has a stroke happened by what kind of hemianopsia the patient has.

(If damage occurs at the chiasm itself, then you can get "heteronymous hemianopsia" that still affects both eyes but causes blindness in opposite sides. If damage occurs upstream of the chiasm, i.e., at one eye's optic nerve or retina, then you're blind just in that eye. And, if you damage even smaller parts of your optic tracts, you can get specific types of quadrantopia (quadr- "quarter" + an- "not" + opia "seeing"), which is basically just hemianopsia but smaller. As with before, if you switch off a tiny part of your left hemisphere's visual tracts, you might become blind in the right-upper or right-lower quarters of both of your eyes.)

In any case, like the original post says, closing one of your eyes never suppresses the visual processing of one of your hemispheres, because each eye's processing is split between both hemispheres. I don't know if this hemispheric sleep theory is really something that Ziz or Zizians believe, but the way the optic chiasm actually works is pretty basic college-level physiology that's been well characterized for more than a century.

While I was reading about Monty Python on The Guardian, I saw this article: "Killings across three states shine spotlight on cultlike ‘Zizian’ group: Police search for member currently on the run from charges linked to homicides across the US". I dug into what that was all about and found this post.

So, in case anyone here was interested, these "Zizian group killings" have made international news.

Edit: Stories about the Zizian murders have also appeared now in Wired and The Wall Street Journal (and the WSJ Podcast).