I've thought about this a lot.
With LSD I've noticed I have an ability to closely inspect my visual perception. I expect you're familiar with the visual step back that you can take with powerful psychedelics - where what you see suddenly stops being your totality of vision, and becomes an image within your vision. You become an observer of your vision - one facet of being tuned to the metaoperator channel.
I think that lots of OEV features are a conjunction of several phenomena, but I agree strongly with what has been said - psychedelics allow you to perceive the cognitive overlay being used to constantly evaluate the properties of your external environment.
At one level, I think that the threshold OEV features are derived from the cellular, honeycomb structure of the retina - this is what gives rise to so many hexagonal and geometric features. Alignment of the incoming image with this sub-grid allows the visual cortex to evaluate horizontals, verticals, perspective(Necker cubes are hexagonal in silhouette!) and symmetry. IME the mind stretches and bends the hexgrid until is matches the features in view, and derives information from the lcoation and degree of distortions.
At another level, and particularly noticeable indoors, where everything is with 0-25ft, I often see a web of fluorescent yellow snot coating everything - this is (IMO) the parietal lobe's way of visually representing distance derived from (broadly) stereoscopy and parallax.
At a further level, I have been able to observe the activity of my peripheral vision - as you know peripheral vision is largely a product of the 'imagination', constructed from expectation and memory on the fly. My direct observation of my peripheral vision seemed to reveal the visual cortext constructing an image by first edge detecting (high constrast), and mapping the broad shapes, then fractally space filling these areas with image patches grabbed from previous glances in that direction.
All of that was revealed to me sitting on the lip of a wooded valley, looking out onto the wooded hillside opposite. I've not tried the same observations at the other extreme, a solidly urban environment.
I think DMT takes this a stage further - whereupon you not only observe your seeing, but are able to visually observe your thinking as well. That's what happens when you're propelled through the screen your vision seems to be projected upon.
Of course all of this overanalysis doesn't detract from the beauty, fluidity and profundity of the strong visual experience!
