During my 2nd DMT session I had a flash of insight on what might be a physiological basis for some of the commonly experienced visuals. I'm new to the forum, so forgive me if this has been proposed/covered before. I didn't find any similar topics in the first few forum pages, nor on the CEVs wiki page.
Some friends of mine described DMT experiences of racing through a tunnel or tunnels; also certain visuals have a circular frame of reference and outer periphery. I wonder if the structure of the eyeball itself is a sort of scaffolding for some of these visuals. If I'm not mistaken, the pupils dilate and blood pressure typically rises during a trip. That could very slightly distort the eyeball. Unless one is in a darkroom, all available light will be amplified by the dilated pupils. Draw, or picture, the first-person view of a trip through a tunnel: round hole up ahead, lines of perspective on all sides converging toward the hole. That looks remarkably like the center of the eye itself: the pupil is the hole, the iris those converging lines. (Closely study your iris in a mirror - it's an interesting structure to look at, whether or not my idea here has any merit.) If the iris is being lit with sparklies (psychoactively generated, and/or assisted by the action of dilated pupils), it might go far toward creating a purely visual experience that the brain can misinterpret as moving through a tunnel.
Without the psychoactive properties of DMT the brain would not be fooled by the illusion, but put the two together and it could get interesting.
This idea is related to another CEV/phosphene concept I had a long time ago. I was recalling dreams in which I was either falling straight down, or levitating upward while looking down. I also recalled a particular phosphene, this one waking: particularly while dehydrated or tired, I would sometimes get a faint visual of the blood rushing to the center of my eye from the outsides; or perhaps from the iris to the pupil - I confess not to know the blood flow map of the eyeball, but I made an educated guess that's what drove the phosphene. The overall effect of this was a circle growing smaller, just about disappearing, and then another large circle appearing at the periphery and shrinking just as the previous one had. Other times it seemed backwards - a small circle grew larger, eclipsed the periphery, and another small circle was born in the center to repeat the process.
The brain operates a lot of other equipment in conjunction with vision, including the gyroscope, which senses up, down, and motion. My hypothesis about dreams of falling or levitating (but not flying directionally) was this: the phosphene driven by that blood flow in the eye was perceived in REM sleep by the brain; if I think in a dream that I am looking down, and I see a shrinking circle, the brain could mistake this for me rising in the air from a circular launch pad - it would simply mistake the circle for an object of constant size, and assume that I must be rising away from it, and then tell the gyroscope this, producing the feeling of levitating upward. Conversely, if I think I am looking down at a circle that is growing larger, and my brain mistakes the circle for being a fixed-size object, it will tell the gyroscope I am falling toward the circle, and I will feel like I'm falling in the dream.
I dunno, just a hypothesis. The third time I was coming down from DMT I seemed to have a bit of tunnel vision, clear in the center but more crystalline/translucent going on outside the center circle, which struck me as the pupil/iris configuration as if seen from further back in the eyeball.