Lets actually talk about the brain for a minute before we give up on it. I'm no brain expert but I have an avid amature interest in brains and I don't see how it's impossible. Here's a few theories:
Our brains are modular composites of parallel processing units. Two really important units are the thalamus and the cortex. Sensory information from the environment is brought into the thalamus and processed, mixed with motor information and memory and sent to the cortex where awareness likely resides. Everything is mapped in parallel, so there are visual bits of the thalamus that project to visual parts of the cortex and auditory thalamus to auditory cortex et cetera. There are even really specialized mappings with corticothalamic feedback loops for written language or spoken language.
This constant, exquisitely balanced feedback between the cortex and the thalamus is mediated by serotonin signaling. Most of the actual communication between these modules relies on glutamate but the feedback is balanced by serotonin.
Psychedelics like psilocin and LSD are known to directly muck up cortico-thalamic feedback by interacting with serotinin receptors. If you change these molecules so that they no longer bind serotonin receptors they are no longer active.
Acute high doses like easily achieved with vaporized DMT may almost abolish normal feedback loops in the brain. I think under these states the visual cortex receives unfiltered information from all over the brain, even other parts of the visual cortex! TMKs hypothesis that you can see your sense of syntax may have a lot of truth.
Abstract 4 and 5 dimensional geometry is totally consistent with unregulated intercortical feedback within the visual cortex. Loops nucleate slow and play out rapidly with almost no dampening mechanism at play. Similar feedback loops are apparent in high dose psilocin visuals but are more dampened and can even be brought under the control of consciousness, to a certain extent.
Realistic visions of people and places with eyes closed may be spurious input from the hippocampus. Abstract symbols may be the language parts of the cortex firing on it's own or maybe it's receiving input from the auditory thalamus. Visuals cause extreme activation of the hypothalamus and are so infused with emotion.
Feebdack blown away and crosstalk takes over. it is not hard to imagine the mechanisms that might be at play. It takes a little work to learn about the brain but it is rewarding. makes trips better, less scary in a way.
Think about it: would you rather be kidnapped and probed by an inexplicable alien intelligence from dimension X or would you rather expose your visual cortex to direct unfiltered information from the core emotional parts of your ancient lizard brain? Both are difficult experiences but the latter may be much easier to integrate.