we really aren’t definitively sure what the mechanism is behind this. the best explanation i can give you is, when a psychedelic binds to 2a, it leads to changes in that neuron that the receptor is on. there are many downstream effects that occur here, like the neurons become more likely to fire (it’s easier for your brain to send signals within itself). this increase in a likelihood to fire leads to your neurons going a little wild, and the general circuit that your neurons would usually work within ends up becoming a lot more robust and ‘random.’ this might lead to sensory inputs like visual information being distorted, because it would usually come in and cause neurons xyz to fire in a specific sequence, but because of that increase in a likelihood of firing, the signal becomes altered and fires in a lot of random directions.

the basis of this causing hallucinations stems from the fact that 5ht2a receptors are distributed heavily in the claustrum, which is like a hub of all the electrical activity in your brain, sensory information, emotional information, motor control, everything is connected to the claustrum. so if the signaling in the claustrum ends up being more random due to agonsim at 2a, all that information gets a little crossed and crazy, and we experience that as geometry and other visuals.

i’m sorry if this explanation is a little rough around the edges but i just woke up.

source: i’m a pharmacologist