Perhaps you can clarify a little more about 'visions'? Drugs that disrupt the serotonergic system (LSD, MDMA, Marijuana, psilocybin) tend to cause sensory-disintegrated (the user is very aware of the abnormality of the experience) psychedelia - spatial and sensory distortion, time distortion, and psychedelia but only complex hallucinations at very high doses. The perceptual senses are primarily involved. Dopaminergic (cocaine, amphetamines, peyote) hallucinations tend to be sensory integrated and complex, meaning that they have enough of a foundation to be believed to be real by the experiencer. These tend to be time-persistent as well. The intellectual and emotional center are primarily involved. Deliriants (anticholinergics like belladonna, datura, atropine) cause severe dementia and delirium which presents as a random unrelated absolute hallucinations that have no bearing on anything. These drugs reduce the level of a major neurotransmitter, acetylcholine so much that the higher brain centers essentially lose their ability to integrate any coherent thought. There's a lot more to it than this if you want to go beyond pharmacology.

If you're asking what determines the content of the visions, that's depends on each individual's subconscience. People may experience similar symptoms, i.e. fear, paranoia, euphoria.The actual visions vary on what each individual might fear, be paranoid about or what makes them happy. If I was deathly afraid of say, chairs, before I take hallucinogen, then I might have visions of hugely ferocious chairs attacking me. If the idea of Santa Claus makes me very happy, then I might have a vision of a Santa Claus type benefactor. Again, it depends on the individual's psychological make-up as to what determines the content of visions or hallucinations. Hope this helps...

The chemicals of the drug interacting with the chemicals in your body...each person varies with visuals. I have friends that never see anything on mushrooms, while I see so many visuals, even after I stop the body high my eyes are still visually tripping, it effects me differently. The surroundings of you also play a major role, you are in a plain environment will differ from a place with lots of lights or colorful flowers or posters, etc. DMT and Datura produce the most profound visual and psychedelic experiences (DO NOT TAKE DATURA, I HIGHLY RECOMEND LSD and MUSHROOMS! for certain people I recomend peyote and mescaline as well.) Always be careful and research everything you are interested in experimenting, and NO YOUR SOURCE

Ever take an inkblot test? Different people see different things - it has much more to do with how a personal conceptualizes and makes sense out of "white noise" kinds of data. Like seeing a picture in a random "ink blot".