Read about the first half of this thread, then stopped, just FYI (in case something to this effect was said in the bottom half).

What do you mean, "just" in the head? All experiences are "in the head" (i.e. mental). When a person with normal vision sees, the experience of seeng is in the head - nobody directly experiences their optic nerves sending certain impulses. Most blind/deaf people still have the parts of the brain that interpret experiences of sight and sound; in computer terms, you can think of this as having broken hardware but working drivers. Hallucinogenic experiences can be explained entirely within scientific understanding of the brain.

Note that I'm not saying I think science is everything. I think the idea of people with disabilities exploring mind-altering substances is certainly very interesting, and I can only imagine what a trip would be like for someone who was born blind (or deaf, for audio hallucinogens), but I think trying to "prove" that you "really see" something "real" is the wrong way of going about it. Salvia trips (and other trips, for that matter) are rather subjective and tend not to match up well with our preconceived notions about what is and is not "real."