We may think that only "crazy" people have hallucinations, but that's not true. Oliver Sacks, professor of neurology at New York University School of Medicine, writes in the New York Times that hallucinations are common and not necessarily a symptom of something wrong with us.
Hallucinations are a sensory perception without stimulus. In other words, your brain is creating a sight or sound or odor without being stimulated by something "out there" to see, hear or smell. Western culture dismisses such experiences as a sign something is wrong, but that ain't necessarily so.
The fact is, all of our sensory experiences are being created in our brains and nervous systems. The way things appear to us, including color and depth; the way sounds "sound" to us, are effects our bodies create in response to objects and sound waves. A being of another species, one with with very different neurological wiring and sensory capabilities, might be right next to us but perceiving an entirely different world.
If we understand sensory experience this way, it isn't so much of a leap to understand that sometimes, without external stimulation, our neurons fire or twitch or whatever neurons do to send signals to the brain to create a sight or sound.
Professor Sacks writes that people who are losing their sight or hearing are prone to visual and auditory hallucinations. He explained to an elderly lady who was "seeing things" that "if the visual parts of the brain are deprived of actual input, they are hungry for stimulation and may concoct images of their own."
Isn't it interesting that a sense organ can be "hungry"? In his teachings on the Five Skandhas, the Buddha taught that our senses, perceptions, and consciousness are all empty of a "self" that lives in our bodies and coordinates the show. And no, consciousness is not "in charge" any more than our noses. The experience of a self is something our bodies re-create from moment to moment.
But back to hallucinations. The question is, should we take hallucinations seriously as "visions," or should we ignore them? Theravada and Zen teachers usually will tell you to not attach significance to them. That's not exactly the same as ignoring them, because it may be that your neurons are trying to tell you something. But that "something" may be pretty mundane -- you're getting sleepy, or you need to adjust your posture.
There's an often-told Zen story about a new monk who sought out his teacher and said, 'Master! I was meditating just now and saw the Buddha!"
"Well, don't let him bother you," the Master replied. "Just keep meditating, and he'll go away."
The "lesson" is that often in our desire to have some transcendent mystical experience, our brains conjure up what we are longing for -- the Buddha, or the Blessed Virgin, or the face of Jesus on a cheese sandwich. These are projections of our grasping nature and our delusions.
Teachers tell us that the deeper dhyanas and enlightenment itself cannot be compared to any kind of sensory experience. My first Zen teacher used to say that if any student tried to describe samadhi by saying "I saw..." or "I felt..." -- it wasn't samadhi.
On the other hand, I do think that once in a great while our neurons send us a signal that is coming from a deeper wisdom, something out of reach of ordinary consciousness. It may be very subtle, just a feeling, or a quickly glimpsed "vision" that has some personal significance. If this ever happens, just accept it and honor whatever the experience communicates, and then let it go. Don't make a Big Deal out of it or "enshrine" it in any way, or the gift will turn into a hindrance.
In some Buddhist traditions there are stories about enlightened masters who develop psychic or other supernatural powers. I'd personally be inclined to understand such stories as fables or allegories, but I realize some of you will disagree. The early texts, such as the Pali Tipitika, give us stories of monks like Devadatta who practiced for the sake of developing supernatural powers and came to a bad end. So even if some enlightened teachers do develop "powers" -- and I'm not saying they do -- such powers are a side effect, not the point.
Although I've been talking about hallucinations as normal experience, don't forget that they can be a sign of real neurological issues that need medical attention. Sensory hallucinations often accompany migraine headaches and seizures. Karen Armstrong, a scholar of religion whose books I like, for years experienced phases of visual distortions, often accompanied by the smell of sulfur. Eventually she was diagnosed with temporal epilepsy.
On the other hand, on long meditation retreats hallucinations can be pretty ordinary. I think most of the time this is a "sensory deprivation" effect, often accompanied by fatigue. Hours of sitting still, resting your eyes on a floor or wall, and your hungry eyes may want to entertain themselves.
In my earlier Zen student days I learned that it was remarkably easy, if I concentrated on it, to give myself the sensation of floating above the meditation pillow. This was true even though I knew good and well I wasn't really floating, but I could entertain myself by "pretend floating" when I was getting tired or bored. Needless to say, this is not a recommended Zen practice. I'm just mentioning it to illustrate that sometimes even strong hallucinations have absolutely no spiritual significance.
I also think that sometimes, when your concentration is getting stronger, the parts of your brain creating sight and other sensation become quieter, or something. You might "see" the floor move or the wall melt. If that happens, don't stop at that point to enjoy the "show," but keep concentrating.
The moral is, "visions" do happen, sort of, but they're something like the scenery along the spiritual path, not the path itself. Don't stop to admire them. And, anyway, in a way it's all a hallucination.