I’ll just mention a few points I personally regard as important:
First, such things as this cannot possibly explain all of the visions that people have had. For example, it cannot possibly explain the miracle of the sun at Fatima, which was witnessed by over twenty thousand people. Second, it can’t possibly explain any of the other visions that have been witnessed by more than one person, for the same reason. In fact, it would even have a hard time disproving visions people have had when other people have been around to witness any seizures or other sorts of things that supposedly account for the visions. In short, it only accounts for those visions that people have claimed to have alone and with no corroborating evidence.
This brings me to point number two: the way the Catholic Church handles visions. Now there are all sorts of people claiming to have visions out there, and the reality is that most of these people probably are lying, reading too much into some natural phenomenom, or experiencing some sort of brain-seizure-lobe-problem thing. However, the Catholic Church stands above these for one very important reason: when a supposed vision is brought to Her, She spends a great deal of time investigating it. In fact, the Catholic Church in a sense starts off with the assumption that the vision was not in fact real, and recognizes that it may be only after finding no natural explanation. The Church has medical tests done on the people claiming to have visions. If they had epileptic tendencies or that sort of thing, they would find that out. The Church is harsher on these things than even the most hardened atheist. She’s very thorough and cautious when dealing with these sorts of things. This is yet another reason why the Catholic Church stands above all of the religious mumbo-jumbo out there and represents the true God.
Third, there are miraculous occurences out there that we can all see. There are bodies of saints that have not decayed over hundreds of years worth of time. There are Eucharistic miracles, such as the miracle of Lanciano which was been examined scientifically by a team of over 70 unbiased doctors and scientists of the World Health Organization. Some of these Eucharistic Miracles have been filmed, such as the miracle of Betania (in which a host turns into a heart and beats at intervals) and the miracle of Naju (in which a host turned into flesh and blood in a woman’s mouth), and many others. Simply search for “Eucharistic Miracle” on google video search and view the results. Unless every human being that watches these videos has some sort of temporal lobe electrical storm or seizure every time they watch these videos, then the concept doesn’t disprove anything. In fact, if this happened, it would be such a miracle as to defy natural explanation itself!
There’s no question some of these things can cause false visions, but they simply can’t explain away everything. Those who are closed to truth will claim they do, but anyone who is really open to the truth - whatever it happens to be - can’t brush these things off.