It's a total mystery,". He says brain studies are shedding light on the pieces of the puzzle and might one day solve the mystery.

VOA: standard.2010.01.15

Studies have shown that activation of the trigeminal nerve may come from a part of the brain called the hypothalamus.

VOA: special.2011.05.03

Some previous studies have suggested that challenging your brain with mentally stimulating activities might help.

VOA: standard.2010.06.17