Some synesthetes report seeing bright flashes when hearing a loud noise. However, in the same person, bright flashes of light are not reported as being loud. I've read of other examples like this; for instance, the number 1 may always be yellow, but yellow is not the number 1.
Has there been any research as to the neurological reasons why synesthesia is asymmetric? Alternately, is my anecdotal assumption that it is asymmetric incorrect and there are actually frequent symmetric presentations of the condition?