Your voice is focussed only when in its entire range it is intense enough to feel started and stopped in the same spot- the center of your skull. - Vocal Wisdom: Maxims of Giovanni Battista Lamperti by William Earl Brown (1931)
Is there any way to explain what otherwise seems to be nonsense? After all, it's been determined that the sinus cavities of the head are not resonators.
I can think of one thing: bone conduction. The well-known pedagogue Richard Miller wrote about this aspect (see here) in his book The Art of Singing.
Miller notes that monitoring sensations in the head is to be encouraged with the caveat of refraining of trying to put those sensations there. And this makes logical sense. Though the Old School may have talked about a column of air extending into the head, the larynx is the final arbiter. Air cannot 'go' anywhere but out of the mouth unless the voice is nasal and the soft palate lowered. Then the nasal cavity does play a part, but hardly one that is desirable since the nasal voice was considered one of two chief defects by the Old School (the other chief defect is the guttural voice). Miller, like James Stark in Bel Canto: A History of Vocal Pedagogy, suggests that whatever is felt by the singer is personal and therefore unreliable. Stark even called sensations in the head 'resonance imagery.' Imagery - by definition - is imaginary, and using this word takes the phenomena out of the range of what can be legitiimized since what is imaginary is not real.
However, what if we take G.B. Lamperti at his word? He tells Mr. Brown that the voice is focussed when it is "intense enough to feel started and stopped in the same spot- the center of the skull." What would cause the voice to be intense enough to be felt in the head? That seems to be the question to ask. And that takes us back to bone conduction. It also takes us back to my posts on the opening of the ear and the neurological connection between the face, inner ear and the stapedius muscle (see here and here).
My thinking goes like this: if the muscles of the face and head (in fact the whole vocal apparatus) have a part in innervating the muscles of the ear as regarding intensity of sound, would this not affect the perception of bone conduction by the singer? As well, might the acquisition of a ringing tone give the singer the illusion - an auditory mirage if you will- of resonance in the head for the simple reason that the ear canals are highly sensitive to this range of frequency and are centrally located?
More questions than answers of course.
When I showed my young boy soprano how to 'call' and then vocalize on [i] he spontaneously told me that he heard a buzzing. Where? I asked. He pointed to the center of his head.
"It sounds like bees!"
You can't argue with a nine year old.