Thanks for the ideas.
My experience, not based on lab research but rather the concentrated
individual testing of may hundreds of music students over a period of a
decade is that there is not 'real' continuum. There are a number of
musicians who lodge a reference note in their mind (such as the violinist
who can sing A, D, G and E in the octave of the violin strings) but do not
demonstrate strong octave equivalence, and it is my understanding from
Diana's work that AP is octave equivalent.
My experience is that there are two groups, AP, and non-AP. The
non-AP have skills ranging from nil (poor pitch discrimination), through
varying degrees of relative pitch, and a group of 'reference note'
hearers. The reference note hearers may or may not have good relative
In my experience with people 17 to 77, AP cannot be taught or
trained. Reference pitch can be. I used to start every class with the
singing of D above middle C. Towards the end of each year (2 - 3 classes
per year for over 20 years), one day I would have the class sit and
without playing, I would ask them to imagine themselves walking into class
and preparing to sing the "D".
When everyone was settled, I would play a note on the piano, and ask
if it was "the D". I would get up to 80% "correct" responses. In some cases
when I would play (for example) the Eb, some people would say, "That's too
high." They would work out how much too high (a minor second), but they
did not "hear" Eb, as my AP students did.
I found virtuoso performers with dreadful relative pitch, but upon
their having "learned" the melody, were fine. They couldn't find a
descending major sixth without singing a descending minor scale out loud.
My 'non-lab' testing cycle was a 10-15 minute individual examination
six times per year, which amounts to up to 50 hours per year for a decade.
I don't have a control group and the only documentation I have is my
database of the grading of the exams, and the limiting factor of course, as
in any such experiment / test, is the limitations on my own pitch
perception, and I guess you just have to take my word that I have ok
relative pitch. My reference tones are all a fourth off.