I am surprised that no one seems to have commented in this in terms of
synesthesia, which reportedly affects between 1 in 500 and 1 in 25,000,
as a neurological phenomenon, according to Jorg Jewanski in the New Grove
II, who adds that "true synaesthesia meets at least four of the following
five criteria: it must be involuntary but elicited, projected, durable
and discrete, memorable, emotional ...  Broader uses of the term may
include more "voluntary" associations.

It would be easy for prople to talk past one another on this subject.
I myself have no direct experience of it whatever.

Jim Tobin