A pharyngealised velar [k\] or uvular
[q\] (to use an ad hoc simplification of the SAMPA transcription) would
work particularly well as the original phonetic value of *q (and, likewise, [x\]
or [X\] for *h2). Pharyngeal constriction (whether involving tongue-root
retraction, larynx raising or sphincteric pharyngeal contraction), has the
effect of raising F1 and lowering F2 and F3, thus producing an acoustic effect
of lowness and backness that may spread to an adjacent frontish vowel (*e >
*a) even if the position of the dorsum is actually high. Some conventional
notations make pharyngealised dorsals appear prohibited, since they would have
to be [+high] and [+low] at the same time, but since pharyngealisation is
actually independent of the shape of the body of the tongue, the problem is only
notational (and therefore illusory). Such sounds have been reported from a
number of languages.
>What are the typological grounds for the statement that
uvulars >trigger a-colouring (and not o-colouring, for instance)? What
would >be a possible (neuro)physiological explanation? Just
Ladefoged & Maddieson (Sounds of the World's Languages, p.
36) describe a study where in an Arabic speaker uvular /q/ lowers F2 of a
following /a/ or /i/, while (slightly) raising F2 of a following /u/, as
compared to velar /k/. This amounts to a backing of the vowel (F1 is
correlated with high/low, (F2-F1) with front/back). The effect is
comparable to that of pharyngealization, or "[A]-colouring" (Arabic /q/ is
historically a pharyngealized /k/).