Khalil Iskarous: Locus equation slopes and intercepts are an acoustic expression of articulator synergy

This study investigated the articulatory roots of locus equations, regression lines relating F2 at the start of a Consonat-Vowel
(CV) transition to F2 at the middle of the vowel, with C fixed and V varying. Several studies have shown that consonants of
different places of articulation tend to exhibit a certain pattern of locus equation slopes and intercepts, with slope descending
from labial to velar to alveolar, and intercept magnitude increasing in the same order. It has been observed that slope
correlates with coarticulation resistance. It is shown in this study, through the use of formulas from the theory of bivariate
regression that express regression slopes and intercepts in terms of standard deviations and averages of the variables, that the
slope directly encodes a measure of coarticulation resistance, that has been independently motivated in the speech production
literature. It is also shown that intercepts are directly related to the degree to which the tongue body assists the formation of
the constriction for the consonant. Moreover, it is shown that the linearity of locus equations originates in articulation. It is
concluded that slopes and intercepts of acoustic locus equations are measures of articulator synergy.