Mid vowel assimilation in siSwati

Previous analyses of the behaviour of siSwati mid vowels show conflicting results. According to Ziervogel and Mabuza, and Taljaard and Snyman, siSwati mid vowels are raised to close mid [e, o] when preceding the high vowels [i, u] (e.g. [likhe?i] ?lift? (n.), [in?ovu] ?elephant?) but remain open mid [?, ?] before [-high] vowels (e.g. [li?emba] ?hope n.?, [ibola] ?football?), suggesting that siSwati has a vowel height assimilation and/or ATR assimilation. However, Kockaert, in his acoustic analysis of the same vowels in the same environments, disputes this description. He concludes that there is no significant difference in the F1, F2, and F3 frequency values of these vowels. Results of an experiment that I have conducted show that the phonological environment does influence the quality of siSwati mid vowels. The change though is not evidence of harmony but that of co-articulation.