ATR allophones or undershoot in Kera?

Kera (a Chadic language) has 6 vowels, 3 of which have +/-ATR allophones. [+ATR] vowels appear in non-heads of feet and [-ATR] vowels in heads and elsewhere. This binary classification is sufficient until we examine the acoustic measurements of F1, F2 and duration in footed and non-footed syllables. These results suggest that the variation in quality relates to the duration of the vowel rather than directly to the foot structure. We will consider the evidence for claiming that there is a gradient relationship between the F1 value and the duration. The key data for this claim come from vowels in non-footed syllables at the right edges of phrases and vowel initial syllables. In non-footed syllables the duration of the vowel is longer than a non-head vowel, but shorter than a head vowel. The F1 value for these vowels is equally between the average head and non-head values. Gendrot and Adda-Decker (2006) have demonstrated similar patterns in other languages where a shorter duration means a more centralised vowel. Their results could lead us to suppose that the reason for this gradient is articulatory, due to the need of a certain amount of time for articulators to arrive at the target position, and that all languages may exhibit a similar phonetic pattern. A few counter-examples suggest that this pattern can be over-ridden by phonological factors. In the case of Kera we may well be seeing a process that began as a gradient phonetic change but which is now in the process of being phonologized. Therefore the use of the term ‘allophone’ correctly describes the phonology, but the phonetics also has a role to play in the quality of the vowel.