Directionality in sound change from asymmetries in acoustic distribution

Abstract

Following the work of John Ohala, historical sound changes are thought to take place by misperception of the input on the part of the listener. Any account of sound change based on misperception, though, faces a paradox: if X sounds like Y, Y should also sound like X, and yet we often see sound changes that are only attested in one direction. A potential solution is to think of phonetic categories as distributions in acoustic space, and so asymmetries in sound change (X > Y, *Y > X) come from asymmetries in the spread of the distribution of X and Y. If X is a very variable phonetic category with a thick-tailed distribution, a high proportion of its tokens should cross the perceptual boundary and be misperceived as Y; if Y has a narrow distribution, only a very small proportion of its tokens should be perceived as X. We predict that unidirectional sound changes should involve a change from a high-variance to a low-variance category. This experiment tests a case study of asymmetric nasal place assimilation (VnpV > VmpV, *VmpV > VnpV) on a sample of six speakers in three vowel contexts. In the contexts aa and ii, the sequence np before the change had a higher-variance distribution of F2 transition (a cue to nasal place) than the sequence mp after the change. In uu, the sequence after the change had the higher variance. These results give partial support to our hypothesis.