American Sign Language (ASL) signs that are located on the forehead in their canonical form are often articulated lower during natural signing. Previous studies have examined this phenomenon from a phonetic perspective, treating it as a form of undershoot, and from a variationist sociolinguistic perspective, treating it as a categorical process. This study sees if the findings and explanations of these studies can be extended to the lowering of signs formed in locations other than the forehead. In a corpus of natural conversational signing from six signers, we measure the vertical displacement of over 3000 tokens of signs canonically formed at the face, head, or neck. While there is some apparent evidence for a categorical lowering process in a minority of signs and considerable evidence for undershoot, neither alone can explain the full range of displacement patterns across all signs and locations. Undershoot must be carefully planned and controlled: no matter how sloppy the signing, signers systematically avoid contacting their eyes. The results can be explained if there is a somatosensory forward-modelling mechanism that can veto undesirable gestural scores and whose decisions are incorporated during learning into the phonological distributions representing the locations of signs.
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