Tactile sign languages used by Deafblind signers are most often acquired by signers competent in a visual sign language who can no longer rely on the grammatical system of the visual language as it is, since some of its features are lost due to the loss of vision. A natural question is which repair strategies are adopted to compensate for the loss of the grammatical features of the visual language that can no longer be perceived. We argue that the transformation of LIS (Italian Sign Language) into tactile Italian Sign Language (LISt) is constrained by grammatical principles, rather than reflecting communication strategies that in principle might compensate for the visual loss equally well. Certain innovations are introduced to carry over the grammatical features of LIS to LISt. Even when LISt undergoes processes that make it diverge from LIS, these processes are attested in other natural languages. For example, among the innovations unconsciously introduced by LISt signers we found an instance of cross-modal grammaticalization. Our research suggests that tactile languages have the potential of becoming complete grammatical systems, at least when they build on previous knowledge of a visual sign language.
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