Sign languages frequently incorporate spoken language components through fingerspelling (manual representations of the written alphabet) and mouthing (silent articulation of spoken words).
The claim is specific and empirically testable, directly addressing whether sign languages incorporate elements of spoken and written languages. Multiple high-quality linguistic papers confirm that fingerspelling and mouthing are core components of many signed languages, serving as bridges between sign and spoken/written language modalities. No retrieved papers refute this fact.