Why inner signing is not simply silent speaking
Inner speech is usually described as the voice in your head, but the term can cover any internal use of language. Signed languages are full languages, so a signer can produce language internally in signs rather than sounds. One philosophical analysis argues that inner speech can take place in sign languages as well as voiced languages, and that this matters for how we theorize about inner speech generally [1].
The analysis points to two differences that go beyond the sensory channel. First, the experience of externally signed speech is perspective-relative in a way that externally voiced speech is not. Second, when someone signs, the process of moving the hands is not separate from the speech produced—the movements are the speech—whereas when someone speaks, moving the mouth is distinct from the soundwaves that result [1]. From these differences, the author extrapolates that inner signing may differ in important ways from inner speaking [1]. That is a reasoned prediction, not a measured finding.
What modality may change about inner speech
If inner signing really is different, then several familiar claims about inner speech become more complicated. The same analysis argues that ideas about the content of inner speech, about how words are tokened—that is, how individual words are instantiated in inner experience—and about the role of inner speech in self-knowledge all need rethinking once sign languages are included [1]. For example, if the movements are the speech in signing, then the boundary between producing a word and experiencing it may be less clear than it is for spoken language.
A separate line of work treats inner speech as a semiotic mechanism—a way of making meaning through signs—and examines it in verbal and artistic discourse. It argues that the development of individual meanings follows the internalization of verbal communication, socio-cultural experience, and external representations of different modalities [2]. This supports the broader idea that the modality of expression can shape what inner speech does, though it does not test sign language directly.
What we can and cannot conclude yet
The strongest direct support for inner signing comes from conceptual analysis rather than from experiments. The claim that inner speech occurs in sign languages and that its characteristics and functions are affected by sensory modality is presented as a philosophical argument [1]. The meaning-making account is likewise a theoretical framework rather than a direct comparison of signers and speakers [2].
Other work gestures at the same territory without settling it. A discussion of inner speech and consciousness notes that people who sign are surely self-aware even though they do not speak, which is consistent with inner language not depending on voice [4]. Research on mental imagery for sign language links motor and visual systems through internal simulations and discusses inner signing as a parallel to inner speech, but it does not explicitly examine how modality influences inner speech functions [3]. Work on streams-of-consciousness as intrapersonal communication examines how modality, including typed versus spoken expression, influences the structure and content of thoughts, which is relevant but not specific to sign language [5].
So the practical answer is that inner speech in sign language is plausible and philosophically well-motivated, and modality is expected to shape its functions—especially around self-knowledge and meaning-making—but the empirical picture of exactly how is still open [1][2].
Sources used in this answer
Inner speech and sign languages
Inner speech can occur in sign languages as well as voiced languages, and differences in sensory modality and in how signs are produced may make inner signing differ in important ways, complicating claims about inner speech content, word tokening, and self-knowledge.
Inner speech in meaning-making through verbal and artistic discourses
Inner speech functions as a semiotic mechanism of meaning-making whose development involves internalized verbal communication, socio-cultural experience, and external representations of different modalities.
Mental imagery for sign language:, Linking motor and visual systems through internal simulations
Mental imagery for sign language links motor and visual systems through internal simulations, and inner signing is discussed as a parallel to inner speech, though modality's influence on inner speech functions is not explicitly examined.
Some clarifications on the role of inner speech in consciousness
People who sign are self-aware even though they do not speak, which is consistent with inner language not depending on vocal speech.
Characterizing streams-of-consciousness as a lens into intrapersonal communication
Analyzing streams-of-consciousness across modalities such as typed and spoken expression can reveal how modality influences the structure and content of thoughts related to inner speech and self-talk.