I am interested in how schizophrenia can be expressed in them, and whether it depends on the perception of the world. Maybe they see text pop-up in the air, or something like that?
A quick Google search reveals some answers such as Atkinson, J. R. (2006).
Around half of all deaf people diagnosed with schizophrenia report experiencing “voices,” during which they sense someone communicating with them in the absence of any external stimulus. This closely parallels prevalence rates of auditory verbal hallucinations (AVH) in hearing people with schizophrenia (50–70%). There is no evidence reported for increased frequency of psychotic hallucinations in congenitally deaf people. However, greater proportions of visual and tactile/somatic hallucinations have been noted. Around 50% of deaf people with a diagnosis of schizophrenia report visual hallucinations, and a similar proportion describe tactile/somatic hallucinations, despite the low occurrence of these phenomena in schizophrenia generally (15% and 5%, respectively).
The abstract points out that:
The study of voice-hallucinations in deaf individuals, who exploit the visuomotor rather than auditory modality for communication, provides rare insight into the relationship between sensory experience and how “voices” are perceived. Relatively little is known about the perceptual characteristics of voice-hallucinations in congenitally deaf people who use lip-reading or sign language as their preferred means of communication. The existing literature on hallucinations in deaf people is reviewed, alongside consideration of how such phenomena may fit into explanatory subvocal articulation hypotheses proposed for auditory verbal hallucinations in hearing people.
Atkinson, J. R. (2006). The perceptual characteristics of voice-hallucinations in deaf people: insights into the nature of subvocal thought and sensory feedback loops. Schizophrenia bulletin, 32(4), 701-708. doi: 10.1093/schbul/sbj063 pubmed: PMC2632268