About 10 percent of the hearing impaired get musical hallucinations, and about 10 percent of the visually impaired get visual hallucinations.

Sacks is pointing to a recurring neurological pattern: when sensory input is reduced or lost, the brain may generate internally produced perceptions to “fill in” the missing stream of information. In hearing loss, this can appear as musical hallucinations (tunes, choirs, orchestras) rather than simple ringing; in vision loss, it can appear as complex visual imagery (faces, patterns, scenes), often discussed under Charles Bonnet syndrome. The paired “about 10 percent” figures underscore that these phenomena are not rare curiosities but statistically meaningful consequences of sensory deprivation, reframing hallucinations as a brain-based response to altered input rather than necessarily a sign of psychiatric illness.