Phosphenes induced by sound.

Three adults with acquired unilateral visual impairment noticed phosphenes when they heard noises. They witnessed them only when resting in a dark or dimly illuminated room. The hallucinations persisted for days in a postkeratoplasty patient, for weeks in a patient with optic neuritis, and for months in a patient with compression of the optic nerve. The sound-induced phosphenes in these cases seemed to be a pathologic variety of hypnagogic hallucination. We theorize that under conditions of altered excitability and visual deafferentation of the brain, cells capable of responding to both visual and auditory stimuli become hyperresponsive to sounds.