Auditory-visual synesthesia: sound-induced photisms.

Nine patients with visual loss due to lesions of the optic nerve or chiasm experienced photisms induced by sound. Descriptions of these varied from simple flashes of white light to complicated colorful hallucinations likened to a flame, a petal of oscillating lines, a kaleidoscope, or an ameba; they always appeared within a defective portion of the visual field as demonstrated by perimetry. The provoking sounds were usually those of normal daily life, ranged from soft to loud, and always seemed to be heard by the ear ipsilateral to the eye in which the photism was seen. Sound-induced photisms occurred under circumstances that would promote a startle reaction to sound, and each patient was startled when the photisms occurred. Visual evoked responses demonstrated partial deafferentation of the eye in which photisms were seen in seven patients tested. The phenomenon may occur when the patient with a partially deafferent anterior visual pathway is startled by sound.