Somatic memory and gain increase as preconditions for tinnitus: Insights from congenital deafness

Tinnitus is the conscious perception of sound heard in the absence of physical sound sources internal or external to the body. The characterization of tinnitus by its spectrum reflects the missing frequencies originally represented in the hearing loss, i.e., partially or completely deafferented, region. The tinnitus percept, despite a total hearing loss, may thus be dependent on the persisting existence of a somatic memory for the "lost" frequencies. Somatic memory in this context is the reference for phantom sensations attributed to missing sensory surfaces or parts thereof. This raises the question whether tinnitus can exist in congenital deafness, were somatic representations have not been formed. We review the development of tonotopic maps in altricial and precocial animals evidence for a lack of tinnitus in congenital deafness and the effects of cochlear implants on the formation of tonotopic maps in the congenitally deaf. The latter relates to the emergence of tinnitus in these subjects. The reviewed material is consistent with the hypothesis that tinnitus requires an established and actively used somatotopic map that leads to a corresponding somatic memory. The absence of such experience explains the absence of tinnitus in congenital bilateral and unilateral deafness.