Hearing after Congenital Deafness: Central Auditory Plasticity and Sensory Deprivation

TLDR

Congenitally deaf cats lack inner ear hair cells but retain auditory afferents, making them a model for studying congenital deafness. Kittens received cochlear implants at varying ages and were electrically stimulated for 2–5.5 months with a monopolar single‑channel compressed analogue strategy, after which cortical field potentials to biphasic pulses were recorded. Chronically stimulated cats showed larger middle‑latency cortical responses (≥300 µV) in both hemispheres, with no loss of ipsilateral responsiveness when the untrained ear was stimulated, and a smaller activated area when implantation occurred at 5–6 months, underscoring greater plasticity and a sensitive period in young cats.

Abstract

The congenitally deaf cat suffers from a degeneration of the inner ear. The organ of Corti bears no hair cells, yet the auditory afferents are preserved. Since these animals have no auditory experience, they were used as a model for congenital deafness. Kittens were equipped with a cochlear implant at different ages and electro-stimulated over a period of 2.0-5.5 months using a monopolar single-channel compressed analogue stimulation strategy (VIENNA-type signal processor). Following a period of auditory experience, we investigated cortical field potentials in response to electrical biphasic pulses applied by means of the cochlear implant. In comparison to naive unstimulated deaf cats and normal hearing cats, the chronically stimulated animals showed larger cortical regions producing middle-latency responses at or above 300 microV amplitude at the contralateral as well as the ipsilateral auditory cortex. The cortex ipsilateral to the chronically stimulated ear did not show any signs of reduced responsiveness when stimulating the 'untrained' ear through a second cochlear implant inserted in the final experiment. With comparable duration of auditory training, the activated cortical area was substantially smaller if implantation had been performed at an older age of 5-6 months. The data emphasize that young sensory systems in cats have a higher capacity for plasticity than older ones and that there is a sensitive period for the cat's auditory system.