Various devices have been developed to overcome the widespread phenomenon of different degrees of hearing deficits between mild and profound hearing loss. Basically, we differentiate between acoustic stimulation (hearing aids), restricted to cases with a partially functioning cochlear receptor, and electrical stimulation (cochlear implants), stimulating the auditory nerve directly in cases with profound or total hearing loss. For the first time, animal data have been collected that indicate the possibility of nearly interference-free use of both stimulation types simultaneously. In addition, we have gathered the first clinical patient experience, which confirms the encouraging results. Future implications for patients with severe high-frequency hearing loss are discussed.

Combined electric and acoustic stimulation (EAS) of the auditory system is a new therapy for patients with severe to profound high- and mid-frequency hearing loss but remaining low-frequency hearing. In a prospective study, 13 patients with low-frequency hearing of better than 60 dB below 1 kHz were implanted with a MED-EL COMBI 40+ cochlear implant. Pure tone thresholds as well as monosyllabic word scores and Hochmair-Schulz-Moser sentences in quiet and in noise were measured with hearing aids, cochlear implant alone and in the combined stimulation mode (EAS) in the same ear. Hearing could be partially preserved in 11 out of the 13 patients. All patients scored significantly higher with cochlear implant alone than with hearing aids. Seven patients scored higher in the EAS mode than with cochlear implant alone for sentences in noise, 4 remained unchanged, and 2 could not use EAS. Synergistic effects of EAS were most prominent for hearing in noise with increases of up to 72% as compared to cochlear implant alone.

This study reports successful conservation of hearing after cochlear implantation using a modified surgical technique. Even high levels of hearing could be maintained, showing that implantation of an intracochlear electrode can be performed atraumatically with preservation of functional structures.

The study investigates early postnatal development of local field potentials (LFPs) in the primary auditory cortex of hearing and congenitally deaf cats. In hearing cats, LFPs elicited by electrical intracochlear stimulation demonstrated developmental changes in mid-latency range, including reductions in peak and onset latencies of individual waves and a maturation of their shape and latencies during the first 2 months of life. In long latency range (>80 ms), the P(1)/N(1) response appeared after the fourth week of life and further increased in amplitude and decreased in latency, reaching mature shapes between the fourth and sixth months after birth (p.n.). Cortical activated areas became increasingly smaller during the first 3 months of life, reaching mature values at the fourth month p.n. The layer-specific pattern of synaptic activity matured 4 months p.n. In congenitally deaf cats, the developmental pattern was different. The lowest cortical LFP thresholds were significantly smaller than in hearing controls, demonstrating a "hypersensitivity" to sensory inputs. The development of N(b) waves was delayed and altered and the long latency responses became smaller than in controls at the second and third months. The activated areas remained smaller than in controls until the third month, then they increased rapidly and exceeded the activated areas of age-matched controls. From the fourth month on, the activated areas decreased again and smaller synaptic currents were found in deaf cats than in controls. The presented data demonstrate that functional development of the auditory cortex critically depends on auditory experience.

Unilateral deafness has a high incidence in children. In addition to children who are born without hearing in one ear, children with bilateral deafness are frequently equipped only with one cochlear implant, leaving the other ear deaf. The present study investigates the effects of such single-sided deafness during development in the congenitally deaf cat. The investigated animals were either born with unilateral deafness or received a cochlear implant in one ear and were subjected to chronic monaural stimulation. In chronically stimulated animals, implantation ages were at the following three critical developmental points: 'early' during the peak of functional cortical synaptogenesis in deaf animals; 'intermediate' at the age when synaptic activity in the deaf cats dropped to the level of hearing control cats and finally, 'late' at the age when the evoked synaptic activity fell below the level of hearing control cats. After periods of unilateral hearing, local field potentials were recorded from the cortical surface using a microelectrode at ∼100 recording positions. Stimulation was with cochlear implants at both ears. The measures evaluated were dependent only on the symmetry of aural input: paired differences of onset latencies and paired relations of peak amplitudes of local field potentials. A massive reorganization of aural preference in favour of the hearing ear was found in these measures if the onset of unilateral hearing was early (before or around the peak of functional synaptogenesis). The effect was reduced if onset of unilateral hearing was in the intermediate period, and it disappeared if the onset was late. In early onset of unilateral deafness, the used ear became functionally dominant with respect to local field potential onset latency and amplitude. This explains the inferior outcome of implantations at the second-implanted ear compared with first-implanted ear in children. However, despite a central disadvantage for the deaf ear, it still remained capable of activating the auditory cortex. Appropriate training may thus help to improve the performance at the second-implanted ear. In conclusion, periods of monaural stimulation should be kept as short as possible, and training focused on the deaf ear should be introduced after delayed second implantation in children.