Optical stimulation of the inner ear, the cochlea, is discussed as a possible alternative to conventional cochlear implants with the hypothetical improvement of dynamic range and frequency resolution. In this study nanosecond-pulsed optical stimulation of the hearing and non-hearing inner ear is investigated in vivo over a wide range of optical wavelengths and at different beam delivery locations. Seven anaesthetized guinea pigs were optically stimulated before and after neomycin induced destruction of hair cells. An optical parametric oscillator was tuned to different wavelengths (420 nm–2150 nm, ultraviolet to near-infrared) and delivered 3–5 ns long pulses with 6 µJ pulse energy via a multimode optical fiber located either extracochlearly in front of the intact round window membrane or intracochlearly within the scala tympani. Cochlear responses were measured using registration of compound action potentials (CAPs). With intact hair cells CAP similar to acoustic stimulation were measured at both locations, while the neomycin treated cochleae did not show any response in any case. The CAP amplitudes of the functional cochleae showed a positive correlation to the absorption coefficient of hemoglobin and also to moderate water absorption. A negative correlation of CAP amplitude with a water absorption coefficient greater than 5.5 cm−1 indicates additional phenomena. We conclude that in our stimulation paradigm with ns-pulses the most dominant stimulation effect is of optoacoustic nature and relates to functional hair cells.

Congenital sensory deprivation can lead to reorganization of the deprived cortical regions by another sensory system. Such cross-modal reorganization may either compete with or complement the "original" inputs to the deprived area after sensory restoration and can thus be either adverse or beneficial for sensory restoration. In congenital deafness, a previous inactivation study documented that supranormal visual behavior was mediated by higher-order auditory fields in congenitally deaf cats (CDCs). However, both the auditory responsiveness of "deaf" higherorder fields and interactions between the reorganized and the original sensory input remain unknown. Here, we studied a higher-order auditory field responsible for the supranormal visual function in CDCs, the auditory dorsal zone (DZ). Hearing cats and visual cortical areas served as a control. Using mapping with microelectrode arrays, we demonstrate spatially scattered visual (cross-modal) responsiveness in the DZ, but show that this did not interfere substantially with robust auditory responsiveness elicited through cochlear implants. Visually responsive and auditory-responsive neurons in the deaf auditory cortex formed two distinct populations that did not show bimodal interactions. Therefore, cross-modal plasticity in the deaf higher-order auditory cortex had limited effects on auditory inputs. The moderate number of scattered cross-modally responsive neurons could be the consequence of exuberant connections formed during development that were not pruned postnatally in deaf cats. Although juvenile brain circuits are modified extensively by experience, the main driving input to the cross-modally (visually) reorganized higher-order auditory cortex remained auditory in congenital deafness.

Optical stimulation of the cochlea with laser light has been suggested as an alternative to conventional treatment of sensorineural hearing loss with cochlear implants. The underlying mechanisms are controversially discussed: The stimulation can either be based on a direct excitation of neurons, or it is a result of an optoacoustic pressure wave acting on the basilar membrane. Animal studies comparing the intra-cochlear optical stimulation of hearing and deafened guinea pigs have indicated that the stimulation requires intact hair cells. Therefore, optoacoustic stimulation seems to be the underlying mechanism. The present study investigates optoacoustic characteristics using pulsed laser stimulation for in vivo experiments on hearing guinea pigs and pressure measurements in water. As a result, in vivo as well as pressure measurements showed corresponding signal shapes. The amplitude of the signal for both measurements depended on the absorption coefficient and on the maximum of the first time-derivative of laser pulse power (velocity of heat deposition). In conclusion, the pressure measurements directly demonstrated that laser light generates acoustic waves, with amplitudes suitable for stimulating the (partially) intact cochlea. These findings corroborate optoacoustic as the basic mechanism of optical intra-cochlear stimulation.

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Electroacousticstimulationinsubjectswithresidualhearingisbecomingmorewidelyusedinclinicalpractice.However,littleisknownaboutthe properties of electrically induced responses in the hearing cochlea. In the present study, normal-hearing guinea pig cochleae underwent cochlear implantation through a cochleostomy without significant loss of hearing. Using recordings of unit activity in the midbrain, we were able to investigate the excitation patterns throughout the tonotopic field determined by acoustic stimulation. With the cochlear implant and the midbrain multielectrode arrays left in place, the ears were pharmacologically deafened and electrical stimulation was repeated in the deafened condition.Theresultsdemonstratethat,inadditiontodirectneuronal(electroneuronal)stimulation,inthehearingcochleaexcitationofthehair cells occurs ("electrophonic responses") at the cochlear site corresponding to the dominant temporal frequency components of the electrical stimulus, provided these are Ͻ 12 kHz. The slope of the rate-level functions of the neurons in the deafened condition was steeper and the firing rate was higher than in the hearing condition at those sites that were activated in the two conditions. Finally, in a monopolar stimulation configuration, the differences between hearing status conditions were smaller than in the narrower (bipolar) configurations.