Intracochlear near infrared stimulation: Feasibility of optoacoustic stimulation in vivo.

Intracochlear optical stimulation has been suggested as an alternative approach to hearing prosthetics in recent years. This study investigated the properties of a near infrared laser (NIR) induced optoacoustic effect. Pressure recordings were performed at the external meatus of anaesthetized guinea pigs during intracochlear NIR stimulation. The sound pressure and power spectra were determined. The results were compared to multi unit responses in the inferior colliculus (IC). Additionally, the responses to NIR stimulation were compared to IC responses induced by intracochlear electric stimulation at the same cochlear position to investigate a potentially confounding contribution of direct neural NIR stimulation. The power spectra of the sound recorded at the external meatus (n = 7) had most power at frequencies below 10 kHz and showed little variation for different stimulation sites. The mean spike rates of IC units responding to intracochlear NIR stimulation (n = 222) of 17 animals were significantly correlated with the power of the externally recorded signal at frequencies corresponding to the best frequencies of the IC units. The response strength as well as the sound pressure at the external meatus depended on the pulse peak power of the optical stimulus. The sound pressure recorded at the external meatus reached levels above 70 dB SPL peak equivalent. In hearing animals a cochlear activation apical to the location of the fiber was found. The absence of any NIR responses after pharmacologically deafening and the comparison to electric stimulation at the NIR stimulation site revealed no indication of a confounding direct neural NIR stimulation. Intracochlear optoacoustic stimulation might become useful in combined electro-acoustic stimulation devices in the future.

Cross-Modal Plasticity in Higher-Order Auditory Cortex of Congenitally Deaf Cats Does Not Limit Auditory Responsiveness to Cochlear Implants.

UNLABELLED: 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" higher-order 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. SIGNIFICANCE STATEMENT: In a common view, the "unused" auditory cortex of deaf individuals is reorganized to a compensatory sensory function during development. According to this view, cross-modal plasticity takes over the unused cortex and reassigns it to the remaining senses. Therefore, cross-modal plasticity might conflict with restoration of auditory function with cochlear implants. It is unclear whether the cross-modally reorganized auditory areas lose auditory responsiveness. We show that the presence of cross-modal plasticity in a higher-order auditory area does not reduce auditory responsiveness of that area. Visual reorganization was moderate, spatially scattered and there were no interactions between cross-modally reorganized visual and auditory inputs. These results indicate that cross-modal reorganization is less detrimental for neurosensory restoration than previously thought.