Cochlear implant thresholds: comparison of middle latency responses with psychophysical and cortical-spike-activity thresholds.

The electrically evoked middle latency response (EMLR) is a potentially useful measure of activation of the auditory system by a cochlear prosthesis. The present study compared cochlear prosthesis thresholds determined using EMLR with thresholds determined for psychophysical detection and for spike activity in cortical neurons. In systemically deafened guinea pigs, the difference between EMLR and psychophysical threshold level varied, with differences ranging from -4.6 dB (EMLR threshold more sensitive) to +10.7 dB (psychophysical threshold more sensitive) across animals and phase durations. Threshold differences between EMLR and auditory cortex neural spike responses were similar in magnitude and range (-6 to +15 dB) to those seen for EMLR vs. psychophysical thresholds. These ranges are comparable to the behavioral operating range for a given condition. In 3 of 12 subjects, the EMLR was absent for some or all electrode configurations tested, even at levels well above the threshold for psychophysical detection or cortical neuronal response. These results suggest that neither the EMLR thresholds nor cortical neuronal spike thresholds are an adequate substitute for psychophysical measures of threshold. While not sufficient for use in place of psychophysical measures, EMLR threshold level is strongly correlated with psychophysical threshold level across subjects (R(2)=0.82). Interestingly, plots of thresholds vs. phase duration were roughly parallel for psychophysical and EMLR thresholds, in contrast to the divergence of psychophysical and more peripheral (e.g., electrically evoked auditory brainstem response) evoked neural threshold vs. phase duration functions.

Auditory cortical images of tones and noise bands.

We examined the representation of stimulus center frequencies by the distribution of cortical activity. Recordings were made from the primary auditory cortex (area A1) of ketamine-anesthetized guinea pigs. Cortical images of tones and noise bands were visualized as the simultaneously recorded spike activity of neurons at 16 sites along the tonotopic gradient of cortical frequency representation. The cortical image of a pure tone showed a restricted focus of activity along the tonotopic gradient. As the stimulus frequency was increased, the location of the activation focus shifted from rostral to caudal. When cochlear activation was broadened by increasing the stimulus level or bandwidth, the cortical image broadened. An artificial neural network algorithm was used to quantify the accuracy of center-frequency representation by small populations of cortical neurons. The artificial neural network identified stimulus center frequency based on single-trial spike counts at as few as ten sites. The performance of the artificial neural network under various conditions of stimulus level and bandwidth suggests that the accuracy of representation of center frequency is largely insensitive to changes in the width of cortical images.

Palatal myoclonus as a cause of objective tinnitus: a report of six cases and a review of the literature.

We describe 6 new cases of palatal myoclonus (PM), a rare disorder that is characterized by involuntary rhythmic contraction of the palatal musculature. Although it has been reported that PM is a lifelong condition, one of our patients experienced a complete resolution of PM, which casts doubt on the accepted beliefs about the persistence of PM. Included in this report is the first published case of voluntary PM with objective tinnitus.

Pathophysiological mechanisms in immune inner ear disease.

Immune inner ear disease is a somewhat controversial entity which describes cochleovestibular dysfunction that is related to immune-mediated mechanisms. The diagnosis of this disease is based on clinical presentation and response to various treatment protocols. Unfortunately, the presentation is variable and the treatment empirical, and this has caused much confusion in the diagnosis and management of the condition. To elucidate the variable nature of the disease, it is important to understand that more than one mechanism of immune injury may be involved. This paper attempts to classify clinical and experimental cases of immune inner ear disease with regard to the Gell and Coombs classification scheme of immune-mediated injury. By understanding the different pathophysiological mechanisms involved, the clinician should be better able to diagnose and manage this difficult problem in a directed fashion. The ramifications of the proposed classification system on the diagnosis, treatment and future research of immune inner ear disease are discussed.

Local effects of nitric oxide on vestibular blood flow in the Mongolian gerbil.

There is a paucity of studies regarding the regulation of vestibular blood flow (VBF), despite the possibility that vascular alterations may contribute to specific vestibulopathies. The current experiments used the Mongolian gerbil as an animal model since it provides easy surgical access to the vestibular end-organs and has been previously used for physiologic studies involving inner ear function. VBF changes were measured in the posterior semicircular canal using laser Doppler flowmetry following round window membrane (RWM) application of the nitric oxide donor 1, 3-propanediamine-N-[4-1-(3-aminopropyl)-2-hydroxy-2-nitrosohydrazi no] butyl (spermine NONOate; SPNO) as a vasodilator. The specificity of the responses induced was tested via pretreatment with an NO scavenger, 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazonline-1-oxyl-3-oxide (carboxy-PTIO; cPTIO). cPTIO, SPNO, vehicle (control) or cPTIO/SPNO were applied to the RWM, during which blood pressure and VBF were monitored for baseline, treatment, and recovery conditions. Results showed concentration-dependent increases in flow, probably resulting from NO's vasodilatory action on local vasculature. cPTIO pretreatment was found to attenuate SPNO-induced VBF increases. These findings support a role of NO in maintaining the vestibular microcirculation.