Click-Evoked Human Electrocochleogram. Noninvasive Recording Method, Origin and Physiologic Significance

The I' potential of the brain-stem auditory-evoked potential

We have consistently recorded a positive wave which precedes wave I, and is called I', within the human brain-stem auditory-evoked potential. It is postulated that I' represents initial neural activity of the auditory nerve, which presumably has as its origin auditory nerve dendrites. Thus, I' may represent a summed far-field dendritic potential from currents of excitatory postsynaptic potentials. We report latency and amplitude values of I'.

Electrophysiological monitoring during acoustic neuroma and other posterior fossa surgery

Techniques used to monitor the function of the seventh and eighth cranial nerves during acoustic neuroma and other posterior fossa surgery are reviewed. The auditory brainstem response (ABR), electrocochleogram (ECochG) and direct recording from the auditory nerve (CNAP) were compared. The best technique is the ECochG, although in many cases, the CNAP should be used as a back-up technique. The CNAP is especially useful for the identification of the auditory nerve. Both can provide real-time feedback on the physiological integrity of the auditory nerve. The ABR may be helpful in monitoring brainstem function. For some procedures, optimal monitoring requires the combined recording of all three techniques. Monopolar constant-voltage intracranial stimulation of the facial nerve is helpful for the identification and preservation of the facial nerve. Audio monitoring of spontaneous electromyographic activity provides real-time feedback on the effect of surgical manipulation of the nerve. Monitoring of ephaptic transmission in the facial nerve during microvascular decompression for hemifacial spasm aids in the identification of the offending vessel.

BAEP subcomponents and wave form--relation to click phase and stimulus rate

The latency distributions of normal brain-stem auditory evoked potential (BAEP) components elicited by condensation (C) and rarefaction (R) clicks at 10 and 50 Hz were found to be double- or multi-peaked for II (10 and 50 Hz), III (50 Hz), IV (10 and 50 Hz) and V (10 Hz). A bifid component III was found in 3.5% (10 Hz) and 7.4% (50 Hz) of BAEPs. A bifid II and triple IV/V configuration were occasionally noted. The prevalences of the different IV/V complex configurations were significantly dependent upon click phase and rate. These results suggest that several subcomponents might participate in the generation of the single BAEP components and that a single generator may contribute to different BAEP components in different subjects. Early subcomponents between I and II (Ib) were found in 13% of 10 Hz BAEPs and 27% of 50 Hz BAEPs (P = 0.002) and the latency distribution of Ib seemed to be bimodal. Ib/I relative amplitude frequently increased with 50 Hz stimulation. We suggest that the generator of Ib is partially cochlear (CM) and partially neural (equivalent to N2 of the ECochG-AP) in origin. The existence of subcomponents must be recognized in clinical use of BAEPs both to avoid misinterpretation and to decrease the normal variability in monophasic click evoked BAEP studies.

Cochlear summating potential recorded from the external auditory meatus of normal humans. Amplitude-intensity functions and relationships to auditory nerve compound action potential

The summating potential (SP) and the auditory nerve compound action potential (AP) of the electrocochleogram were recorded from the external auditory meatus (EAM) of 10 normal subjects in response to alternating rarefaction and condensation clicks at 90-120 dB pe SPL. Both logarithmic and power functions suggested a similar strength of relationship between SP amplitude and stimulus intensity in each subject. However, a power function represented a more appropriate model of this relationship for the whole set of subjects than did the logarithmic function. Individual power function exponents varied from 0.35 to 0.56 with a weighted average of 0.46 and a S.E. of 0.03. By contrast, because of major heterogeneity of individual slopes at the high stimulus intensities eliciting EAM-measurable SPs, neither the logarithmic nor the power model provided a common description of the relationship between AP voltage and click strength for all subjects. Remarkable inter-individual variability also characterized the relationships between SP and AP amplitudes at various intensity levels and between the SP/AP amplitude ratio and click intensity. The lawful behavior of SP amplitude with varying click strength indicates that, in appropriate circumstance, non-invasive recordings from the ear canal of normal humans provide quantitative information on certain states of activity of cochlear receptors.