Cochlear summating potential to broadband clicks detected from the human external auditory meatus. A study of subjects with normal hearing for age.

The cochlear summating potential (SP) preceding the auditory nerve compound action potential (AP) was elicited by broadband alternating condensation and rarefaction clicks and recorded by noninvasive electrodes from the external auditory meatus (EAM) of 60 volunteers of both sexes, 12 to 67 years old, who had normal hearing for age. Quantitative data were obtained on: the number of ears displaying measurable SPs; the SP detection level; the SP onset, peak and rise times; the duration of the SP-AP complex; the SP amplitude; and the SP/AP amplitude ratio. Previously unknown relationships were unveiled between the amplitude, but not the temporal, measures of the SP and laterality, sex, age, and audiometrically determined hearing thresholds to 4 to 8 kHz tones. The highest correlations were obtained with these last thresholds, which suggested that receptors in the basal turn of the cochlea played a dominant role in the generation of the EAM-detected SP. To improve on existing techniques for determining abnormal SP elevation, a multiple regression method was devised that utilized sex, age, 4 to 8 kHz hearing thresholds, and AP voltage to establish upper normal limits of SP amplitude for individual subjects and ears.

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.