Exploration of cochlear function by otoacoustic emissions: relationship to pure-tone audiometry

The amplitudes, growth functions and detection thresholds of evoked otoacoustic emissions (EOE) were measured in 44 normally hearing subjects and 138 patients with two categories of cochlear dysfunctions: (a) acoustic trauma; and (b) presbycusis. Separate sets of experiments were also performed: (a) detection of stimulus frequency emissions; and (b) click EOE. EOE properties were studied around 2 kHz, 1 kHz and 0.75 kHz (+/- 0.1 kHz). A partial correlation and multivariate analysis was carried out to investigate the relationships between EOE properties and puretone auditory thresholds (from 0.25 to 8 kHz, half-octave steps). For each experiment and each frequency, only one highly significant correlation was found, linearly relating the n kHz EOE threshold with the hearing threshold at 2n kHz: there was a shift of about one octave between EOE amplitudes and audiometric data. This means that EOE thresholds give no direct information about the local cochlear state. A simplified model has been implemented, which assumes that EOE thresholds and amplitudes are proportional to the total number of residual active sites along the organ of Corti, i.e., to the total length of active basilar membrane towards the base of the cochlea. It is shown that this model accounts for the results revealed by the statistical analysis and closely fits the experimental data.