Electrocochleography and experimentally induced loudness recruitment

The relationship between changes in loudness and the cochlear whole-nerve potential following experimentally produced deafness was studied in an animal model. Reaction time of a subject's response to an auditory stimulus has been shown to be an index of loudness in human experiments and has been adapted to nonhuman primates. In a series of experiments, four macaque monkeys were operantly conditioned to respond to 8-kHz tones over a range of 3--80 dB SPL, and their reaction times to pure tone stimuli were measured. Whole-nerve cochlear action potentials were recorded from chronic inner-ear electrodes. The relationship between behavioral and electrical measures of loudness recruitment were examined in animals with both temporary and permanent noise-induced hearing loss. Loudness recruitment was demonstrated experimentally after a 1-h exposure to a high-intensity 8-kHz octave band of noise. Excellent agreement was observed between the reaction time function and the action potential input-output function at intervals of 0.5, 12, 24, 48, and 84 h after exposure. Permanent hearing loss was produced in some of these animals by a much longer duration of exposure to the 8-kHz octave band of noise. Recruitment was observed in both the behavioral and the electrical measures. Histological studies of these damaged cochleas revealed primarily outer hair cell destruction, with a relative sparing of inner hair cells and nerve supply. The findings of this study are interpreted as strong support for the clinical electrocochleogram as an objective indicator of the presence of loudness recruitment.