Distortion generated by the ear: Its emergence and evolution during development

Physiologically vulnerable active mechanics associated with the transduction of sounds in adults distort cochlear output. Specifically, frequencies not present in the incoming acoustic signal are represented in its output (i.e., the spike trains of auditory nerve fibers). The purpose of this investigation was to study the appearance and evolution of intermodulation distortion during development. Established surgical and electrophysiologic techniques were used to record the extracellular electrical activity of individual auditory nerve fibers from developing kittens. Discharge-rate or synchrony-based responses to two tones (f₁ and f₂) presented simultaneously at various ratios of f₂/f₁ were recorded. The cubic distortion product (CDP; 2f₁-f₂, where f₁<f₂) and the difference tone (f₂-f₁) were the intermodulation distortion products (DPs) emphasized in the investigation. CDPs were first observed nearly 10 days after the earliest auditory nerve fiber responses to simple tone-burst stimuli could be elicited in neonatal cats. Furthermore, the spectrum of DPs generated in the cochlea changed dramatically during the second postnatal week and achieved adultlike characteristics after the first postnatal month. Findings reported here support the notion that cochlear transduction is initially linear in neonatal cats, possibly representing a critical period of auditory development, and that the content of distortion-related spectral information carried in auditory nerve fiber spike trains increases during development, in concert with other peripheral auditory attributes.