The spectral content of the cochlear microphonic measured in scala media of the guinea pig cochlea

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Abstract
Cochlear microphonic (CM) in response to low-frequency tonal stimuli, measured as a function of sound-pressure level (SPL) in scala media of the guinea pig cochlea, was averaged and Fourier analyzed. The slope of the amplitude of the Nth CM harmonic vs. sound intensity in log-log coordinates was approximately N (1 .ltoreq. N .ltoreq. 5) in the first 3 cochlear turns. Notable variations on such a slope rule applied to CM in turn I. CM harmonic phases plotted vs. SPL grouped into 2 distinctive categories expressly delimited by whether the order of the harmonic was even or odd. Some difference between CM recorded between scala media and scala tympani and recorded between scala media and the animals''s neck could be attributed to neural contamination. CM in its saturation region has a hysteretic relation to the input sound pressure. At high sound levels, large physiologically produced acoustic harmonics existed at the animal''s eardrum. An asymmetrical, saturating, single-valued nonlinearity may be a model for CM generation at low sound levels. At higher sound levels a different, more complex, hysteretic nonlinearity seems mandatory.

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