Qualitative and quantitative changes in the guinea pig organ of Corti after pure tone acoustic overstimulation.

A direct relationship between delivered sound energy and morphological overstimulation has been formulated in the so-called equal energy principle. The validity of this principle was systematically tested by studying the effects of acoustic overstimulation on the guinea pig cochlea by means of scanning electron microscopy (SEM) and light microscopy (LM) techniques. The animals in the present study were exposed to 3.85 kHz pure tones of various durations and intensities and allowed a recovery period of 4 weeks before the morphological changes in the cochlea were assessed. A 4-grade scale was devised in order to achieve a qualitative as well as a quantitative SEM evaluation of the cochlear damage. The results of the examinations by LM and SEM were compared and analysed statistically. No significant difference was observed between groups of animals exposed to a sound energy of 9.4 and 37.5 Pa2.h, irrespective of sound intensity (114 or 120 dB SPL). Groups of animals exposed to a higher energy level (150 Pa2.h) differed significantly from those exposed to higher and also lower sound energy levels. Groups exposed to different intensities (108, 114 and 120 dB SPL) did not differ significantly within this energy level.