THE TUNED DISPLACEMENT RESPONSE OF THE HEARING ORGAN IS GENERATED BY THE OUTER HAIR-CELLS

The motile responses of the guinea-pig hearing organ in response to a tone applied to the ear were measured by laser interferometry. Two types of responses can be recorded: (i) a vibration at the frequency of the applied tone; and (ii) a displacement response consisting of a shift in the position of the organ surface. The purpose of this study is to characterize the displacement response. The results are as follows. There is a relationship between the frequency of highest sensitivity (best-frequency) of the displacement response and the site from which it is recorded. High best-frequencies are noticed at more basal locations, low best-frequencies towards the apex. The displacement response is more frequency-selective than the vibration response. The displacement response is observed within physiological sound pressure levels. Its sharpness is dependent on the stimulus intensity, it shows biological variability and can be manipulated by drugs that are known to modify the receptor potential of the sensory cells, or to interfere with outer hair cell motility. These results suggest that the displacement response is an important step in the transduction process in the mammalian hearing organ and that it is generated by the motile action of the outer hair cells.