Characteristics of the travelling wave in the low-frequency region of a temporal-bone preparation of the guinea-pig cochlea

This study provides a detailed quantitative description of the acoustically evoked vibration responses in the low-frequency region of the in vitro gui nea-pig cochlea. Responses of the basilar membrane, the reticular lamina an d Hensen cells were measured with a laser Doppler vibrometer, without the n eed for introducing artificial light reflectors. The apex of the cochlea wa s opened, leaving the helicotrema intact. Two response components were dete cted: a 'fast' component, which was probably caused by the hole in the coch lea, and a 'slow' component, which shared the features of a classical trave lling wave. The velocity response of the "slow" component exhibited a relat ively flat low-frequency slope (15 dB/oct) and a much steeper high-frequenc y roll-off (third turn: -47 dB/oct; fourth turn: -35 dB/oct). The group del ay was dependent on the characteristic frequency. In the fourth turn, the s harpness of the velocity tuning curves (Q(10) (dB): 1.0) was similar to tho se of in vivo mechanical and neural recordings, whereas in the third turn t he tuning (Q(10 dB): 1.1) was much less than for in vivo recordings. The re sults indicate that cochlear amplification, which is responsible for the hi gh sensitivity and sharp tuning in the basal part of the cochlea, is much l ess pronounced in the apical turn of the cochlea. (C) 2000 Elsevier Science B.V. All rights reserved.