Micro-mechanical motion of the guinea-pig organ of Corti.

One-dimensional measurements of sound-evoked motion were made using optical coherence tomography (OCT) in the high frequency, basal turn of the cochlea in deeply anesthetized guinea-pigs. Wideband acoustic stimuli were used to evoke nanometer-scale vibrations of the basilar membrane (BM) and organ of Corti (OoC). Both macro- and micro-mechanical aspects of the responses were analysed, where macro-mechanical refers to 'motions observed on the BM', and micro-mechanical refers to 'motions of OoC and related structures, relative to those observed on the BM'. Both macro- and micro-mechanical motions exhibited compressive nonlinearity across wide dynamic ranges (typically from ∼30 to >80 dB SPL near a site's characteristic frequency or CF). Only the macro-mechanical motion was sharply tuned, however – the micro-mechanical motion, which included a transverse 'squeezing' type of OoC deformation, was low-pass filtered (from well below CF), and not sharply tuned in its own right. The micro-mechanical motion was also delayed, accumulating ∼0.5 cycles of phase-shift with respect to the macro-mechanics between 0 Hz and CF. Putative cell-length changes were also derived in the Deiters' (DC) and outer hair cell (OHC) regions: Both DC and OHC length changes exceeded macro-mechanical movements by factors of ∼2-4 at low frequencies and moderate sound levels, but decreased in relative magnitude with increasing frequency and/or sound level. At low frequencies, OHCs became longer and shorter, and DCs became shorter and longer, when the BM moved towards the scala tympani and scala vestibuli, respectively. These phase relationships reversed at frequencies closer to CF, in keeping with the ∼0.5 cycles of phase-accumulation in the micro-mechanics. These observations are discussed with respect to previous findings, including a recent theoretical study that considered frequency-dependent changes in the mode-shape of a wave propagating along a simple physical waveguide [1]. [ABSTRACT FROM AUTHOR]