Figure 2. Measurements of waves on Reissner’s membrane.

(A) Sound stimulation of an in vivo preparation of the guinea pig’s cochlea vibrates the Reissner’s membrane as measured near the cochlear apex. The phase accumulation over the region of measurement indicates the presence of traveling waves propagating from base to apex (left to right). (B) Waves on the Reissner’s membranes of different rodents display a similar dependence of wavelength on the stimulus frequency. For stimulation at frequencies exceeding 1 kHz the wavelength decreases as f^−2/3. The black line, which represents the behavior expected from theory, reveals a crossover from this scaling at high frequencies to scaling as f⁻¹ at low frequencies. This transition occurs near a wavelength λ = 2h or a frequency of 1 kHz. The measurements from chinchillas and those marked (1) from guinea pigs were performed in vivo; the experiments on gerbils and those marked (2) from guinea pigs employed in vitro preparations. (C) The sensitivity of Reissner’s membrane waves to acoustic stimulation is about 10 nm·Pa⁻¹ for frequencies up to 5 kHz and declines for greater frequencies. Four different experiments, represented by different symbols, were performed on guinea pig cochleas in vivo. For an animation of the waves measured on Reissner’s membrane see Movie S1.