Laser-feedback measurements of turtle basilar membrane motion using direct reflection

Area of Science:

• Auditory Neuroscience
• Comparative Physiology
• Bioacoustics

Background:

• Mammalian hearing relies on the basilar membrane's tonotopic organization for frequency discrimination.
• Lower vertebrates, like turtles, may employ different mechanisms, such as electrical resonance in auditory receptor cells, for frequency selectivity.

Purpose of the Study:

• To investigate the mechanical properties of the turtle basilar membrane and its role in frequency separation.
• To determine if the turtle basilar membrane exhibits frequency-dependent spatial patterns similar to mammals.

Main Methods:

• Utilized laser-feedback interferometry to measure vibrational amplitude and phase of the turtle basilar membrane.
• Applied nanometer displacements to the eardrum to simulate auditory stimuli.
• Analyzed phase-angle and amplitude tuning curves as a function of frequency and position.

Main Results:

• The turtle basilar membrane's vibrational characteristics broadly mirrored the middle-ear filter.
• No significant variation in phase angle or best frequency was observed across different positions on the basilar membrane within specimens.
• Input-output functions of the basilar membrane were predominantly linear.
• The middle ear exhibited negative gain (-2 to -6), while the central basilar membrane showed positive gain (4 to 18).

Conclusions:

• The turtle basilar membrane's mechanical properties do not appear to be the primary basis for frequency discrimination.
• Frequency separation in turtles likely relies more on cellular mechanisms like electrical resonance rather than basilar membrane tonotopy.
• Laser-feedback interferometry is a viable non-invasive technique for studying basilar membrane mechanics.