Tom Duke
Teaching
Miscellaneous
Active amplification: The main focus of hearing research in recent years has been the nature of the active process that enhances sound detection in the inner ear. We have advanced the general concept of self-tuned criticality to explain how the active system works. The cochlea contains a set of force-generating dynamical systems, each of which is maintained at the threshold of an oscillatory instability by feedback control. Poised at the critical point, on the verge of vibrating, each oscillator is especially responsive to periodic disturbances at its own characteristic frequency. The active amplification provided by the set of critical oscillators is ideally suited to the ear's needs, since it provides frequency selectivity, exquisite sensitivity and a wide dynamic range.
Interference in the ear: The intrinsic nonlinearity of the active mechanism causes tones of different frequency to interfere with one another in the cochlea. In order to provide a framework for understanding how the ear processes the more complex sounds of speech and music, we have examined the response of a critical oscillator to two tones. Our calculations indicate how the response to one tone is suppressed by the presence of a second tone of similar frequency. They also show how a characteristic spectrum of distortion products is generated. Based on this analysis, we have suggested that psychophysical phenomena such as the sensation of dissonance and auditory illusions might be attributed to the physical nature of the peripheral detection apparatus.
Detection apparatus: Hair cells of the inner ear detect mechanical stimuli by deflections of the hair bundle, which open tension-gated transduction channels in the cell membrane to admit ions from the endolymph. We have developed a model in which the interaction of calcium ions with the transduction channels can generate an oscillatory instability of the hair bundle. The frequency of spontaneous oscillations is governed, principally, by the architecture of the bundle. Working on a slower time scale, myosin motors attached to the channel act to tune this dynamical system to the critical point, at which it just begins to oscillate. The two adaptation mechanisms together create an active amplifier.
collaborators:
Frank
Julicher, MPI Complex Systems, Dresden
