Modeling the nonlinear active cochlea: Mathematics and analog VLSI

MODELING THE NONLINEAR ACTIVE COCHLEA: MATHEMATICS AND ANALOG VLSI Bo Wen Supervisor: Kwabena A. Boahen The human auditory system vastly outperforms any machine in efficiency and robustness in perceiving sound. Complex and delicate, its front end, namely the cochlea, senses and processes sound in a nonlinear active fashion, exhibiting remarkable sensitivity and extraordinary frequency discrimination. The mechanism through which the mammalian cochlea achieves its incredible capability is postulated as the cochlear amplifier, which is generally believed to originate from outer hair cells electromotility. However, the detail of the cochlear amplifier remains unclear. Using both analytic and synthetic approaches, the present work provided a plausible basis for realization of the cochlear amplifier, thus advancing our understanding of cochlear mechanics. We proposed a novel cochlear amplifier mechanism based on the cochlea’s microanatomy, as well as outer hair cell (OHC) motility, to account for the cochlea’s characteristic behavior. This mechanism, active bidirectional coupling (ABC), considers both the basal tilt of OHCs and the apical tilt of phalangeal processes as critical for feeding OHC motile forces (with saturating property) forward and backward onto the basilar membrane, thereby enhancing the cochlea’s functioning. The ABC-based mathematical cochlear model produces responses that are comparable to physiological measurements. Theoretical model analysis reveals that ABC leads to negative damping basal to the response peak over a small longitudinal cochlear region. The tilted structure works as a v spatial filter, amplifying the cochlear traveling wave only when its wavelength becomes comparable to the tilt distance. Inspired by the biology, we proceeded to build a nonlinear active silicon cochlea—a very large scale integration (VLSI) physical cochlear model—for achieving real-time lowpower cochlear processing. Designed in current mode and operating in Class AB, this microchip implements the ABC mechanism, together with a silicon auditory nerve, in 0.25 μm complementary metal-oxide-semiconductor (CMOS) technology. The resultant new architecture addresses the shortcomings of existing silicon cochleae, filter banks in cascade and in parallel. Analog current representing the basilar membrane’s velocity drives the silicon auditory nerve, which encodes sound in digital pulses, mimicking neuronal spikes, as the final output of the cochlea.