Lazzaro and Mead — Circuit Models of Sensory Transduction in the Cochlea CIRCUIT MODELS OF SENSORY TRANSDUCTION IN THE COCHLEA

Nonlinear signal processing is an integral part of sensory transduction in the nervous system. Sensory inputs are analog, continuous-time signals with a large dynamic range, whereas central neurons encode information with limited dynamic range and temporal specificity, using fixed-width, fixed-height pulses. Sensory transduction uses nonlinear signal processing to reduce real-world input to a neural representation, with a minimal loss of information. An excellent example of nonlinear processing in sensory transduction occurs in the cochlea, the organ that converts the sound energy present at the eardrum into the first neural representation of the auditory system, the auditory nerve. Humans can process sound input over a 120-dB dynamic range, yet the firing rate of an auditory-nerve fiber can encode only about 25 dB of sound intensity. Humans can sense binaural time differences of the order of ten microseconds, yet an auditory-nerve fiber can fire at most once per millisecond. Using limited neural resources, the cochlea creates a representation that preserves the information essential for sound localization and understanding. Moreover, this neural code expresses auditory information in a way that facilitates feature extraction by higher neural structures. We are building silicon integrated circuits that model sensory transduction in the cochlea, both to explore the general computational principles of the cochlea, and to create potentially useful devices for sound understanding, for sound localization, and for cochlear prostheses. In this paper, we describe the architecture and operation of an integrated circuit that models, to a limited degree, the evoked responses of the auditory nerve. The chip receives as input a time-varying voltage corresponding to sound input, and computes outputs that correspond to the responses of individual auditory-nerve fibers. The chip models the structure as well as the function of the cochlea; all subcircuits in the chip have anatomical correlates. The chip computes all outputs in real time, using analog continuous-time processing.