Bo Wen, Kwabena A. Boahen
We present a novel cochlear model implemented in analog very large scale integration (VLSI) technology that emulates nonlinear active cochlear behavior. This silicon cochlea includes outer hair cell (OHC) electromotility through active bidirectional coupling (ABC), a mech- anism we proposed in which OHC motile forces, through the mi- croanatomical organization of the organ of Corti, realize the cochlear ampliﬁer. Our chip measurements demonstrate that frequency responses become larger and more sharply tuned when ABC is turned on; the de- gree of the enhancement decreases with input intensity as ABC includes saturation of OHC forces.
1 Silicon Cochleae
Cochlear models, mathematical and physical, with the shared goal of emulating nonlinear active cochlear behavior, shed light on how the cochlea works if based on cochlear mi- cromechanics. Among the modeling efforts, silicon cochleae have promise in meeting the need for real-time performance and low power consumption. Lyon and Mead developed the ﬁrst analog electronic cochlea , which employed a cascade of second-order ﬁlters with exponentially decreasing resonant frequencies. However, the cascade structure suf- fers from delay and noise accumulation and lacks fault-tolerance. Modeling the cochlea more faithfully, Watts built a two-dimensional (2D) passive cochlea that addressed these shortcomings by incorporating the cochlear ﬂuid using a resistive network . This par- allel structure, however, has its own problem: response gain is diminished by interference among the second-order sections’ outputs due to the large phase change at resonance .
Listening more to biology, our silicon cochlea aims to overcome the shortcomings of exist- ing architectures by mimicking the cochlear micromechanics while including outer hair cell (OHC) electromotility. Although how exactly OHC motile forces boost the basilar mem- brane’s (BM) vibration remains a mystery, cochlear microanatomy provides clues. Based on these clues, we previously proposed a novel mechanism, active bidirectional coupling (ABC), for the cochlear ampliﬁer . Here, we report an analog VLSI chip that implements this mechanism. In essence, our implementation is the ﬁrst silicon cochlea that employs stimulus enhancement (i.e., active behavior) instead of undamping (i.e., high ﬁlter Q ).
The paper is organized as follows. In Section 2, we present the hypothesized mechanism (ABC), ﬁrst described in . In Section 3, we provide a mathematical formulation of the