An Analogue VLSI Implementation of the Meddis Inner Hair Cell Model
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An Analogue VLSI Implementation of the Meddis Inner Hair Cell Model Alistair McEwan Computer Engineering Laboratory, School of Electrical and Information Engineering, University of Sydney, NSW 2006, Australia Email: [email protected]
Andre´ van Schaik Computer Engineering Laboratory, School of Electrical and Information Engineering, University of Sydney, NSW 2006, Australia Email: [email protected] Received 18 June 2002 and in revised form 23 September 2002 The Meddis inner hair cell model is a widely accepted, but computationally intensive computer model of mammalian inner hair cell function. We have produced an analogue VLSI implementation of this model that operates in real time in the current domain by using translinear and log-domain circuits. The circuit has been fabricated on a chip and tested against the Meddis model for (a) rate level functions for onset and steady-state response, (b) recovery after masking, (c) additivity, (d) two-component adaptation, (e) phase locking, (f) recovery of spontaneous activity, and (g) computational efficiency. The advantage of this circuit, over other electronic inner hair cell models, is its nearly exact implementation of the Meddis model which can be tuned to behave similarly to the biological inner hair cell. This has important implications on our ability to simulate the auditory system in real time. Furthermore, the technique of mapping a mathematical model of first-order differential equations to a circuit of log-domain filters allows us to implement real-time neuromorphic signal processors for a host of models using the same approach. Keywords and phrases: analogue circuits, analogue computing, neuromorphic engineering, audio processing.
1.
INTRODUCTION
Inner hair cells (IHCs) are mechanical to neural transducers located within the cochlea and play an important role in biological sound processing. Sound captured by the eardrum is translated into movement of the cochlear fluid, which in turn causes the basilar membrane to vibrate (see Figure 1). This vibration is converted into a neural signal by the IHCs and results in the firing of the auditory nerve cells. The transduction of the sound signal by the IHC is nonlinear and exhibits several time constants of adaptation. To mimic the IHC processing, past silicon cochleae (see [1, 2]) have used nonlinear lowpass filters to produce the adaptation characteristic of the IHC. These IHC circuits responded favourably to a simple set of stimuli but failed with more complex stimuli [1]. We present measurement results of an analogue VLSI implementation of the Meddis IHC model [3], which is the most descriptive computational model for the function of the IHC [4]. In the circuit presented here, we use log-domain lowpass filters [5] to map the differential equations of the Meddis IHC model to circuits on a silicon chip. This technique allows the Meddis model to run in real time. In our model, we have
furthermore increased the flexibility of the Meddis model by maintaining control of all time constants while introducing indepe
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