The University of Melbourne Departments of Otolaryngology and Electrical Engineering (UMDOLEE) receiving and stimulating component of a multiple-electrode cochlear implant hearing prosthesis produces constant stimulation. It has a stimulating pulse shape that minimizes the production of toxic substances and loss of metal from the electrodes, and this is achieved with a biphasic rectangular waveform where the first phase is negative with respect to ground. The duration of each stimulus phase in 180 msec, which is long enough to allow low levels of current stimulation, and short enough to permit rates of 1000 pulses/second to be achieved. In order to be consistent with our present understanding of the perception of pitch, the device permits the independent stimulation of a number of electrodes. Furthermore, to electrically isolate the stimulus to small areas, there is the capacity to vary the current and set the threshold independently at individual electrodes. The phase and amplitude of the pulses to neighbouring electrodes with also be varied to assist in localizing the current flow. The pattern to stimulation to individual or groups of electrodes can also be altered to enable studies to be carried out to determine ways of conveying frequency and intensity information over a more normal dynamic range.

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The occurrence of threshold phenomena at membranes is of fundamental importance in studies of excitation of living cells. This paper presents the derivation of generalized expressions for solution concentration as a function of time and distance in a simple plane membrane+ solution system under constant current polarization, considering both thick and thin membranes ; the conditions of occurrence of threshold effects are deduced. From these expressions are obtained the relation between potential difference and time for two reversible probe electrodes, one on either side of the membrane: this permits an investigation of the degree of charge and the structure of real membranes under conditions of rising potential gradient.

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