Electrical stimulation of the auditory nerve: direct current measurement in vivo

Abstract

Neural prostheses use charge recovery mechanisms to ensure the electrical stimulus is charge balanced. Nucleus cochlear implants short all stimulating electrodes between pulses in order to achieve charge balance, resulting in a small residual direct current (DC). In the present study we sought to characterize the variation of this residual DC with different charge recovery mechanisms, stimulation modes, and stimulation parameters, and by modeling, to gain insight into the underlying mechanisms. In an acute study with anaesthetised guinea pigs, DC was measured in four platinum intracochlear electrodes stimulated using a Nucleus C124M cochlear implant at moderate to high pulse rates (1200-14,500 pulses/s) and stimulus intensities (0.2-1.75 mA at 26-200 microseconds/phase). Both monopolar and bipolar stimulation modes were used, and the effects of shorting or combining a capacitor with shorting for charge recovery were investigated. Residual DC increased as a function of stimulus rate, stimulus intensity, and pulse width. DC was lower for monopolar than bipolar stimulation, and lower still with capacitively coupled monopolar stimulation. Our model suggests that residual DC is a consequence of Faradaic reactions which allow charge to leak through the electrode tissue interface. Such reactions and charge leakage are still present when capacitors are used to achieve charge recovery, but anodic and cathodic reactions are balanced in such a way that the net charge leakage is zero.

Silicon-Integrated High-Density Electrocortical Interfaces

“…One of the most straightforward strategies is to employ serial dcblocking capacitors, inherently forcing the net dc to be zero all the time. This method has been employed for neural stimulation applications [97]- [99] due to its intrinsic safety when area permits. However, the required blocking capacitance is often prohibitively large for onchip integration, and is thus inadequate for high-density and/or miniaturized implants.…”

Cochlear Implants: System Design, Integration, and Evaluation

“…• Charge balance check to prevent unbalanced stimulation and DC stimulation because they generate gases, toxic oxychlorides, corrosion products, and associated pH changes that can cause tissue damage [99].…”

Neurotrophins and electrical stimulation for protection and repair of spiral ganglion neurons following sensorineural hearing loss

“…The output of the stimulator delivered 100 µs/phase charge balanced biphasic current pulses to a bipolar electrode pair at a stimulus rate of 1200 pulses per second (pps) and was amplitude-modulated to a depth of 50% at 30 Hertz. Electrode shorting and capacitive coupling were used to ensure complete charge recovery (Huang et al, 1999). The amplitude of the stimulus waveform was set so that the minimum current level equalled the post-operative EABR threshold (i.e.…”